Messier 23 – The NGC 6494 Open Star Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 23 open star cluster. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that other astronomers wouldn’t make the same mistake. Consisting of 100 objects, the Messier Catalog has come to be viewed as a major milestone in the study of Deep Space Objects.

One of these objects is Messier 23 (aka. NGC 6494), a large open star cluster that is located in the constellation Sagittarius. Given its luminosity, it can be found quite easily in the rich star fields of the summer Milky Way using small telescopes and even binoculars.

Description:

Located some 2,150 light years (659 Parsecs) away from Earth, this vast cloud of 176 confirmed stars stretches across 15 to 20 light years of space. At an estimated 220 to 300 million years old, Messier 23 is on the “senior citizen” list of galactic open clusters in our galaxy. At this age, its hottest stars reach spectral type B9, and it even contains a few blue straggler candidates.

Messier 23. Atlas Image mosaic obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
Mosaic image obtained as part of the Two Micron All Sky Survey (2MASS). Credit: UofM/IPAC/Caltech/NASA/NSF

Given that M23 has spent many centuries sweeping through the interstellar medium, astronomers have wondered how this would affect its metal content. Using UBV photometry, astronauts examined the metallicity of M23, and determined that it had no discernible effect. As W.L. Sanders wrote of the cluster in 1990:

“UBV photometric observations of 176 stars in the galactic cluster NGC 6494 are presented and analyzed. The effect of a gas poor environment on the metal abundance of NGC 6494 is studied. It is determined that the metallicity of NGC 6494, which has a delta(U – B) value = + 0.02, is not affected by the interarm region in which it dwelled.”

At the same time, astronomers have discovered that some of M23’s older stars – the red giants – are suffering mass loss. As G. Barbaro (et al.) of the Istituto di Fisica dell’Universita put it in 1969:

“A statistical research on evolved stars beyond hydrogen exhaustion is performed by comparing the H-R diagrams of about 60 open clusters with a set of isochronous curves without mass loss derived from Iben’s evolutionary tracks and time scales for Population I stars. Interpreting the difference in magnitude between the theoretical positions thus calculated and the observed ones as due to mass loss, when negative, the results indicate that this loss may be conspicuous only for very massive and red stars. However, a comparison with an analogous work of Lindoff reveals that the uncertainties connected with the bolometric and color corrections may invalidate by a large amount the conclusions which might be drawn from such research.”

Close-up image of the core of M23, showing some of its brightest member stars. Credit: Sharp/NOAO/AURA/NSF
Close-up of the core of M23, showing some of its brightest member stars. Credit: Sharp/NOAO/AURA/NSF

However, the most recent studies show that we have to determine radial velocities before we can really associate red giants as being cluster members. J.C. Mermilliod of Laboratoire d’Astrophysique de l’Ecole wrote in his 2008 study, “Red giants in open clusters“:

“The present material, combined with recent absolute proper motions, will permit various investigation of the galactic distribution and space motions of a large sample of open clusters. However, the distance estimates still remain the weakest part of the necessary data. This paper is the last one in this series devoted to the study of red giants in open clusters based on radial velocities obtained with the CORAVEL instruments.”

History of Observation:

This neat and tidy galactic star cluster was one of the original discoveries of Charles Messier. As he recorded of the cluster when first viewing it, which occurred on June 20th, 1764:

“In the night of June 20 to 21, 1764, I determined the position of a cluster of small stars which is situated between the northern extremity of the bow of Sagittarius and the right foot of Ophiuchus, very close to the star of sixth magnitude, the sixty-fifth of the latter constellation [Oph], after the catalog of Flamsteed: These stars are very close to each other; there is none which one can see easily with an ordinary refractor of 3 feet and a half, and which was taken for these small stars. The diameter of all is about 15 minutes of arc. I have determined its position by comparing the middle with the star Mu Sagittarii: I have found its right ascension of 265d 42′ 50″, and its declination of 18d 45′ 55″, south.”

