Messier 69 – the NGC 6637 Globular Cluster

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the globular cluster known as Messier 69.

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is known as Messier 69 (NGC 6637), a globular cluster located in the constellation Sagittarius. Located about about 29,700 light-years away from Earth, this cluster lies close to Messier 70 (both of which were discovered Charles Messier on August 31st, 1780). Both objects lie close to the galactic center, and M69 is one of the most metal-rich globular clusters known.

Description:

At about 29,700 light years from Earth, this 61 light year diameter ball of stars is one of the faintest of the Messier objects and very close to our galactic center. It was formed quite early in our galactic history and is one of the most metal rich of all globular clusters. As Robert Zinn and Pierre DeMarque of Yale University’s Department of Astronomy wrote in a 1996 study:

“We have observed the metal-rich globular clusters NGC 6624 and NGC 6637 (M69) using the planetary camera of the WFPC2 on the Hubble Space Telescope (HST). Observations of the Ca II triplet lines in giant stars in these clusters show that NGC 6624 and NGC 6637 have metallicities on the Zinn and West scale of [Fe/H] = -0.63 ± 0.09 and -0.65 ± 0.09, only slightly more metal rich than 47 Tuc [Fe/H] = -0.71 ± 0.07. For clusters of identical (or nearly so) metallicity, one can make a direct comparison of the color-magnitude diagrams to derive the relative ages of the clusters. The positions of NGC 6624 and NGC 6637 in the Galaxy suggest that they belong to the bulge population of globular clusters. The only other bulge clusters that have been dated so far are the more metal rich clusters NGC 6528 and NGC 6553, which also appear to be very old. Consequently, the age-metallicity relation of the bulge may be very steep. The close similarity of the ages and metallicities of NGC 6624 and NGC 6637 to the thick-disk globular clusters 47 Tuc and NGC 6352 indicates that the age-metallicity relations of these populations intersect. We briefly discuss the possibility that these populations had a common origin.”

This dazzling image shows the globular cluster Messier 69, as viewed through the NASA/ESA Hubble Space Telescope. Credit: NASA/ESA/HST

One very odd thing about M69 is its lack of variable stars. Harlow Shapley didn’t find any and the number of known variable stars debatable, with a few of them being Mira-type variable stars with periods of about 200 days. As J.D. Gregorsok (et al) indicated in a 2003 study:

“We present time-series VI photometry of the metal-rich globular cluster NGC 6637. Our color-magnitude diagrams show a predominantly red-clump horizontal branch morphology with hints of a blue horizontal branch extension as seen in NGC 6388 and NGC 6441. We discovered at least four new long-period variable stars in addition to recovering the nine variable stars already discovered. We discuss the cluster membership probabilities of the variables, and present their light curves.”

Are studies like this important? You bet. Because neighboring globular cluster M70 is so close in distance, there’s a distinct chance the two might be physical neighbors. Only through studies can we understand if they truly formed together or not. As A. Rosenberg (et al) explained in a 2000 study:

“Among the many tools we have to investigate the properties of a stellar population, the color-magnitude diagrams (CMD) are the most powerful ones, as they allow to recover for each individual star its evolutionary phase, giving precious information on the age of the entire stellar system, its chemical content, and its distance. This information allows us to locate the system in the space, giving a base for the distance scale, study the formation histories of the Galaxy, and test our knowledge of stellar evolution models.”

The Messier 69 globular cluster, as imaged by the Hubble Space Telescope. Credit: NASA/ESA/Hubble Space Telescope

History of Observation:

M69 was discovered by Charles Messier and added to his catalog on August 31, 1780, the same night he found M70. In his notes he states: “Nebula without star, in Sagittarius, below his left arm and near the arc; near it is a star of 9th magnitude; its light is very faint, one can only see it under good weather, and the least light employed to illuminate the micrometer wires makes it disappear: its position has been determined from Epsilon Sagittarii: this nebula has been observed by M. de La Caille, and reported in his Catalog; it resembles the nucleus of a small Comet. (diam 2′)”.

While Messier was mistaken about LaCaille’s position, there was no mistaking the observations of Sir William Herschel who first resolved this globular cluster – from a very northern position! “1784, 20 feet telescope. Very bright, pretty large, easily resolvable, or rather an already resolved cluster of minute stars. It is a miniature of the 53d of the Connoissance [M53].” His son John would go on to add it to the General Catalog and describe it as a “blaze of stars”, while Messier’s error would continue on for many years as a debate on LaCaille’s position.

But you know where to find it!

Locating Messier 69:

Because the constellation of Sagittarius is so low for the northern hemisphere, it is best to wait until it is at culmination (its highest point) before trying for this small globular cluster. Begin by identifying the familiar teapot asterism and draw a mental line between its southernmost stars – Zeta and Epsilon. About one third the distant between Epsilon and Zeta, you will see a conspicuous pair of stars that will show easily in your binoculars or telescope finderscope. M69 is less than a degree north of the northernmost of this pair.

The location of Messier 69 in the Sagittarius constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

In binoculars, M69 will appear almost stellar and very faint – like a hairy star that won’t quite resolve. To a small telescope it will appear cometary and begin resolution in apertures around 8″. It requires dark, transparent skies and is not well suited to moonlight or urban lighting situations.

And here are the quick facts on this Messier Object to help you get started:

Object Name: Messier 69
Alternative Designations: M69, NGC 6637
Object Type: Class V Globular Cluster
Constellation: Sagittarius
Right Ascension: 18 : 31.4 (h:m)
Declination: -32 : 21 (deg:m)
Distance: 29.7 (kly)
Visual Brightness: 7.6 (mag)
Apparent Dimension: 9.8 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Messier 68 – the NGC 4590 Globular Cluster

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the globular cluster known as Messier 68.

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is the globular cluster known as Messier 68. Located roughly 33,000 light-years away in the Constellation of Hydra, this cluster is orbiting through the Milky. In addition to being one of the most metal-poor globular clusters, it may be undergoing core collapse, and is believed to have been acquired from a satellite galaxy that merged with the Milky Way in the past.

Description:

At a distance of approximately 33,000 light-years, the M68 globular cluster contains at least 2,000 stars, including 250 giants and 42 variables – one of which is actually a foreground star and not a true member. Spanning 106 light years in diameter and coming towards us at a speed of 112 kilometers per second, about 250 giant stars are happily perking away – enjoying their chemically abundant status. As Jae-Woo Lee (et al), indicated in a 2005 study:

“We present a detailed chemical abundance study of seven giant stars in M68, including six red giants and one postasymptotic giant branch (AGB) star. We find significant differences in the gravities determined using photometry and those obtained from ionization balance, which suggests that non-LTE (NLTE) affects are important for these low-gravity, metal-poor stars. We adopt an iron abundance using photometric gravities and Fe II lines to minimize those effects, finding [Fe/H] = -2.16 ± 0.02 ( = 0.04). For element-to-iron ratios, we rely on neutral lines versus Fe I and ionized lines versus Fe II (except for [O/Fe]) to also minimize NLTE effects. We find variations in the abundances of sodium among the program stars. However, there is no correlation (or anticorrelation) with the oxygen abundances. Furthermore, the post-AGB star has a normal (low) abundance of sodium. Both of these facts add further support to the idea that the variations seen among some light elements within individual globular clusters arise from primordial variations and not from deep mixing. M68, like M15, shows elevated abundances of silicon compared with other globular clusters and comparable-metallicity field stars. But M68 deviates even more in showing a relative underabundance of titanium. We speculate that in M68 titanium is behaving like an iron-peak element rather than its more commonly observed adherence to enhancements seen in the so-called -elements such as magnesium, silicon, and calcium. We interpret this result as implying that the chemical enrichment seen in M68 may have arisen from contributions from supernovae with somewhat more massive progenitors than those that contribute to abundances normally seen in other globular clusters.”

Image of Messier 68, taken by the NASA/ESA Hubble Space Telescope oCredit: Hubble/NASA/ESA

One of the most unusual features of Messier 68 is its position in the grand scheme of things – opposite our galactic center. We know that globular clusters lay almost exclusively within the galactic halo, so what could cause this? As Yoshiaki Sofue of the University of Tokoyo’s Department of Astronomy explained in a 2008 study:

“We construct a Galacto-Local Group rotation curve, combining the Galactic rotation curve with a diagram, where galacto-centric radial velocities of outer globular clusters and member galaxies of the Local Group are plotted against their galacto-centric distances. In order for the Local Group to be gravitationally bound, an order of magnitude larger mass than those of the Galaxy and M31 is required. This fact suggest that the Local Group contains dark matter filling the space between the Galaxy and M31. We may consider that there are three components of dark matter. First, the galactic dark matter which defines the mass distribution in a galaxy controlling the outer rotation curve; second, extended dark matter filling the entire Local Group having a velocity dispersion as high as ~200 km s^-1, which gravitationally stabilize the Local Group; and finally, uniform dark matter having much higher velocities originating from supergalactic structures. The third component, however, does not significantly affect the structure and dynamics of the present Local Group. We may therefore speculate that at any place in the Galaxy, there are three different components of dark matter having different velocities or different temperatures. They may behave almost independently from each other, but are interacting by their gravity.”