The M23 open star cluster, as it appears in the night sky, flanked by M8 (Lagoon), M16 (Eagle), M17 (Omega), M20 (Trifid) and other deep sky objects. Credit & Copyright: Fernando Cabrerizo/NASA
The M23 open star cluster, as it appears in the night sky (a patch of red), flanked by M8 (Lagoon), M16 (Eagle), M17 (Omega), M20 (Trifid) and other deep sky objects. Credit & Copyright: Fernando Cabrerizo/NASA

While William Herschel did not publish his observations of Messier’s objects, he was still an avid observer. So of course, he had to look at this cluster, and wrote the following observations in his personal notes:

“A cluster of beautiful scattered, large stars, nearly of equal magnitudes (visible in my finder), it extends much farther than the field of the telescope will take in, and in the finder seems to be a nebula of a lengthened form extending to about half a degree.”

In July of 1835, Admiral Smyth would make an observation of Messier 23 and once again add his colorful remarks to the timeline:

“A loose cluster in the space between Ophiuchus’s left leg and the bow of Sagittarius. This is an elegant sprinkling of telescopic stars over the whole field, under a moderate magnifying power; the most clustering portion is oblique, in the direction sp to nf [south preceding to north following, SW to NE], with a 7th-magnitude star in the latter portion. The place registered it that of a neat pair, of the 9th and 10th magnitudes, of a lilac hue, and about 12″ apart. This object was discovered by Messier 1764, and it precedes a rich out-cropping of the Milky Way. The place is gained by differentiating the cluster with Mu Sagittarii, from which it bears north-west, distant about 5 deg, the spot being directed to by a line from Sigma on the shoulder, through Mu at the tip of the bow.”

Remember when observing Messier 23 that it won’t slap you in the face like many objects. Basically, it looks like a stellar scattering of freckles across the face of the sky when fully-resolved. It’s actually one of those objects that’s better to view with binoculars and low power telescopes.

messier-23-location

Locating Messier 23:

M23 can be easily found with binoculars about a finger’s width north and two finger widths west of Mu Sagittarii. Or, simply draw a mental line between the top star in the teapot lid (Lambda) and Xi Serpentis. You’ll find a slight compression in the star field about halfway between these two stars that shows up as an open cluster with binoculars.

Using a finderscope, the object will appear nicely as a hazy spot. And for those using telescopes of any size, you’ll need to use fairly low magnification to help set this cluster apart from the surrounding star field, and it will resolve well to almost all instruments.

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

Object Name: Messier 23
Alternative Designations: M23, NGC 6494
Object Type: Open Star Cluster
Constellation: Sagittarius
Right Ascension: 17 : 56.8 (h:m)
Declination: -19 : 01 (deg:m)
Distance: 2.15 (kly)
Visual Brightness: 6.9 (mag)
Apparent Dimension: 27.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon 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 22 – The Sagittarius Nebula

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Sagittarius Cluster (aka. Messier 22). Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that others wouldn’t make the same mistake. Consisting of 100 objects, this “Messier Catalog” would come to be viewed by posterity as a major milestone in the study of Deep Space Objects.

One of these objects is the Sagittarius Cluster, otherwise known as Messier 22 (and NGC 6656). This elliptical globular cluster, is located in the constellation Sagittarius, near the Galactic bulge region. It is one of the brightest globulars visible in the night sky, and was therefore one of the first of its kind to be discovered and later studied.

Description:

Located around 10,400 light years from our Solar System, in the direction of Sagittarius, M22 occupied a volume of space that is 200 light years in diameter and is receding away from us at 149 kilometers per second. M22 has a lot in common with many other clusters of its type, which includes being a gravitationally bound sphere of stars, and that most of its stars are all about the same age – about 12 billion years old.

Messier 22, showing its proximity to Messier 28 and Kaus Borealis. Credit: Wikisky
Messier 22, showing its proximity to Messier 28 and Kaus Borealis. Credit: Wikisky

It is part of our galactic halo, and may once have been part of a galaxy that our Milky Way cannibalized. But it’s there that the similarities end. For example, it consists of at least 70,000 individual stars, only 32 of which are variable stars. It also spans an incredible 32 arc minutes in the sky and ranks as the fourth brightness of all the known globular clusters in our galaxy.