And that fact is carried out by further studies. As Roberto Capuzzo Dolcetta (et al) demonstrated in a study:

“Globular clusters moving in the Milky Way, as well as small galaxies swallowed by the strong tidal field of the Milky Way, develop tidal tails. This project is a part of a larger program of study devoted to the study of the evolution of Globular Cluster Systems in galaxies and of the mutual feedback between the parent galaxy and its GCS, on both the small and large scale. This project is part of an ongoing program devoted to test if and how tidal interaction with the parent galaxy may affect kinematics of stars close to the tidal radius of some galactic globular clusters and explain the flat observed profile of the velocity dispersion radial profile at large radii. The study of the dynamical interaction of globular clusters (hereafter GCs) with the galactic tidal field represents a modern and current astrophysical concern at the light of recent high resolution observations. The globular cluster system (hereafter GCS) results to be less peaked than that of halo stars in our Galaxy, in M31, M87 and M89, as well as in three galaxies of the Fornax cluster and 18 elliptical galaxies. The most probable explanation for this finding is that the two systems (halo and GCS) originally had the same profile and that, afterwards, the GCS evolved due to two complementary effects, mainly: tidal interaction with the galactic field and dynamical friction, which induces massive GCs to decay in the central galactic region in less than 10^8 years. External tidal fields have also the effect of inducing the evolution of the shape of the mass function of individual clusters, because of the preferential loss of low-mass stars as a consequence of mass segregation. Strong evidence that the tidal field plays a fundamental role in the evolution of mass functions was achieved by the discovery that their slopes correlate more strongly with the cluster location in the Milky Way than with the cluster metallicity. But the strongest evidences of the interaction of GCs with the galactic field have been found in the last decade, with the detection of haloes and tails surrounding many GCs.”

Messier 68 globular cluster by Hubble Space Telescope; 3.32? view. Credit: NASA & ESA (Hubble Space Telescope)

Is it true that Messier 68 may indeed by a “left over” from another galaxy? Yes, indeed. As M. Catelan argued in a 2005 study:

“We review and discuss horizontal branch (HB) stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters in the HB morphology metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC—an argument which, due to its strong reliance on the ancient RR Lyrae stars, is essentially independent of the chemical evolution of these systems after the very earliest epochs in the Galaxy’s history.”

History of Observation:

M68 was discovered by Charles Messier on April 9, 1780 who described it as; “Nebula without stars below Corvus and Hydra; it is very faint, very difficult to see with the refractors; near it is star of sixth magnitude”. The first resolution of the individual stars was, of course, attributed to Sir William Herschel. As he wrote in his notes at the time:

“A beautiful cluster of stars, extremely rich, and so compressed that most of the stars are blended together; it is near 3′ broad and about 4′ long, but chiefly round, and there are very few scattered stars about. This oval cluster is also approaching to the globular form, and the central compression is carried to a high degree. The insulation is likewise so far advanced that it admits of an accurate description of the contour.”

Thanks to a rather strange error on Admiral Smyth’s part, for many years it was believed to be the discovery of Pierre Mechain. As Smyth wrote in his notes:

“A large round nebula on Hydra’s body, under Corvus, discovered in 1780 by Mechain. In 1786, Sir William Herschel’s powerful 20-foot reflector resolved it into a rich cluster of small stars, so compressed that most of the components are blended together. It is about 3′ broad, and 4′ long; and he estimated that its profundity may be of the 344th order. It is posited nearly mid-way between two small stars, one in the np [NW] and the other in the sf [SE] quadrant, a line between which would bisect the nebula. It is very pale, but so mottled that a patient scrutiny leads to the inference, that it has assumed a spherical figure in obedience to attractive forces. Differentiated with Beta Corvi, from which it bears south by east, within 3 deg distance.”

This error took nearly a century to correct! Don’t take a century to view this lovely globular cluster yourself...

The location of Messier 68 in the Hydra constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 68:

The brighter stars of the northern winter season make finding this small globular cluster quite easy for both binoculars and telescopes – begin first by identifying the lopsided rectangle of the constellation of Corvus and focus your attention on its southeastern most star – Beta. Our target is located about three finger-widths southeast of Beta Corvi and just a breath northeast of the double star A8612.

It will show as a faint, round glow in binoculars, and small telescopes will perceive individual members. Large telescopes will fully resolve this small globular to the core! Messier Object 68 is well suited to any sky conditions when the stars of Corvus are visible.

And here are the quick facts on this Messier Object to help you get started:

Object Name: Messier 68
Alternative Designations: M68, NGC 4590
Object Type: Class X Globular Cluster
Constellation: Hydra
Right Ascension: 12 : 39.5 (h:m)
Declination: -26 : 45 (deg:m)
Distance: 33.3 (kly)
Visual Brightness: 7.8 (mag)
Apparent Dimension: 11.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Farewell Kepler. Welcome TESS

At 6:51 EDT on Wednesday, April 18th, a SpaceX Falcon 9 rocket blasted off from Florida’s Cape Canaveral. It was carrying NASA’s TESS: the Transiting Exoplanet Survey Satellite. From what we can tell, the mission went without a hitch, with the first stage returning to land on its floating barge in the Atlantic Ocean, and stage 2 carrying on to send TESS into its final orbit.

This is a changing of the guard, as we’re now entering the final days for NASA’s Kepler Space Telescope. It’s running out of fuel and already crippled by the loss of its reaction wheels. In just a few months NASA will shut it down for good.

That is sad, but don’t worry, with TESS on its way, the exoplanet science journey continues: searching for Earth-sized worlds in the Milky Way.

It’s hard to believe that we’ve only known about planets orbiting other stars for just over 20 years now. The first extrasolar planet found was the hot jupiter 51 Pegasi B, which was discovered in 1995 by a team of Swiss astronomers.

They found this world using the radial velocity method, where the gravity of the planet pulls its star back and forth, changing the wavelength of the light we see ever so slightly. This technique has been refined and used discover many more planets orbiting many more stars.

But another technique has been even more successful: the transit technique. This is where the light from the star is carefully measured over time, watching for any dip in brightness as a planet passes in front.

In a series of papers, Professor Loeb and Michael Hippke indicate that conventional rockets would have a hard time escaping from certain kinds of extra-solar planets. Credit: NASA/Tim Pyle
In a series of papers, Professor Loeb and Michael Hippke indicate that conventional rockets would have a hard time escaping from certain kinds of extra-solar planets. Credit: NASA/Tim Pyle

At the time that I’m writing this article in April, 2018, there are 3,708 confirmed planets with several thousand more candidates that need additional confirmation.

Planets are everywhere, in all shapes and sizes. From the familiar gas giants, rocky worlds and ice giants we have in the Solar System, to the unusual hot jupiters and super earths. Astronomers have even found comets in other solar systems, planets like Saturn but with ring systems that dwarf our neighbouring planet. The hunt is even on for exomoons. Moons orbiting planets orbiting other stars.

NASA’s Kepler Space Telescope was the most productive planet hunting instrument ever built. Of those 3,708 planets discovered so far, Kepler turned up 2,342 worlds.

Artist's concept of the Kepler mission with Earth in the background. Credit: NASA/JPL-Caltech
Artist’s concept of the Kepler mission with Earth in the background. Credit: NASA/JPL-Caltech

Kepler was launched back in March 2009, and began operations on May 12, 2009. It used its 1.4 meter primary mirror to observe a 12-degree region of the sky. Just for comparison, the Moon takes up about half a degree. So a region containing hundreds of times the size of the Moon.

Kepler was placed into an Earth-trailing orbit around the Sun, with a period of 372.5 days. With a longer year, the telescope slowly drifts behind the Earth by about 25 million km per year.

As I mentioned earlier, Kepler was designed to use the transit technique, searching for planets passing in front of their stars in this very specific region of the sky. While previous exoplanet surveys had only found the more massive planets, Kepler was sensitive enough to see worlds with half the mass of Earth orbiting other stars.

The number of confirmed exoplanets, by year. Credit: NASA
The number of confirmed exoplanets, by year. Credit: NASA

And everything was going great until July 14, 2012 when one of the spacecraft’s four reaction wheels failed. These are gyroscopes that allow the spacecraft to change its orientation without propellant. No problem, Kepler was designed to only need three. Then a second wheel failed on May 11, 2013, bringing an end to its main mission.

What the Kepler engineers came up with is one of the most ingenious spacecraft rescues in the history of spaceflight. They realized that they could use light pressure from the Sun to perfectly stabilize the telescope and keep it pointed at a region of the sky.

How the K2 mission rescued Kepler. Image credit: NASA
How the K2 mission rescued Kepler. Image credit: NASA

This allowed Kepler to keep working, observing even larger portions of the sky, but its orbit around the Sun would only let it watch one region for a shorter period of time. Instead of scanning Sun-like stars, Kepler focused its attention on red dwarf stars, which can have Earth sized worlds orbiting them every few days.

This was known as the K2 era, and during this time it turned up an additional 307 confirmed, and 480 unconfirmed planets.

But Kepler is running out of time now. About a month ago NASA announced that Kepler’s almost out of fuel. This fuel is important because one important maneuver it needs to make is to point itself back and Earth and upload all the data it’s been gathering. NASA figures that’s just a few months away now, and when it happens, they’ll instruct the telescope to point at Earth for one last time, transmit its final data, and then shut down forever.

And today TESS blasted off successfully, making its way to take over where Kepler leaves off.

It’s carrying NASA’s Transiting Exoplanet Survey Satellite, or TESS, the sequel to Kepler, taking the search for exoplanets to the next level.

The TESS mission has been around in some form since 2006 when it was originally conceived as a privately funded mission by Google, the Kavli Foundation and MIT.

Over the years, it was proposed to NASA, and in 2013, it was accepted as one of NASA’s Explorer Missions. These are missions with a budget of $200 million or less. WISE and WMAP are other examples of Explorer Missions.

But there are a bunch of differences between Kepler and TESS.

Remember when I said that Kepler was observing a 12 x 12 – degree region of the sky? TESS will be surveying the entire sky, an area 400 times larger than what Kepler observed.

It has a set of 4 separate identical telescopes with CCD cameras, each of which are 16.8 megapixels. They’re arrayed to give TESS a 24-degree square view of the sky. TESS will break up the sky into 26 different sectors and study the region for at least 27 days, switching from bright star to bright star every two minutes.

Artist Illustration of TESS and its 4 telescopes. Credit: NASA/MIT
Artist Illustration of TESS and its 4 telescopes. Credit: NASA/MIT

While Kepler was doing a deep dive into one specific region of the sky, TESS is going to be observing the 500,000 brightest stars in the sky, which are 30 to 100 times brighter than the kinds of stars Kepler was looking at. Many of which will be stars like our own Sun.