And four must be its lucky number, because it is also one of only four globular clusters known to contain a planetary nebula. Recent Hubble Space Telescope investigations of Messier 22 have led to the discovery of an astonishing discovery. For starters, in 1999, astronomers discovered six planet-sized objects floating around inside the cluster that were about 80 times the mass of Earth!

Using a technique known as microlensing, which measures the way gravity bends the light of the background stars, the Hubble Space Telescope was able to determine the existence of the gas giant. Even though the Hubble can’t resolve them because the angle at which the light bends is about 100 times smaller than the telescope’s angular resolution, scientist know they are there because the gravity “powers up” the starlight, making it brighter each time a body passes in front of it.

Because a microlensing event is very rare and totally unpredictable, the Hubble team needed to monitor 83,000 stars every three days for nearly four months. Luckily, a sharp peak in brightness was all the proof they needed that they were on the right track.

The center of the globular cluster Messier 22, also known as M22, as observed by the NASA/ESA Hubble Space Telescope. Credit: ESA/HST/NASA
The center of the globular cluster Messier 22, also known as M22, as observed by the NASA/ESA Hubble Space Telescope. Credit: ESA/HST/NASA

Said Kailash Sahu, of the Space Telescope Science Institute, Baltimore, MD, of the discovery in 2007: “Hubble’s excellent sharpness allowed us to make this remarkable new type of observation, successfully demonstrating our ability to see very small objects. This holds tremendous potential for further searches for dark, low-mass objects.”

During their study time, the Hubble team caught six microlensing events that lasted less than 20 hours and one which endured for 18 days. By calculating the times of the eclipses and the spikes in brightness, astronomers could then estimate the mass of the object passing in front of the star. These wandering rogues might be planets torn away from their parent stars by the huge amounts of gravitational influence from so many closely packed suns – or (in the case of the long event) simply a smaller mass star passing in front of another.

They could be brown dwarfs, or even a totally new type of object. As co-investigator Nino Panagia of the European Space Agency and Space Telescope Science Institute said: “Since we know that globular clusters like M22 are very old, this result opens new and exciting opportunities for the discovery and study of planet-like objects that formed in the early universe,”

Two black holes were also discovered in M22 and confirmed by the Chandra X-ray telescope in 2012. The objects have between 10 and 20 solar masses, and their discovery suggests that there may be 5 to 100 black holes within the cluster (and maybe some multiple black holes as well). The presence of black holes and their interaction with the stars of M22 could explain the cluster’s unusually large central region.

These are optical images of M22 and the candidate companion stars to the radio sources M22-VLA1 and M22-VLA2: the globular cluster M22, on the left, and the location of the radio sources on archival Hubble images. Credit: Doug Matthews/Adam Block/NOA/AURA/NSF/HST/NASA/ESA
Optical images of M22 and the candidate companion stars to the radio sources M22-VLA1 and M22-VLA2. Credit: Doug Matthews/Adam Block/NOA/AURA/NSF/HST/NASA/ESA

Other objects of interesting include two black holes – M22-VLA1 and M22-VLA2 – both of which are part of binary star systems. Each has a companion star and is pulling matter from it. This gas and dust, in turn, forms an accretion disk around each black hole, creating emissions that scientists used to confirm their existence.

Messier 22 is one of only four known globular clusters that contain a planetary nebula. This nebula – catalogued as GJJC1 or IRAS 18333-2357 – is rather small and young, being only 3 arcseconds in diameter and 6,000 years old. It was discovered in 1986 using the infrared satellite IRAS, and identified as a planetary nebula in 1989.

History of Observation:

Chances are, magnificent Messier 22 was probably the first globular cluster to ever be recorded in the history of astronomy, most likely by Abraham Ihle in 1665. Over the years it has been included in many historic observations, including Edmund Halley’s list of 6 objects published 1715, and observed by De Chéseaux (his Number 17) and Le Gentil, as well as by Abbe Nicholas Louis de la Caille, who included it in his catalog of southern objects (as Lacaille I.12).