It’ll be capable of surveying the entire sky over the course of two years, which is an area 400 times larger than Kepler observed. And astronomers are expecting that the mission will turn up thousands of extrasolar planets, 500 of which will be Earth-sized or super-Earth-sized.

Illustration of the TESS field of view. Credit: NASA/MIT
Illustration of the TESS field of view. Credit: NASA/MIT

By performing this wide survey of the sky with bright stars, TESS will be finding the close extrasolar planets. If a bright star has planets passing in front of it from our perspective, TESS will find it. It will create the definitive catalog of nearby planets.

Since these worlds are much brighter in the sky, it’ll be easier for the world’s ground and space-based observatories to do follow up observations. Astronomers will be able to measure the size, mass, density and even the atmospheres of extrasolar worlds. Just wait until James Webb gets its detectors on some of these worlds.

In addition to its primary job of finding planets, NASA has invited Guest Investigators to use the spacecraft for other science research, such as finding quasars, tracking stellar rotation, and observing the variations of dwarf stars. Anything that has a change in brightness will a great target for TESS.

One interesting feature of the TESS mission will be its orbit, taking it on a path that no other mission has ever used. It’s called a “P/2 lunar-resonant” orbit, and takes the spacecraft on an elliptical trajectory that takes half as long as the Moon to orbits the Earth – 13.7 days.

Simulation of the TESS orbit. Credit: NASA/MIT
Simulation of the TESS orbit. Credit: NASA/MIT

At its closest point to Earth, it’ll be 35,785 km above the surface and take three hours to transmit all its data to ground stations. Then it’ll fly out to the highest point, at an altitude of 373,300 km, out of the hazards of the Van Allen Belts.

By the time the TESS mission wraps up, we’re going to know a lot about the extrasolar planets in our nearby neighborhood. Well, a lot about the planets that perfectly line up with their stars from our perspective. And sadly, this is only a couple of percent of the star systems out there.

We’re going to need other techniques to find the rest, which I’m sure we’ll be covering in future articles.

Note: this is the transcript from a video we posted. Watch it here.

Messier 67 – the King Cobra Open Star Cluster

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the big snake – the King Cobra Cluster (aka. Messier 67).

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is the open star cluster known as Messier 67, aka. the King Cobra Cluster. Located in the Cancer Constellation, and with age estimates ranging from 3.2 and 5 billion years, this cluster is one of the oldest clusters known. And at a distance of roughly 2610 and 2930 (800 – 900 pc) from Earth, it is the closest of any of the older open star clusters.

Description:

At 3.2 billion years, Messier 67 is billed as one of the oldest star clusters known and the oldest of all the Messier clusters. Containing perhaps 500 stars, and about 100 stars similar to our own Sun, this cloud contains no main sequence stars bluer than spectral type F, since the brighter stars of that age have already left the main sequence – no matter how it may appear! Among its 150 white dwarf stars, there’s only about 30 blue stragglers…

Messier 67 (aka. the King Cobra Cluster), one of the oldest known open star clusters. . Credit & Copyright: Processing – Noel Carboni, Imaging – Greg Parker/NASA

As Xiao-Bin Zhan (et al) indicated in a 2005 study:

“We present results of a time-series CCD photometry of two blue stragglers in the open cluster M67 that are also oscillating variables, S1280 and S1284. The observations obtained on 11 nights confirmed the Delta Scuti-like variability of the two stars. Four and five main pulsating frequencies are detected for S1280 and S1284, respectively, through a power spectral analysis. A preliminary mode identification indicates that the two stars are both in radial oscillation. Based on the nature of oscillation, the physical parameters of the two stars are determined, and their evolutionary status discussed.”

As you look at this old open cluster, realize that it’s a great study field for stellar evolution. As Jarrod R. Hurley (et al) explained in their 2005 study:

“The old open cluster M67 is an ideal testbed for current cluster evolution models because of its dynamically evolved structure and rich stellar populations that show clear signs of interaction between stellar, binary and cluster evolution. Here we present the first truly direct N-body model for M67, evolved from zero age to 4 Gyr taking full account of cluster dynamics as well as stellar and binary evolution. Our preferred model starts with 12000 single stars and 12000 binaries placed in a Galactic tidal field at 8.0 kpc from the Galactic Centre. Our choices for the initial conditions and for the primordial binary population are explained in detail. At 4 Gyr, the age of M67, the total mass has reduced by 90% as a result of mass loss and stellar escapes. The mass and half-mass radius of luminous stars in the cluster are a good match to observations although the model is more centrally concentrated than observations indicate. The stellar mass and luminosity functions are significantly flattened by preferential escape of low-mass stars. We find that M67 is dynamically old enough that information about the initial mass function is lost, both from the current luminosity function and from the current mass fraction in white dwarfs. The model contains 20 blue stragglers at 4 Gyr which is slightly less than the 28 observed in M67. Nine are in binaries. The blue stragglers were formed by a variety of means and we find formation paths for the whole variety observed in M67. Both the primordial binary population and the dynamical cluster environment play an essential role in shaping the population. A substantial population of short-period primordial binaries (with periods less than a few days) is needed to explain the observed number of blue stragglers in M67.”

History of Observation:

According to Johann Elert Bode, M67 was originally discovered by Johann Gottfried Koehler before the year 1779, but his telescope was so primitive that little more than the light could be made out. According to historical records his listed it as his object nineteen, describing it as, “A rather conspicuous nebula in elongated figure, near Alpha of Cancer.”

Charles Messier independently rediscovered M67, resolved it into stars, and cataloged it on April 6, 1780: “Cluster of small stars with nebulosity, below the southern claw of Cancer.” It was observed again by Caroline Herschel, and many times by Sir William, ending up getting its General Catalog designation for John Herschel. Of all the folks in history who described it… Dreyer said it best when he said it was a “Remarkable; cluster; very bright; very large; extremely rich.”

Locating Messier 67:

Finding M67 is easy in both binoculars and a telescope once you’ve identified the upside down Y shape of the constellation of Cancer. Simply take aim at the easternmost star in the Y, and you’ll find this delightful open cluster about a finger width to the west.

The location Messier 67 in the Cancer constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

In binoculars and very small telescopes you’ll spy a rich concentration that will appear almost galaxy-like, while larger apertures will fully resolve this cloud of stellar points. M67 is well suited to urban skies and moderate moonlit conditions.

Enjoy the magnificent M67 yourself!

And here are the quick facts on this Messier Object to help you get started:

Object Name: Messier 67
Alternative Designations: M67, NGC 2682
Object Type: Open Galactic Star Cluster
Constellation: Cancer
Right Ascension: 08 : 50.4 (h:m)
Declination: +11 : 49 (deg:m)
Distance: 2.7 (kly)
Visual Brightness: 6.1 (mag)
Apparent Dimension: 30.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Messier 66 – the NGC 3627 Intermediate Spiral Galaxy

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the intermediate spiral galaxy known as Messier 66.

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is the intermediate elliptical galaxy known as Messier 66 (NGC 3627). Located about 36 million light-years from Earth in the direction of the Leo constellation, this galaxy measures 95,000 light-years in diameter. It is also the brightest and largest member of the Leo Triplet of galaxies and is well-known for its bright star clusters, dust lanes, and associated supernovae.

Description:

Enjoying life some 35 million light years from the Milky Way, the group known as the “Leo Trio” is home to bright galaxy Messier 66 – the easternmost of the two M objects. In the telescope or binoculars, you’ll find this barred spiral galaxy far more visible and much easier to see details within its knotted arms and bulging core.

Hubble image of the intermediate spiral galaxy Messier 66. Credits: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration/Davide De Martin/Robert Gendler

Because of interaction with its neighboring galaxies, M66 shows signs of a extremely high central mass concentration as well as a resolved noncorotating clump of H I material apparently removed from one of the spiral arms. Even one of its spiral arms got it noted in Halton Arp’s collection of Peculiar Galaxies! So exactly what did it collide with?As   Xiaolei Zhang (et al) indicated in a 1993 study:

“The combined CO and H I data provide new information, both on the history of the past encounter of NGC 3627 with its companion galaxy NGC 3628 and on the subsequent dynamical evolution of NGC 3627 as a result of this tidal interaction. In particular, the morphological and kinematic information indicates that the gravitational torque experienced by NGC 3627 during the close encounter triggered a sequence of dynamical processes, including the formation of prominent spiral structures, the central concentration of both the stellar and gas mass, the formation of two widely separated and outwardly located inner Lindblad resonances, and the formation of a gaseous bar inside the inner resonance. These processes in coordination allow the continuous and efficient radial mass accretion across the entire galactic disk. The observational result in the current work provides a detailed picture of a nearby interacting galaxy which is very likely in the process of evolving into a nuclear active galaxy. It also suggests one of the possible mechanisms for the formation of successive instabilities in postinteraction galaxies, which could very efficiently channel the interstellar medium into the center of the galaxy to fuel nuclear starburst and Seyfert activities.”

Ah, yes! Star forming regions… And what better way to look deeper than through the eyes of the Spitzer Space Telescope? As R. Kennicutt (University of Arizona) and the SINGS Team observed:

“M66’s blue core and bar-like structure illustrates a concentration of older stars. While the bar seems devoid of star formation, the bar ends are bright red and actively forming stars. A barred spiral offers an exquisite laboratory for star formation because it contains many different environments with varying levels of star-formation activity, e.g., nucleus, rings, bar, the bar ends and spiral arms. The SINGS image is a four-channel false-color composite, where blue indicates emission at 3.6 microns, green corresponds to 4.5 microns, and red to 5.8 and 8.0 microns. The contribution from starlight (measured at 3.6 microns) in this picture has been subtracted from the 5.8 and 8 micron images to enhance the visibility of the dust features.”