Atlas image mosaic of Messier 22 obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology. Credit: NASA/NSF
Atlas image mosaic of Messier 22 obtained as part of the Two Micron All Sky Survey (2MASS). Credit:UoM/IPAC/Caltech/NASA/NSF

However, it was Charles Messier who made it famous when he cataloged it as M22 on June 5th, 1764. As he said of the object at the time:

“I have observed a nebula situated a bit below the ecliptic, between the head and the bow of Sagittarius, near the star of seventh magnitude, the twenty-fifth of that constellation, according to the catalog of Flamsteed. That nebula didn’t appear to me to contain any star, although I have examined it with a good Gregorian telescope which magnified 104 times: it is round, and one sees it very well with an ordinary refractor of 3 feet and a half; its diameter is about 6 minutes of arc. I have determined its position by comparing with the star Lambda Sagittarii: its right ascension has been concluded as 275d 28′ 39″, and its declination as 24d 6′ 11”. It was Abraham Ihle, a German, who discovered this nebula in 1665, when observing Saturn. M. le Gentil has examined it also, and he has made an engraving of the configuration in the volume of the Memoirs of the Academy, for the year 1759, page 470. He observed it on August 29, 1747, under good weather, with a refractor of 18 feet length: He also observed it on July 17, and on other days. “It always appeared to me,” he says, “very irregular in its figure, hair and distributing in space of rays of light all over its diameter.”

While Messier’s description is a wonder, let us remember that he was a comet hunter by profession. Once more, it was the observer Admiral Smythto whom we are indebted for the most detailed and vivid description of the cluster:

“A fine globular cluster, outlying that astral stream, the Via Lactea [Milky Way], in the space between the Archer’s head and bow, not far from the point of the winter solstice, and midway between Mu and Sigma Sagittarii. It consists of very minute and thickly condensed particles of light, with a group of small stars preceding by 3m, somewhat in a crucial form. Halley ascribes the discovery of this in 1665, to Abraham Ihle, the German; but it has been thought this name should have been Abraham Hill, who was one of the first council of the Royal Society, and was wont to dabble with astronomy. Hevelius, however, appears to have noticed it previous to 1665, so that neither Ihle nor Hill can be supported.

“In August, 1747, it was carefully drawn by Le Gentil, as seen with an 18-foot telescope, which drawing appears in the Mémoires de l’Académie for 1759. In this figure three stars accompany the cluster, and he remarks that two years afterwards he did not see the preceding and central one: I, however, saw it very plainly in 1835. In the description he says, “Elle m’a toujours parue tres-irrégulière dans sa figure, chevelue, et rependant des espèces de rayons de lumière tout autout de son diamètre.” This passage, I quote, “as in duty bound;” but from familiarity with the object itself, I cannot say that I clearly understand how or why his telescope exhibited these “espèces de rayons.” Messier, who registered it in 1764, says nothing about them, merely observing that it is a nebula without a star, of a round form; and Sir William Herschel, who first resolved it, merely describes it as a circular cluster, with an estimated profundity of the 344th order. Sir John Herschel recommends it as a capital test for trying the space-penetrating power of a telescope.

“This object is a fine specimen of the compression on which the nebula-theory is built. The globular systems of stars appear thicker in the middle than they would do if these stars were all at equal distances from each other; they must, therefore, be condensed toward the centre. That the stars should be accidentally disposed is too improbable a supposition to be admitted; whence Sir William Herschel supposes that they are thus brought together by their mutual attractions, and that the gradual condensation towards the centre must be received as proof of a central power of such kind.”

Messier 22 location. Image: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of Messier 22 in the night sky. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 22:

From its position almost on the ecliptic plane, bright globular cluster M22 is easy to find in optics of all sizes. The most important clue is simply identifying the Sagittarius “teapot” shape. Once you’ve located it, just choose the “lid” star, Lambda (Kaus Borealis) and look about a fingerwidth (2 degrees) due northeast. In binoculars, if you center on Lambda, M22 will appear in the 10:00 region of your field of view.

In a finderscope, you will need to hop from Lambda northeast to 24 Sagittari and you’ll see it as a faint fuzzy nearby also to the northeast. From a dark sky location, Messier Object 22 can also sometimes be spotted with the unaided eye! No matter what size optics you use, this large, very luminous ball of stars is quite appealing. A joy to binocular users and an exercise in resolution to telescopes.

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

Object Name: Messier 22
Alternative Designations: M22, NGC 6656
Object Type: Class VII Globular Star Cluster
Constellation: Sagittarius
Right Ascension: 18 : 36.4 (h:m)
Declination: -23 : 54 (deg:m)
Distance: 10.4 (kly)
Visual Brightness: 5.1 (mag)
Apparent Dimension: 32.0 (arc min)

Go on… Magnificent Messier 22 is waiting for you to appreciate it!

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon 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 21 (M21) – The NGC 6531 Open Star Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 21 open star cluster. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that other astronomers wouldn’t make the same mistake. Consisting of 100 objects, the Messier Catalog has come to be viewed as a major milestone in the study of Deep Space Objects.

One of these objects is Messier 21 (aka. NGC 6531), an open star cluster located in the Sagittarius constellation. A relatively young cluster that is tightly packed, this object is not visible to the naked eye. Hence why it was not discovered until 1764 by Charles Messier himself. It is now one of the over 100 Deep Sky Objects listed in the Messier Catalog.

Description:

At a distance of 4,250 light years from Earth, this group of 57 various magnitude stars all started life together about 4.6 million years ago as part of the Sagittarius OB1 stellar association. What makes this fairly loose collection of stars rather prized is its youth as a cluster, and the variation of age in its stellar members. Main sequence stars are easy enough to distinguish in a group, but low mass stars are a different story when it comes to separating them from older cluster members.

Messier 21 (NGC 6531). Atlas Image mosaic obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
Atlas mosaic image of Messier 21 (NGC 6531) obtained as part of the Two Micron All Sky Survey (2MASS). Credit: 2MASS/UofM/IPAC/Catech/NASA/NSF

As Byeong Park of the Korean Astronomy Observatory said in a 2001 study of the object:

“In the case of a young open cluster, low-mass stars are still in the contraction phase and their positions in the photometric diagrams are usually crowded with foreground red stars and reddened background stars. The young open cluster NGC 6531 (M21) is located in the Galactic disk near the Sagittarius star forming region. The cluster is near to the nebula NGC 6514 (the Trifid nebula), but it is known that it is not associated with any nebulosity and the interstellar reddening is low and homogeneous. Although the cluster is relatively near, and has many early B-type stars, it has not been studied in detail.”

But study it in detail they did, finding 56 main sequence members, 7 pre-main sequence stars and 6 pre-main sequence candidates. But why did this cluster… you know, cluster in the way it did? As Didier Raboud, an astronomer from the Geneva Observatory, explained in his 1998 study “Mass segregation in very young open clusters“:

“The study of the very young open cluster NGC 6231 clearly shows the presence of a mass segregation for the most massive stars. These observations, combined with those concerning other young objects and very recent numerical simulations, strongly support the hypothesis of an initial origin for the mass segregation of the most massive stars. These results led to the conclusion that massive stars form near the center of clusters. They are strong constraints for scenarii of star and stellar cluster formation.” say Raboud, “In the context of massive star formation in the center of clusters, it is worth noting that we observe numerous examples of multiple systems of O-stars in the center of very young OCs. In the case of NGC 6231, 8 stars among the 10 brightest are spectroscopic binaries with periods shorter than 6 days.”