Colour composite image of the spiral galaxy M66 (or NGC 3627) obtained with the FORS1 and FORS2 multi-mode instruments (at VLT MELIPAL and YEPUN, respectively). Credit: ESO

Messier 66 has also been deeply studied for evidence of forming super star clusters, too. As David Meier indicated:

“Super star clusters are thought to be precursors of globular clusters and are some of the most extreme star formation regions in the universe. They tend to occur in actively starbursting galaxies or near the cores of less active galaxies. Radio super star clusters cannot be seen in optical light because of extreme extinction, but they shine brightly in infrared and radio observations. We can be certain that there are many massive O stars in these regions because massive stars are required to provide the UV radiation that ionizes the gas and creates a thermally bright HII regions. Not many natal SSCs are currently known, so detection is an important science goal in its own right. In particular, very few SSCs are known in galactic disks. We need more detections to be able to make statistical statements about SSCs and fill in the mass range of forming star clusters. With more detections, we will be able to investigate the effects of other environments (e.g. bars, bubbles, and galactic interaction) on SSCs, which could potentially be followed up in the far future with the Square Kilometer Array to discover their effects on individual forming massive stars.”

But there’s still more. Try magnetic properties in M66’s spiral patterns. As M. Soida (et al) indicated in their 2001 study:

“By observing the interacting galaxy NGC 3627 in radio polarization we try to answer the question; to which degree does the magnetic field follow the galactic gas flow. We obtained total power and polarized intensity maps at 8.46 GHz and 4.85 GHz using the VLA in its compact D-configuration. In order to overcome the zero-spacing problems, the interferometric data were combined with single-dish measurements obtained with the Effelsberg 100-m radio telescope. The observed magnetic field structure in NGC 3627 suggests that two field components are superposed. One component smoothly fills the interarm space and shows up also in the outermost disk regions, the other component follows a symmetric S-shaped structure. In the western disk the latter component is well aligned with an optical dust lane, following a bend which is possibly caused by external interactions. However, in the SE disk the magnetic field crosses a heavy dust lane segment, apparently being insensitive to strong density-wave effects. We suggest that the magnetic field is decoupled from the gas by high turbulent diffusion, in agreement with the large Hi line width in this region. We discuss in detail the possible influence of compression effects and non-axisymmetric gas flows on the general magnetic field asymmetries in NGC 3627. On the basis of the Faraday rotation distribution we also suggest the existence of a large ionized halo around this galaxy.”

History of Observation:

Both M65 and M66 were discovered on the same night – March 1, 1780 – by Charles Messier, who described M66 as, “Nebula discovered in Leo; its light is very faint and it is very close to the preceding: They both appear in the same field in the refractor. The comet of 1773 and 1774 has passed between these two nebulae on November 1 to 2, 1773. M. Messier didn’t see them at that time, no doubt, because of the light of the comet.”

Both galaxies would be observed and cataloged by the Herschel family and further expounded upon by Admiral Smyth:

“A large elongated nebula, with a bright nucleus, on the Lion’s haunch, trending np [north preceding, NW] and sf [south following, SE]; this beautiful specimen of perspective lies just 3deg south-east of Theta Leonis. It is preceded at about 73s by another of a similar shape, which is Messier’s No. 65, and both are in the field at the same time, under a moderate power, together with several stars. They were pointed out by Mechain to Messier in 1780, and they appeared faint and hazy to him. The above is their appearance in my instrument.

“These inconceivably vast creations are followed, exactly on the same parallel, ar Delta AR=174s, by another elliptical nebula of even a more stupendous character as to apparent dimensions. It was discovered by H. [John Herschel], in sweeping, and is No. 875 in his Catalogue of 1830 [actually, probably an erroneous position for re-observed M66]. The two preceding of these singular objects were examined by Sir William Herschel, and his son [JH] also; and the latter says, “The general form of elongated nebulae is elliptic, and their condensation towards the centre is almost invariably such as would arise from the superposition of luminous elliptic strata, increasing in density towards the centre. In many cases the increase of density is obviously attended with a diminution of ellipticity, or a nearer approach to the globular form in the central than in the exterior strata.” He then supposes the general constitution of those nebulae to be that of oblate spheroidal masses of every degree of flatness from the sphere to the disk, and of every variety in respect of the law of their density, and ellipticity towards the centre. This must appear startling and paradoxical to those who imagine that the forms of these systems are maintained by forces identical with those which determine the form of a fluid mass in rotation; because, if the nebulae be only clusters of discrete stars, as in the greater number of cases there is every reason to believe them to be, no pressure can propagate through them. Consequently, since no general rotation of such a system as one mass can be supposed, Sir John suggests a scheme which he shows is not, under certain conditions, inconsistent with the law of gravitation. “It must rather be conceived,” he tells us, ” as a quiescent form, comprising within its limits an indefinite magnitude of individual constituents, which, for aught we can tell, may be moving one among the other, each animated by its own inherent projectile force, and deflected into an orbit more or less complicated, by the influence of that law of internal gravitation which may result from the compounded attractions of all its parts.”

Messier 66 location. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 66:

Even though you might think by its apparent visual magnitude that M66 wouldn’t be visible in small binoculars, you’d be wrong. Surprisingly enough, thanks to its large size and high surface brightness, this particular galaxy is very easy to spot directly between Iota and Theta Leonis. In even 5X30 binoculars under good conditions you’ll easy see both it and M65 as two distinct gray ovals.

A small telescope will begin to bring out structure in both of these bright and wonderful galaxies, but to get a hint at the “Trio” you’ll need at least 6″ in aperture and a good dark night. If you don’t spot them right away in binoculars, don’t be disappointed – this means you probably don’t have good sky conditions and try again on a more transparent night. The pair is well suited to modestly moonlit nights with larger telescopes.

May you equally be attracted to this galactic pair!

And here are the quick facts on M66 to help you get started:

Object Name: Messier 66
Alternative Designations: M66, NGC 3627, (a member of the) Leo Trio, Leo Triplet
Object Type: Type Sb Spiral Galaxy
Constellation: Leo
Right Ascension: 11 : 20.2 (h:m)
Declination: +12 : 59 (deg:m)
Distance: 35000 (kly)
Visual Brightness: 8.9 (mag)
Apparent Dimension: 8×2.5 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

What’ll It Take to Find Life? Searching the Universe for Biosignatures

An artist's interpretation of HD 189733. It looks nice and blue, but it's actually a nightmare world that could be raining glass with 2 km/s winds. Credit: ESO/M. Kornmesser


The supertelescopes are coming, enormous ground and space-based observatories that’ll let us directly observe the atmospheres of distant worlds. We know there’s life on Earth, and our atmosphere tells the tale, so can we do the same thing with extrasolar planets? It turns out, coming up with a single biosignature, a chemical in the atmosphere that tells you that yes, absolutely, there’s life on that world, is really tough.

I’ve got to admit, I’ve been pretty bad for this in the past. In old episodes of Astronomy Cast and the Weekly Space Hangout, even here in the Guide to Space, I’ve said that if we could just sample the atmosphere of a distant world, we could say with conviction if there’s life there.

Just detect ozone in the atmosphere, or methane, or even pollution and you could say, “there’s life there.” Well, future Fraser is here to correct past Fraser. While I admire his naive enthusiasm for the search for aliens, it turns out, as always, things are going to be more difficult than we previously thought.

Astrobiologists are actually struggling to figure out a single smoking gun biosignature that could be used to say there’s life out there. And that’s because natural processes seem to have clever ways of fooling us.

What are some potential biosignatures, why are they problematic, and what will it take to get that confirmation?

Let’s start with a world close to home: Mars.

For almost two decades, astronomers have detected large clouds of methane in the atmosphere of Mars. Here on Earth, methane comes from living creatures, like bacteria and farting cows. Furthermore, methane is easily broken down by sunlight, which means that this isn’t ancient methane leftover from billions of years ago. Some process on Mars is constant replenishing it.

But what?

Well, in addition to life, methane can form naturally through volcanism, when rocks interact with heated water.

NASA tried to get to the bottom of this question with the Spirit and Opportunity rovers, and it was expected that Curiosity should have the tools on board to find the source of the methane.

Panoramic image of the Curiosity rover, from September 2016. The pale outline of Aeolis Mons can be seen in the distance. Credit: NASA/JPL-Caltech/MSSS
Panoramic image of the Curiosity rover, from September 2016. The pale outline of Aeolis Mons can be seen in the distance. Credit: NASA/JPL-Caltech/MSSS
Over the course of several months, Curiosity did detect a boost of methane down there on the surface, but even that has led to a controversy. It turns out the rover itself was carrying methane, and could have contaminated the area around itself. Perhaps the methane it detected came from itself. It’s also possible that a rocky meteorite fell nearby and released some gas that contaminated the results.

The European Space Agency’s ExoMars mission arrived at Mars in October, 2016. Although the Schiaparelli Lander was destroyed, the Trace Gas Orbiter survived the journey and began mapping the atmosphere of Mars in great detail, searching for places that could be venting methane, and so far, we don’t have conclusive results.

In other words, we’ve got a fleet of orbiters and landers at Mars, equipped with instruments designed to sniff out the faintest whiff of methane on Mars.

Artist’s impression visualising the separation of the ExoMars entry, descent and landing demonstrator module, Schiaparelli, from the Trace Gas Orbiter (TGO). Credit: ESA

There’s some really intriguing hints about how the methane levels on Mars seem to rise and fall with the seasons, indicating life, but astrobiologists still don’t agree.

Extraordinary claims require extraordinary evidence and all that.

Some telescopes can already measure the atmospheres of planets orbiting other stars. For the last decade, NASA’s Spitzer Space Telescope has been mapping out the atmospheres of various worlds. For example, here’s a map of the hot jupiter HD 189733b

Spitzer temperature map of HD 189733b (NASA)
Spitzer temperature map of HD 189733b (NASA)
. The place sucks, but wow, to measure an atmosphere, of another planet, that’s pretty spectacular.