Credit: earthsky.org
Achernar, the flattest star known, is classified as be star. Credit: earthsky.org

But are there any other surprises hidden inside? You bet! Try Be-stars, a class of rapidly rotating stars that end up becoming flattened at the poles. As Virginia McSwain of Yale University’s Department of Astronomy wrote in a 2005 study, “The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters“:

“Be stars are a class of rapidly rotating B stars with circumstellar disks that cause Balmer and other line emission. There are three possible reasons for the rapid rotation of Be stars: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence (MS) evolution of B stars. To test the various formation scenarios, we have conducted a photometric survey of 55 open clusters in the southern sky. We use our results to examine the age and evolutionary dependence of the Be phenomenon. We find an overall increase in the fraction of Be stars with age until 100 Myr, and Be stars are most common among the brightest, most massive B-type stars above the zero-age main sequence (ZAMS). We show that a spin-up phase at the terminal-age main sequence (TAMS) cannot produce the observed distribution of Be stars, but up to 73% of the Be stars detected may have been spun-up by binary mass transfer. Most of the remaining Be stars were likely rapid rotators at birth. Previous studies have suggested that low metallicity and high cluster density may also favor Be star formation.”

History of Observation:

Charles Messier discovered this object on June 5th, 1764. As he wrote in his notes on the occassion:

“In the same night I have determined the position of two clusters of stars which are close to each other, a bit above the Ecliptic, between the bow of Sagittarius and the right foot of Ophiuchus: the known star closest to these two clusters is the 11th of the constellation Sagittarius, of seventh magnitude, after the catalog of Flamsteed: the stars of these clusters are, from the eighth to the ninth magnitude, environed with nebulosities. I have determined their positions. The right ascension of the first cluster, 267d 4′ 5″, its declination 22d 59′ 10″ south. The right ascension of the second, 267d 31′ 35″; its declination, 22d 31′ 25″ south.”

Messier 21. Credit: Wikisky
Close up of the Messier 21 star cluster. Credit: Wikisky

While Messier did separate the two star clusters, he assumed the nebulosity of M20 was also involved with M21. In this circumstance, we cannot fault him. After all, his job was to locate comets, and the purpose of his catalog was to identify those objects that were not. In later years, Messier 21 would be revisited again by Admiral Smyth, who would describe it as follows:

“A coarse cluster of telescopic stars, in a rich gathering galaxy region, near the upper part of the Archer’s bow; and about the middle is the conspicuous pair above registered, – A being 9, yellowish, and B 10, ash coloured. This was discovered by Messier in 1764, who seems to have included some bright outliers in his description, and what he mentions as nebulosity, must have been the grouping of the minute stars in view. Though this was in the power of the meridian instruments, its mean apparent place was obtained by differentiation from Mu Sagittarii, the bright star about 2 deg 1/4 to the north-east of it.”

Locating Messier 21:

Once you have become familiar with the Sagittarius region, finding Messier 21 is easy. It’s located just two and a half degrees northwest of Messier 8 – the “Lagoon Nebula” – and about a half a degree northeast of Messier 20 – the “Trifid Nebula“. If you are just beginning to astronomy, try starting at the teapot’s tip star (Lambda) “Al Nasl”, and starhopping in the finderscope northwest to the Lagoon.

Credit IAU/Sky & Telescope magazineRoger Sinnott & Rick Fienberg
The location of M21 in the Sagittarius constellation. Credit: IAU/Sky & Telescope magazineRoger Sinnott & Rick Fienberg

While the nebulosity might not show in your finder, optical double 7 Sagittari, will. From there you will spot a bright cluster of stars two degrees due north. These are the stars embedded withing the Trifid Nebula, and the small, compressed area of stars to its northeast is the open star cluster M21. It will show well in binoculars under most sky conditions as a small, fairly bright concentration and resolve well for all telescope sizes.

And here are the quick facts, for your convenience:

Object Name: Messier 21
Alternative Designations: M21, NGC 6531
Object Type: Open Star Cluster
Constellation: Sagittarius
Right Ascension: 18 : 04.6 (h:m)
Declination: -22 : 30 (deg:m)
Distance: 4.25 (kly)
Visual Brightness: 6.5 (mag)
Apparent Dimension: 13.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon 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 20 (M20) – The Trifid Nebula

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Trifid Nebula (aka. Messier 20). Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that others wouldn’t make the same mistake. Consisting of 100 objects, the Messier Catalog would come to be viewed by posterity as a major milestone in the study of Deep Space Objects.

One of these objects is the Trifid Nebula (aka. Messier 20, NGC 6514), a star-forming region of ionized gas located in the Scutum spiral arm of the Milky Way, in the direction of the southern Sagittarius constellation. A bright object that is a favorite amongst amateur astronomers, this object is so-named because it is a combination open star cluster, emissions nebula, reflection nebula, and a dark nebula that looks like it consists of three lobes.

Description:

Almost everyone who is familiar with space images has likely seen a beautiful color image of this emission and reflection nebula. However, when looking at M20 through a telescope, what you will see will be less colorful. Why? When it comes to photographs, exposure times and wavelengths cause different colors to become visible.

Composite image comparing visible-light views from Hubble of the Trifid Nebula with an infrared view from NASA’s Spitzer Space Telescope of the glowing Trifid Nebula. Credit: NASA/JPL-Caltech/J. Rho (SSC/Caltech)
Composite image comparing visible-light views from Hubble of the Trifid Nebula with an infrared view from NASA’s Spitzer Space Telescope of the glowing Trifid Nebula. Credit: NASA/JPL-Caltech/J. Rho (SSC/Caltech)

Photographically, the red emission nebula contained within Messier 20 has a bright blue star cluster in it central portion. It glows red because the ultraviolet light of the stars ionizes the hydrogen gas, which then recombines and emits the characteristic red hydrogen-alpha light captured on film. Further away, the radiation from these hot, young stars becomes too weak to ionize the hydrogen. Now the gas and dust glows blue by reflection!

No matter how it is observed, the Trifid – or “three lobed” – nebula has a distinctive set of dark dust lanes which divide it. These also have a classification of their own, and were cataloged by E.E. Barnard as a dark nebula – Barnard 85 (B 85). In 1999 the Hubble Space Telescope took a look deep into the Trifid nebula at some of its star forming regions (see below).

What it found was a stellar jet poking its way into the cloud, like a fabulous twisted antenna. Inside the exhaust column is a new star waiting to be born, yet sometime over the next 10,000 years the central massive star will probably erode away all of its material before it can fully form. Nearby, a stalk stands waiting.

Close up on the interiotr of the Trifid Nebula. Credit: NASA/HST
Close up on the interior of the Trifid Nebula, showing the star forming region and a stellar jet. Credit: NASA/HST

Like the jet, it is also a stellar nursery – one with an EGG (evaporating gaseous globule) at its tip – a condensed cloud of gas able to survive so far. As Jeff Hester of the Department of Physics & Astronomy explained:

“If our interpretation is correct, the microjet may be the last gasp from a star that was cut off from its supply lines 100,000 years ago. The vast majority of stars like our sun form not in isolation, but in the neighborhood of massive, powerful stars. HST observations of the Trifid Nebula provide a window on the nature of star formation in the vicinity of massive stars, as well as a spectacular snapshot of the “ecology” from which stars like our sun emerge.”

We know that Messier 20 contains new stars, but what about old stars? Are there surprises buried within these bright folds that still await discovery? According to F. Yusef-Zadeh (et al) and a 2000 study titled “Radio continuum emission from the central stars of M20 and the detection of a new supernova remnant near M20“, the answer is yes:

“We report the discovery of a new candidate barrel-shaped supernova remnant (SNR) lying adjacent to M20 and two shell-type features to the north and east of SNR W28. Future observations should clarify whether the nonthermal shell fragment is either part of W20 or yet another previously unidentified shell-type SNR.”

The Trifid nebula (M20, NGC NGC 6514) in pseudocolor. Image taken with the Palomar 1.5-m telescope. The field of view is 16’ ´ 16’. Red shows [S II] ll 6717+6731. Green shows Ha l 6563. Blue shows [O III] l 5007. The WFPC2 field of view is indicated. Image: Jeff Hester (Arizona State University), Palomar telescope.
The Trifid nebula (M20, NGC NGC 6514) in pseudocolor. Image taken with the Palomar 1.5-m telescope. Credit: Jeff Hester (Arizona State University)/Palomar telescope

History of Observation:

Charles Messier discovered this object on June 5th, 1764. As he recorded of the object in his notes:

“In the same night I have determined the position of two clusters of stars which are close to each other, a bit above the Ecliptic, between the bow of Sagittarius and the right foot of Ophiuchus: the known star closest to these two clusters is the 11th of the constellation Sagittarius, of seventh magnitude, after the catalog of Flamsteed: the stars of these clusters are, from the eighth to the ninth magnitude, environed with nebulosities. I have determined their positions. The right ascension of the first cluster, 267d 4′ 5″, its declination 22d 59′ 10″ south. The right ascension of the second, 267d 31′ 35″; its declination, 22d 31′ 25″ south.”