They perform this feat by measuring the chemicals of the star while the planet is passing in front of it, and then measure it when there’s no planet. That tells you what chemicals the planet is bringing to the party.

They also were able to measure the atmosphere of HAT-P-26b, which is a relatively small Neptune-sized world orbiting a nearby star, and were surprised to find water vapor in the atmosphere of the planet.

Does that mean there’s life? Wherever we find water on Earth we find life. Nope, you can totally get water without having life.

When it launches in 2019, NASA’s James Webb Space Telescope is going to take this atmospheric sensing to the next level, allowing astronomers to study the atmospheres of many more worlds with a much higher resolution.

Illustration showing the possible surface of TRAPPIST-1f, one of the newly discovered planets in the TRAPPIST-1 system. Credits: NASA/JPL-Caltech
Illustration showing the possible surface of TRAPPIST-1f, one of the newly discovered planets in the TRAPPIST-1 system. Credits: NASA/JPL-Caltech

One of the first targets for Webb will be the TRAPPIST-1 system with its half-dozen planets orbiting in the habitable zone of a red dwarf star. Webb should be able to detect ozone, methane, and other potential biosignatures for life.

So what will it take to be able to view a distant world and know for sure there’s life there.

Astrobiologist John Lee Grenfell from the German Aerospace Centre recently created a report, going through all the exoplanetary biosignatures that could be out there, and reviewed them for how likely they were to be an indication of life on another world.

The first target will be molecular oxygen, or O2. You’re breathing it right now. Well, 21% of every breath, anyway. Oxygen will last in the atmosphere of another world for thousands of years without a source.

It’s produced here on Earth by photosynthesis, but if a world is being battered by its star, and losing atmosphere, then the hydrogen is blown off into space, and molecular oxygen can remain. In other words, you can’t be certain either way.

How about ozone, aka O3? O2 is converted into O3 through a chemical process in the atmosphere. It sounds like a good candidate, but the problem is that there are natural processes that can produce ozone too. There’s an ozone layer on Venus, one on Mars, and they’ve even been detected around icy moons in the Solar System.

There’s nitrous oxide, also known as laughing gas. It’s produced as an output by bacteria in the soil, and helps contribute to the Earth’s nitrogen cycle. And there’s good news, Earth seems to be the only world in the Solar System that has nitrous oxide in its atmosphere.

But scientists have also developed models for how this chemical could have been generated in the Earth’s early history when its sulfur-rich ocean interacted with nitrogen on the planet. In fact, both Venus and Mars could have gone through a similar cycle.

In other words, you might be seeing life, or you might be seeing a young planet.

Ligeia Mare, shown in here in data obtained by NASA’s Cassini spacecraft, is the second largest known body of liquid on Saturn’s moon Titan. It is filled with liquid hydrocarbons, such as ethane and methane, and is one of the many seas and lakes that bejewel Titan’s north polar region. Credit: NASA/JPL-Caltech/ASI/Cornell

Then there’s methane, the chemical we spent so much time talking about. And as I mentioned, there’s methane produced by life here on Earth, but it’s also on Mars, and there are liquid oceans of methane on Titan.

Astrobiologists have suggested other hydrocarbons, like ethane, isoprene, but these have their own problems too.

What about the pollutants emitted by advanced civilizations? Astrobiologists call these “technosignatures”, and they could include things like chlorofluorocarbons, or nuclear fallout. But again, these chemicals would be hard to detect light years away.

Astronomers have suggested that we should search for dead earths, just to set a baseline. These would be worlds located in the habitable zone, but clearly life never got going. Just rock, water and a non-biologically created atmosphere.

The problem is that we probably can’t even figure out a way to confirm that a world is dead either. The kinds of chemicals you’d expect to see in the atmosphere, like carbon dioxide could be absorbed by oceans, so you can’t even make a negative confirmation.

One method might not even involve scanning atmospheres at all. The vegetation here on Earth reflects back a very specific wavelength of light in the 700-750 nanometer region. Astrobiologists call this the “red edge”, because you’ll see a 5X increase in reflectivity compared to other surfaces.

Although we don’t have the telescopes to do this today, there are some really clever ideas, like looking at how the light from a planet reflects onto a nearby moon, and analyze that. Searching for exoplanet earthshine.

In fact, back in the Earth’s early history, it would have looked more purple because of Archaean bacteria.

There’s a whole fleet of spacecraft and ground observatories coming online that’ll help us push further into this question.

ESA’s Gaia mission is going to map and characterize 1% of the stars in the Milky Way, telling us what kinds of stars are out there, as well as detect thousands of planets for further observation.

A conceptual image of the Transiting Exoplanet Survey Satellite. Image Credit: MIT
A conceptual image of the Transiting Exoplanet Survey Satellite.
Image Credit: MIT

The Transiting Exoplanet Space Survey, or TESS, launches in 2018, and will find all the transiting Earth-sized and larger exoplanets in our neighborhood.

The PLATO 2 mission will find rocky worlds in the habitable zone, and James Webb will be able to study their atmospheres. We also talked about the massive LUVOIR telescope that could come online in the 2030s, and take these observations to the next level.

And there are many more space and ground-based observatories in the works.

As this next round of telescopes comes online, the ones capable of directly measuring the atmosphere of an Earth-sized world orbiting another star, astrobiologists are going to struggling to find a biosignature that provides a clear sign there’s life there.

Instead of certainty, it looks like we’re going to have the same struggle to make sense of what we’re seeing. Astronomers will be disagreeing with each other, developing new techniques and new instruments to answer unsolved questions.

It’s going to take a while, and the uncertainty is going to be tough to handle. But remember, this is probably the most important scientific question that anyone can ask: are we alone in the Universe?

The answer is worth waiting for.

Source: John Lee Grenfell: A Review of Exoplanetary Biosignatures.

Hat tip to Dr. Kimberly Cartier for directing me to this paper. Follow her work on EOS Magazine.

Messier 65 – the NGC 3623 Intermediate Spiral Galaxy

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the intermediate spiral galaxy known as Messier 65.

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is the intermediate spiral galaxy known as Messier 65 (aka. NGC 3623), which is located about 35 million light-years from Earth in the Leo constellation. Along with with Messier 66 and NGC 3628, it is part of a small group of galaxies known as the Leo Triplet, which makes it one of the most popular targets among amateur astronomers.

Description:

Enjoying life some 35 million light years from the Milky Way, the group known as the “Leo Trio” is home to bright galaxy Messier 65 – the westernmost of the two M objects. To the casual observer, it looks like a very normal spiral galaxy and thus its classification as Sa – but M65 is a galaxy which walks on the borderline. Why? Because of close gravitational interaction with its nearby neighbors. Who can withstand the draw of gravity?!

The Messier 65 intermediate spiral galaxy. Credit: ESO/INAF-VST/OmegaCAM/Astro-WISE/Kapteyn Institute

Chances are very good that Messier 65 is even quite a bit larger than we see optically as well. As E. Burbidge (et al) said in a 1961 study:

“A fragmentary rotation-curve for NGC 3623 was obtained from measures of the absorption features Ca ii X 3968 and Na I X 5893 and the emission lines [N ii] X 6583 and Ha. The measures from two outer regions are discordant if only circular velocities are assumed, and it is concluded that the measured velocity of one of these regions-the only prominent H ii region in the galaxy-has a large non-circular component. The approximate mass derived from the velocity in the outer arm relative to the center is 1.4 X 1011 M0. It is concluded that the total mass is larger than this, perhaps between 2 and 3 X 1011 M0. This would suggest that the mass-to-light ratio in solar units (photographic) for this galaxy, which is intermediate in type between Sa and Sb, lies between 10 and 20.”

But just how much interaction has been going on between the three galaxies which coexist so closely? Sometimes it takes things like studying in multicolor photometry data to understand. As Zhiyu Duan of the Chinese Academy of Sciences Astronomical Observatory indicated in a 2006 study:

“By comparing the observed SEDs of each part of the galaxies with the theoretical ones generated by instantaneous burst evolutionary synthesis models with different metallicities (Z = 0.0001, 0.008, 0.02, and 0.05), two-dimensional relative age distribution maps of the three galaxies were obtained. NGC 3623 exhibits a very weak age gradient from the bulge to the disk. This gradient is absent in NGC 3627. The ages of the dominant stellar populations of NGC 3627 and NGC 3628 are consistent, and this consistency is model independent (0.5-0.6 Gyr, Z = 0.02), but the ages of NGC 3623 are systematically older (0.7-0.9 Gyr, Z = 0.02). The results indicate that NGC 3627 and NGC 3628 have undergone synchronous evolution and that the interaction has likely triggered starbursts in both galaxies. The results indicate that NGC 3627 and NGC 3628 have undergone synchronous evolution and that the interaction has likely triggered starbursts in both galaxies. For NGC 3623, however, the weak age gradient may indicate recent star formation in its bulge, which has caused its color to turn blue. Evidence is found for a potential bar existing in the bulge of NGC 3623, and my results support the view that NGC 3623 does interact with NGC 3627 and NGC 3628.”

Messier 65, as imaged by the Hubble Space Telescope. Credit: NASA,/ESA/Hubble Space Telescope

So, let’s try looking at things in a slightly different color – integral-field spectroscopy. As V.L. Afanasiev (et al) said in a 2004 study:

“The mean ages of their circumnuclear stellar populations are quite different, and the magnesium overabundance of the nucleus in NGC 3627 is evidence for a very brief last star formation event 1 Gyr ago whereas the evolution of the central part of NGC 3623 looks more quiescent. In the center of NGC 3627 we observe noticeable gas radial motions, and the stars and the ionized gas in the center of NGC 3623 demonstrate more or less stable rotation. However, NGC 3623 has a chemically distinct core – a relic of a past star formation burst – which is shaped as a compact, dynamically cold stellar disk with a radius of ?250-350 pc which has been formed not later than 5 Gyr ago.”

Now, let’s take a look at that gas – and the properties for the gases that exist and co-exist in the galactic trio. As David Hogg (et al) explained in a 2001 study:

“We have studied the distribution of cool, warm, and hot interstellar matter in three of the nearest bright Sa galaxies. New X-ray data for NGC 1291, the object with the most prominent bulge, confirm earlier results that the ISM in the bulge is dominated by hot gas. NGC 3623 has a lesser amount of hot gas in the bulge but has both molecular gas and ionized hydrogen in the central regions. NGC 2775 has the least prominent bulge; its X-ray emission is consistent with an origin in X-ray binary stars, and there is a strict upper limit on the amount of molecular present in the bulge. All three galaxies have a ring of neutral hydrogen in the disk. NGC 3623 and NGC 2775 each have in addition a molecular ring coincident with the hydrogen ring. We conclude that even within the morphological class Sa there can be significant differences in the gas content of the bulge, with the more massive bulges being likely to contain hot, X-ray–emitting gas. We discuss the possibility that the X-ray gas is part of a cooling flow in which cool gas is produced in the nucleus.”

The Leo Triplet, with M65 at the upper right, M66 at the lower right, and NGC 3628 at the upper left. Credit: Scott Anttila. Credit: Wikipedia Commons/Anttler

Even more studies have been done to take a look a disc properties associated with M65. According to M. Bureau (et al);

“NGC 3623 (M 65) is another highly-inclined galaxy in the Leo group, but it is of much later type than NGC 3377, SABa(rs). It is part of the Leo triplet with NGC 3627 and NGC 3628 but does not appear to be interacting. NGC 3623’s kinematics an has barely been studied and observations provide a glimpse of its dynamics. The large-scale velocity reveals minor-axis rotation, in agreement with the presence of a bar. In addition, a quasi edge-on disk is present in the center, where the iso velocity contours flatten out abruptly.”

History of Observation:

Both M65 and M66 were discovered on the same night – March 1, 1780 – by Charles Messier, who described M65 as “Nebula discovered in Leo: It is very faint and contains no star.” Sir William Herschel would later observe M65 as well, describing it as “A very brilliant nebula extended in the meridian, about 12′ long. It has a bright nucleus, the light of which suddenly diminishes on its border, and two opposite very faint branches.”

However, it would be Lord Rosse who would be the first to see structure: “March 31, 1848. – A curious nebula with a bright nucleus; resolvable; a spiral or annular arrangement about it; no other portion of the nebula resolved. Observed April 1, 1848 and April 3, with the same results.”

Locating Messier 65:

Even though you might think by its apparent visual magnitude that M65 wouldn’t be visible in small binoculars, you’d be wrong. Surprisingly enough, thanks to its large size and high surface brightness, this particular galaxy is very easy to spot directly between Iota and Theta Leonis. In even 5X30 binoculars under good conditions you’ll easy see both it and M66 as two distinct gray ovals.

Messier 65 location. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

A small telescope will begin to bring out structure in both of these bright and wonderful galaxies, but to get a hint at the “Trio” you’ll need at least 6″ in aperture and a good dark night. If you don’t spot them right away in binoculars, don’t be disappointed – this means you probably don’t have good sky conditions and try again on a more transparent night. The pair is well suited to modestly moonlit nights with larger telescopes.

Capture one of the Trio tonight! And here are the quick facts on this Messier Object:

Object Name: Messier 65
Alternative Designations: M65, NGC 3623, (a member of the) Leo Trio, Leo Triplet
Object Type: Type Sa Spiral Galaxy
Constellation: Leo
Right Ascension: 11 : 18.9 (h:m)
Declination: +13 : 05 (deg:m)
Distance: 35000 (kly)
Visual Brightness: 9.3 (mag)
Apparent Dimension: 8×1.5 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Messier 64 – The Black Eye Galaxy

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at that “evil” customer known as Messier 64 – aka. the “Black Eye Galaxy”!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is known as Messier 64, which is also known as the “Black Eye” or “Evil Eye Galaxy”. Located in the Coma Berenices constellation, roughly 24 million light-years from Earth, this spiral galaxy is famous for the dark band of absorbing dust that lies in front of the galaxy’s bright nucleus (relative to Earth). Messier 64 is well known among amateur astronomers because it is discernible with small telescopes.

Description:

Residing about 19 million light years from our home galaxy, the “Sleeping Beauty” extends across space covering an area nearly 40,000 light years across, spinning around at a speed of 300 kilometers per second. Toward its core is a counter-rotating disc approximate 4,000 light years wide and the friction between these two may very well be the contributing factor to the huge amounts of starburst activity and distinctive dark dust lane.

Infrared image taken by the Hubble Space Telescope, which penetrated the dust clouds swirling around the centers of the M64 galaxy. Credits: Torsten Boeker, Space Telescope Science Institute and NASA/ESA

Stars themselves appear to be forming in two waves, first evolving outside following the density gradient where abundant interstellar matter was waiting, and then evolving slowly. As the material from the mature stars began beig pushed back by their stellar winds, supernovae, and planetary nebulae, increased amounts of interstellar matter once again compressed, beginning the process of star formation again. This “second wave” may very well be represented by the dark, obscuring dust lane we see.

But, M64 isn’t without it share of turmoil. Its dual rotation may have started as a collision when two galaxies merged some billion years ago – or so theory would suggest. But did it? As Robert Braun and Rene Walterbos explained in their 1995 study:

“This galaxy is known to contain two nested, counter rotating, gas disks of a few 108 solar mass each, with the inner disk extending to approximately 1 kpc and the outer disk extending beyond. The stellar kinematics along the major axis, extending across the transition region between the two gas disks, show no hint of velocity reversal or increased velocity dispersion.  The stars always rotate in the same sense as the inner gas disk, and thus it is the outer disk which ‘counterrotates’. The projected circular velocities inferred from the stellar kinematics and from the H I disks agree to within approximately 10 km/s, supporting other evidence that the stellar and gaseous disks are coplanar to approximately 7 deg. This upper limit is comparable to the mass of detected counter rotating gas. This low mass of counter rotating material, combined with the low-velocity dispersion in the stellar disk, implies that NGC 4826 cannot be the product of a retrograde merger of galaxies, unless they differed by at least an order of magnitude in mass. The velocities of the ionized gas along the major axis are in agreement with that of the stars for R less than 0.75 kpc. The subsequent transition toward apparent counter rotation of the ionized gas is spatially well resolved, extending over approximately 0.6 kpc in radius. The kinematics of this region are not symmetric with respect to the galaxy center. On the southeast side there is a significant region in which vproj (H II) much less than vcirc approximately 150 km/s, but sigma (H II) approximately 65 km/s. The kinematic asymmetries cannot be explained with any stationary dynamical model, even is gas inflow or warps were invoked. The gas in this transition region shows a diffuse spatial structure, strong (N II) and (S II) emission, as well as the high-velocity dispersion. These data present us with the conundrum of explaining a galaxy in which a stellar disk, and two counter rotating H I disks, at smaller and much larger radii, appear in equilibrium and nearly coplanar, yet in which the transition region between the gas disks is not in steady state.”

So is all what it really appears to be? Are new stars being born in the darkness? As A. Majeed (et al) indicated in their 1999 study:

“The Evil Eye galaxy (NGC 4826; M64) is distinguished by an asymmetrically placed, strongly absorbing dust lane across its prominent bulge. We obtained a long-slit spectrum of NGC 4826, with the slit across the galaxy’s nucleus, covering equal parts of the obscured and the unobscured portions of the bulge. By comparing the spectral energy distributions at corresponding positions on the bulge, symmetrically placed with respect to the nucleus, we were able to study the wavelength dependent effects of absorption, scattering, and emission by the dust, as well as the presence of ongoing star formation in the dust lane. We report the detection of strong extended red emission (ERE) from the dust lane within about 15 arcsec distance from the nucleus of NGC 4826. The ERE band extends from 5400 A to 9400 A, with a peak near 8800 A. The integrated ERE intensity is about 75 % of that of the estimated scattered light from the dust lane. The ERE shifts toward longer wavelengths and diminishes in intensity as a region of star formation, located beyond 15 arcsec distance, is approached. We interpret the ERE as originating in photoluminescence by nanometer-sized clusters, illuminated by the galaxy’s radiation field, in addition to the illumination by the star-forming complex within the dust lane. When examined within the context of ERE observations in the diffuse ISM of our Galaxy and in a variety of other dusty environments such as nebulae, we conclude that the ERE photon conversion efficiency in NGC 4826 is as high as found elsewhere, but that the size of the nanoparticles in NGC 4826 is about twice as large as those thought to exist in the diffuse ISM of our Galaxy.”

Messier 64 (“Black Eye Galaxy”) imaged using amateur telescope. Credit: Jeff Johnson.

But the debate is still on. As R.A. Walterbos (et al) expressed in their 1993 study:

“The close to coplanar orientation of the gas disks is one aspect which is in good agreement with what is expected on the basis of a merger model for the counter-rotating gas. The rotation direction of the inner gas disk with respect to the stars, however, is not. In addition, the existence of a well defined exponential disk probably implies that if a merger did occur it must have been between a gas-rich dwarf and a spiral, not between two equal mass spirals. The stellar spiral arms of NGC 4826 are trailing over part of the disk and leading in the outer disk. Recent numerical calculations by Byrd et al. for NGC 4622 suggest that long lasting leading arms could be formed by a close retrograde passage of a small companion. In this scenario, the outer counter-rotating gas disk in NGC 4826 might be the tidally stripped gas from the dwarf. However, in NGC 4826 the outer arms are leading, while it appears that in NGC 4622 the inner arms are leading. A realistic N-body/hydro simulation of a dwarf-spiral encounter is clearly needed. It may also be possible that the counter-rotating outer gas disk is due to gradual infall of gas from the halo, rather than from a discrete merger event.”