While Messier did separate the two star clusters, he did not note so many different portions to the nebula – but, he did note nebulosity. In this circumstance, we cannot fault him. His purpose was to locate comets, after all; and the reason for the catalog was to list objects that were not. In later years, it would be Sir William Herschel who would take a closer look at Messier 20 and discover much more. As he wrote of the nebula:

“If it was supposed that double nebulae at some distance from each other would frequently be seen, it will now on the contrary be admitted that an expectation of finding a great number of attracting centers in a nebulosity of no great extent is not so probable; and accordingly observation has shewn that greater combinations of nebular than those of the foregoing article are less frequently to be seen. The following list however contains 20 treble, 5 quadruple, and 1 sextuple nebulae of this sort. Among the treble nebulae there is one, namely H V.10 [M20], of which the nebulosity is not yet separated. Three nebulae seem to join faintly together, forming a kind of triangle; the middle of which is less nebulous, or perhaps free of nebulosity; in the middle of the triangle is a double star of the 2nd or 3rd class; more faint nebulosities are following.”

A close detail of the Trifid Nebula, showing the "Pillar region". Credit: NASA and Jeff Hester (Arizona State University).
A close detail of the Trifid Nebula, showing a “Pillar” region. Credit: NASA/Jeff Hester (Arizona State University).

While William went on to catalog four separate areas in his books, it was his son John to whom we owe the famous name that we know it by today. “A most remarkable object. Very large; trifid, three nebulae with a vacuity in the midst, in which is centrally situated the double star Sh 379, the nebula is 7′ in extent. A most remarkable object.”

Just remember when you observe that sky conditions are everything and that not even a large telescope can make it appear if the sky isn’t right. Even Admiral Smyth has his share of troubles spotting it. Said he of the Trifid Nebula:

“I lowered the telescope a couple of degrees, and gazed for the curious trifid nebula, 41 H. IV [H IV.41]; but though I could make out the delicate triple star in the centre of its opening, the nebulous matter resisted the light of my telescope, so that its presence was only indicated by a peculiar glow. Pretty closely preceding this is No. 20 M., an elegant cruciform group of stars, discovered in 1764, which he considered to be surrounded with nebulosity.”

Locating Messier 20:

Once you have become familiar with the Sagittarius region, finding Messier 20 is easy, since it is located just 2 degrees northwest of Messier 8 – the “Lagoon” Nebula. However, at magnitude 9, it isn’t an easy to spot with small binoculars, and not always easy for a small telescope either. Because we often see it depicted in pictures as bright and beautiful, we simply assume M20 will jump out of the sky; but you’ll find that its a lot fainter and more elusive than you might think.

The Sagittarius constellation. Credit: iau.org
The Sagittarius constellation. Credit: iau.org

If you are a beginner to astronomy, try starting at the teapot’s tip star (Lambda), “Al Nasl”, and starhopping in the finderscope northwest to the Lagoon. While the nebulosity might not show in your finder, the optical double star 7 Sagittari, will. From there you will spot a bright cluster of stars two degrees due north. These are the stars embedded withing the Trifid and the small, compressed area of stars to its northeast is the open star cluster of Messier 21.

Center your finderscope on the north and south oriented pair of stars and observe. Remember that you will need a moonless night and that sky conditions will need to be right to see the dark dustlanes! And here are the quick facts about M20, for your convenience:

Object Name: Messier 20
Alternative Designations: M20, NGC 6514, Trifid Nebula
Object Type: Emission