History of Observation:

M64 was discovered by Edward Pigott on March 23, 1779, just 12 days before Johann Elert Bode found it independently on April 4, 1779. Roughly a year later, Charles Messier independently rediscovered it on March 1, 1780 and cataloged it as M64. Said Pigot:

“.. on the 23rd of March [1779], I discovered a nebula in the constellation of Coma Berenices, hitherto, I presume, unnoticed; at least not mentioned in M. de la Lande’s Astronomy, nor in M. Messier’s ample Catalogue of nebulous Stars [of 1771]. I have observed it in an acromatic instrument, three feet long, and deduced its mean R.A. by comparing it to the following stars Mean R.A. of the nebula for April 20, 1779, of 191d 28′ 38″. Its light being exceedingly weak, I could not see it in the two-feet telescope of our quadrant, so was obliged to determine its declination likewise by the transit instrument. The determination, however, I believe, may be depended upon to two minutes: hence, the declination north is 22d 53″1/4. The diameter of this nebula I judged to be about two minutes of a degree.”

However, Pigott’s discovery got published only when read before the Royal Society in London on January 11, 1781, while Bode’s was published during 1779 and Messier’s in late summer, 1780. Pigott’s discovery was more or less ignored and recovered only by Bryn Jones in April 2002! (May the good Mr. Pigot know that he was remembered here and his reports placed first!!)

Messier 64, the Black Eye Galaxy. Credit: Miodrag Sekulic

So how did it get the name “Black Eye Galaxy”? We have Sir William Herschel to thank for that: “A very remarkable object, much elongated, about 12′ long, 4′ or 5′ broad, contains one lucid spot like a star with a small black arch under it, so that it gives one the idea of what is called a black eye, arising from fighting.” Of course, John Herschel perpetuated it when he wrote in his own notes:

“The dark semi-elliptic vacancy (indicated by an unshaded or bright portion in the figure,) which partially surrounds the condensed and bright nucleus of this nebula, is of course unnoticed by Messier. It was however seen by my Father, and shown by him to the late Sir Charles Blagden, who likened it to the appearance of a black eye, an odd, but not inapt comparison. The nucleus is somewhat elongated, and I have a strong suspicion that it may be a close double star, or extremely condensed double nebula.”

Locating Messier 64:

Locating M64 isn’t particularly easy. Begin by identifying bright orange Arcturus and the Coma Berenices star cluster (Melotte 111) about a hand span to the general west. As you relax and let your eyes dark adapt, you will see the three stars that comprise the constellation of Coma Berenices, but if you live under light polluted skies, you may need binoculars to find its faint stars. Once you have confirmed Alpha Comae, star hop approximately 4 degrees north/northwest to 35 Comae. You will find M64 around a degree to the northeast of star 35.

While Messier 64 is binocular possible, it will require very dark skies for average binoculars and will only show as a very small, oval contrast change. However, in telescopes as small as 102mm, its distinctive markings can be seen on dark nights with good clarity. Don’t fight over it… There’s plenty of dark dustlane in this Sleeping Beauty to go around!

The location of Messier 64 in the Coma Berenices constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

And here are the quick facts on this Messier Object to help you get started:

Object Name: Messier 64
Alternative Designations: M64, NGC 4826, The Black Eye Galaxy, Sleeping Beauty Galaxy, Evil Eye Galaxy
Object Type: Type Sb Spiral Galaxy
Constellation: Coma Berenices
Right Ascension: 12 : 56.7 (h:m)
Declination: +21 : 41 (deg:m)
Distance: 19000 (kly)
Visual Brightness: 8.5 (mag)
Apparent Dimension: 9.3×5.4 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Messier 63 – the Sunflower Galaxy

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the “Sunflower Galaxy”, otherwise known as Messier 63.

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is the spiral galaxy known as Messier 63 – aka. the Sunflower Galaxy. Located in the Canes Venatici constellation, this galaxy is located roughly 37 million light-years from Earth and has an active nucleus. Messier 63 is part of the M51 Group, a group of galaxies that also includes Messier 51 (the ‘Whirlpool Galaxy’), and can be easily spotted using binoculars and small telescopes.

Description:

Messier 63 is what is known as a a flocculent spiral galaxy, consisting of a central disc surrounded by many short spiral arm segments – one not connected by a central bar structure. Drifting along in space some 37,000 light years from our own galaxy, we known it interacts gravitationally with M51 (the Whirlpool Galaxy) and we also know that its outer regions are rotating so quickly that if it weren’t for dark matter – it would rip itself apart.

Infrared image of the Sunflower Galaxy (Messier 63) taken by the Spitzer Space Telescope. Credit: NASA/JPL-Caltech/SINGS Team

As Michele D. Thornley and Lee G. Mundy, of the Maryland University Department of Astronomy, indicated in a 1997 study:

“The morphology and inematics described by VLA observations of H I emission and FCRAO and Berkeley-Illinois-Maryland Association (BIMA) Array observations of CO emission provide evidence for the presence of low-amplitude density waves in NGC 5055. The distribution of CO and H I emission suggests enhanced gas surface densities along the NIR spiral arms, and structures similar to the giant molecular associations found in the grand design spirals M51 and M100 are detected. An analysis of H I and H? velocity fields shows the kinematic signature of streaming motions similar in magnitude to those of M100 in both tracers. The lesser degree of organization along the spiral arms of NGC 5055 may be due to the lower overall gas surface density, which in the arms of NGC 5055 is a factor of 2 lower than in M100 and a factor of 6 lower than in M51; an analysis of gravitational instability shows the gas in the arms is only marginally unstable and the interarm gas is marginally stable. The limited extent of the spiral arm pattern is consistent with an isolated density wave with a relatively high pattern speed.”

There very well could be a massive object hidden within. As Sebastien Blais-Ouellette of the Universite de Montreal said in a 1998 study:

“In a global kinematical study of NGC 5055 using high resolution Fabry-Perot, intriguing spectral line profiles have been observed in the center of the galaxy. These profiles seem to indicate a rapidly rotating disk with a radius near 365 pc and tilted 50 deg with respect to the major axis of the galaxy. In the hypothesis of a massive dark object, a naive keplerian estimate gives a mass around 10^7.2 to 10^7.5 M.”

Infrared image of the M63 galaxy made by Médéric Boquien, using data retrieved on the SINGS project public archives of the Spitzer Space Telescope. Credit: NASA/JPL-Caltech

But that’s not all they’ve found either… How about a lopsided, chemically unbalanced nucleus! As V.L. Afanasiev (et al) pointed out in their 2002 study:

“We have found a resolved chemically distinct core in NGC 5055, with the magnesium-enhanced region shifted by 2″.5 (100 pc) to the south-west from a photometric center, toward a kinematically identified circumnuclear stellar disk. Mean ages of stellar populations in the true nucleus, defined as the photometric center, and in the magnesium-enhanced substructure are coincident and equal to 3-4 Gyr being younger by several Gyr with respect to the bulge stellar population.”

Yep. It might be beautiful, but it’s warped. As G. Battaglia of the Kapteyn Astronomical Institute indicated in a 2005 study:

“NGC 5055 shows remarkable overall regularity and symmetry. A mild lopsidedness is noticeable, however, both in the distribution and kinematics of the gas. The tilted ring analysis of the velocity field led us to adopt different values for the kinematical centre and for the systemic velocity for the inner and the outer parts of the system. This has produced a remarkable result: the kinematical and geometrical asymmetries disappear, both at the same time. These results point at two different dynamical regimes: an inner region dominated by the stellar disk and an outer one, dominated by a dark matter halo offset with respect to the disk.”

Sunflower Galaxy (Messier 63). Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

History of Observation:

Messier Object 63 was the very first discovery by Charles Messier’s friend and assistant Pierre Mechain, who turned it up on June 14, 1779. While Mechain himself did not write the notes, Messier did:

“Nebula discovered by M. Mechain in Canes Venatici. M. Messier searched for it; it is faint, it has nearly the same light as the nebula reported under no. 59: it contains no star, and the slightest illumination of the micrometer wires makes it disappear: it is close to a star of 8th magnitude, which precedes the nebula on the hour wire. M. Messier has reported its position on the Chart of the path of the Comet of 1779.”

Messier 63 would go on to be observed and resolved by Sir William Herschel and cataloged by his son John. It would be descriptively narrated by Admiral Symth and exclaimed over by many astronomers – one of the best of which was Lord Rosse: “Spiral? Darkness south flowing nucleus.” Of all the descriptions, perhaps the best belongs to Curtis, who first photographed it with the Crossley Reflector at Lick Observatory: “Has an almost stellar nucleus. The whorls are narrow, very compactly arranged, and show numerous almost stellar condensations.”

Locating Messier 63:

The beautiful Sunflower Galaxy is among one of the easiest of the Messier objects to find. It’s located almost precisely between Cor Caroli (Alpha Canes Venetici) and Eta Ursa Majoris. With the slightest of optical aid, stars 19, 20 and 23 CnV will show easily in finderscope or binoculars and M63 will be positioned right around two degrees away towards Eta UM.

The location of Messier 63 in the Canes Venatici constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

While this spiral galaxy has a nice overall brightness, it’s going to be very faint for binoculars, only showing as the tiniest contrast change in smaller models. However, even a modest telescope will easily see a faint oval shape with a concentrated nucleus. The more aperture you apply, the more details you will see. As size approaches 8″ and larger, expect to see spiral structure!

Power up… And look for the spiral in the Sunflower!

Object Name: Messier 63
Alternative Designations: M63, NGC 5055, Sunflower Galaxy
Object Type: Type Sb Spiral Galaxy
Constellation: Canes Venatici
Right Ascension: 13 : 15.8 (h:m)
Declination: +42 : 02 (deg:m)
Distance: 37000 (kly)
Visual Brightness: 8.6 (mag)
Apparent Dimension: 10×6 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

The Dorado Constellation

Welcome to another edition of Constellation Friday! Today, in honor of the late and great Tammy Plotner, we take a look at that fishiest of asterisms – the Dorado constellation. Enjoy!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

Since that time, many additional constellations have been discovered, such as Dorado. This southern constellation, which was discovered in the 16th century by Dutch navigators, is now one of the 88 constellations recognized by the International Astronomical Union (IAU). It is bordered by the constellations of Caelum, Horologium, Hydrus, Mensa, Pictor, Reticulum, and Volans.

Name and Meaning:

Because of its southerly position, Dorado was unknown to the ancient Greeks and Romans so no classical mythological connection exists. However, there are some very nice tales and history associated with this constellation. The name Dorado is Spanish for mahi-mahi, or the dolphin-fish. The mahi-mahi has a opalescent skin that turns blue and gold as the fish dies.

Image of the night sky taken at the European Southern Observatory’s Very Large Telescope in Chile. The Large and Small Magellanic Clouds are visible in the night sky. Credit: ESO, Y. Beletsky

This may very well be the reason Dorado is sometimes called the goldfish is certain stories and legends. Because the early Dutch explorers observed the mahi-mahi chasing swordfish, Dorado was added to their new sky charts following the constellation of the flying fish, Volans. Some very old star atlases refer to Dorado as Xiphias, another form of swordfish, but clearly its “fishy” nature stands!

History of Observation:

Dorado was one of twelve constellations named by Dutch astronomer Petrus Plancius, based on the observations of Dutch sailors that explored the southern hemisphere during the 16th century. It first appeared on a celestial globe published circa 1597-8 in Amsterdam. Dorado was taken a bit more seriously when it was included by Johann Bayer in 1603 in his star atlas, Uranometria, where it appeared under its current name.

It has endured to become one of the 88 modern constellations adopted and approved by the International Astronomical Union.

Notable Objects:

Covering 179 square degrees of sky, it consists of three main stars and contains 14 Bayer/Flamsteed designated stellar members. Dorado has several bright stars and contains no Messier objects. The brightest star in the constellation is Alpha Doradus, a binary star that is approximately 169 light years distant. This binary system is one of the brightest known, and is composed of a blue-white giant (classification A0III) and a blue-white subgiant (B9IV).

The Tarantula Nebula (NGC 2070) located in the southern Dorado constelaltion. Credit: ESO

Beta Doradus, the second brightest star in the constellation, is a Cepheid variable star located approximately 1,050 light years from Earth. Its spectral type varies from white (F-type) to yellow (G-type), like our Sun. Gamma Doradus is another variable, which serves as a prototype for stars known as Gamma Doradus variables, and is approximately 66.2 light years distant.

Another interesting character is HE 0437-5439, an unbound hypervelocity star in Dorado discovered in 2005. This star appears to be receding at the speed of 723 km/s (449 mi/s), and is therefore no longer gravitationally bound to the Milky Way. It is approximately 200,000 light years distant and is a main sequence star belonging to the spectral type BV (a white-blue subdwarf).

Most notable is the Large Magellanic Cloud (LMC), an irregular galaxy located in the constellations Dorado and Mensa. This satellite galaxy to the Milky Way is roughly 1/100 times as massive as our galaxy, with an estimated ten billion times the mass of the Sun. Located about 157,000 light years away, the LMC is home to several impressive objects – like the Tarantula Nebula and the Ghost Head Nebula.

There are no meteor showers associated with the constellation.

The Ghost Head Nebula (NGC 2080), . Credit: ESA/NASA/Mohammad Heydari-Malayeri

Finding Dorado:

The South Ecliptic Pole lies within Dorado and it is bordered by the constellations of Caelum, Horologium, Reticulum, Hydrus, Mensa, Volans and Pictor. It is visible at latitudes between +20° and -90° and is best seen at culmination during the month of January. Let’s begin our explorations with binoculars and Alpha Doradus – the “a” symbol on our map. One of the reasons this star shines so brightly is because it’s not one – but two.

Don’t get your telescope out just yet, because Alpha is separated by only only a couple tenths of a second of arc and both members are about a magnitude apart. Located about 175 light years away from our solar system, this tight pair averages a distance between each other that’s equal to about the same distance as Saturn from our Sun. That’s not particularly unusual for a binary star, but what is unusual is the primary star. Alpha Dor A’s spectrum is “peculiar” – very rich in silicon. It seems to be concentrated in a stellar magnetic spot!

Let’s have a look at Cephid variable star Beta Doradus – the “B” symbol on our map. Beta is an evolved super giant star and every 9.942 days it reaches a maximum brightness of magnitude 3.46 then drops to magnitude 4.08. While these types of changes are so slight they would be difficult to follow with just the eye, that doesn’t mean what happens isn’t important. By studying Cephids we understand “period-luminosity” relation. The pulsation period of a Cepheid gives us absolute brightness, and comparing it with apparent brightness gives us distance. That way, when we find a Cepheid variable star in another galaxy, we can tell just how far away that galaxy is!

Now, let’s go from one end of the constellation to the other with binoculars as we start with Delta Doradus – the “8” shape on our map. If you were on the Moon, this particular star would be the south “pole star” – just like Polaris is to the north on Earth! Sweep along the body of the fish and end at Gamma Doradus – the “Y” shape on our map. Guess what? Another variable star! But this one isn’t a Cepheid. Gamma Doradus variables are variable stars which display variations in luminosity due to non-radial pulsations of their surface.

The stars are typically young, early F or late A type main sequence stars, and typical brightness fluctuations are 0.1 magnitudes with periods on the order of one day. This is a relatively new class of variable stars, having been first characterised in the second half of the 1990s, and details on the underlying physical cause of the variations remains under investigation. We call these mysterious strangers Oscillating Blue Stragglers.

Don’t put away your binoculars yet. We have to look at R Doradus! Here we have a red giant Mira variable star that’s about 200 to 225 light years away from Earth. The visible magnitude of R Doradus varies between 4.8 and 6.6, which makes the variable changes easy to follow with binoculars, but when viewed in the infrared it is one of the brightest stars in the sky. However, this isn’t what the most interesting part is.

With the exception of our own Sun, R Doradus is currently believed to be the star with the largest apparent size as viewed from Earth. The stellar diameter of R Doradus could be as much as 585 million kilometers. That’s upwards to 400 times larger than Sol – yet it has about the same mass! If placed at the center of the Solar System, the orbit of Mars would be entirely contained within the star. Too cool…

Dorado contains a huge amount of deep sky objects very well suited for binoculars, small and large telescopes. So many, in fact, our small star chart would be so cluttered that it would be impossible to read designations. One of the most notable of all is the Large Magellanic Cloud, one of our Milky Way Galaxy’s neighbors and members of our local galaxy group. In itself, it is an irregular dwarf galaxy, distorted by tidal interaction with the Milky Way and may have once been barred spiral galaxies.

The Magellanic Clouds’ radial velocity and proper velocity were recently accurately measured by a team from the Harvard-Smithsonian Center for Astrophysics to produce a 3-D velocity measurement that clocked their passage through the Milky Way galaxy in excess of 480km/s (300 miles per second) using input from Hubble Telescope. This unusually high velocity seems to imply that they are in fact not bound to the Milky Way, and many of the presumed effects of the Magellanic Clouds have to be revised. Be sure to explore the LMC for its own host of nebula and star forming regions. It was host to a supernova (SN 1987A), the brightest observed in over three centuries!

For the telescope, there are many objects in Dorado that you don’t want to miss. (This article would be 10 pages long if I listed them all, so let’s just highlight a few.) For galaxy group fans, why not choose NGC 1566 (RA 04h 20m 00s Dec -56 56.3′) NGC 1566 is a spiral galaxy that dominates the Dorado Group and it is also a Seyfert galaxy as well. At the center of the cluster, look for interacting galaxies NGC 1549 and NGC 1553.

These two bright members are lenticular galaxy NGC 1553 (RA 04h 16m 10.5s Dec -55 46′ 49″), and elliptical galaxy NGC 1549 (RA 04h 15m 45.1s Dec -55 35′ 32″). Their interaction appears to be in the early stage and can be seen in optical wavelengths by faint but distinct irregular shells of emission and a curious jet on the northwest side. Chandra X-ray imaging of NGC 1553 show diffuse hot gas making up 70% of the emissions, dotted with many point-like sources (low-mass X-ray binaries) making up the rest.

Similar to Messier 60, these bright spots are binary star systems of black holes and neutron stars most of which are located in globular clusters and reflect this old galaxy’s very active past. In these systems, material pulled off a regular star is heated and gives off X-rays as it falls toward the accompanying black hole or neutron star.

The location of the southern Constellation Dorado. Credit: IAU/Sky&Telescope magazine

 

Turn your telescope towards NGC 2164 (RA 05h 58m 53s Dec -68 30.9′). Here we are resolving an open star cluster / globular cluster that’s in another galaxy, folks! Also nearby you’ll find faint open cluster NGC 2172 (RA 5 : 59.9 Dec -68 : 38) and galactic star cluster NGC 2159 (05 57.8, -68 38). What a treat to study in another galaxy!

Would you like to study another complex? Then let’s take a look at NGC 2032 (RA 05h 35m 21s Dec -67 34.1′). Better known as the “Seagull Nebula” this complex that contains four separate NGC designations: NGC 2029, NGC 2032, NGC 2035 and NGC 2040. Spanning across an open star cluster, there are many nebula types here including emission nebula, reflection nebula and HII regions. It is also bissected by a dark nebula, too!

Of course, no telescope trip through Dorado would be complete without stopping by NGC 2070 (RA 05h 38m 37s Dec -69 05.7′) – the “Tarantula Nebula”. Located about 180,000 light years from our solar system and first recorded by Nicolas Louis de Lacaille in 1751, this huge HII region is an extremely luminous object. Its luminosity is so bright that if it were as close to Earth as the Orion Nebula, the Tarantula Nebula would cast shadows. In fact, it is the most active starburst region known in our Local Group of galaxies! At its core lies the extremely compact cluster of stars that provides the energy to make the nebula visible. And we’re glad it does!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources: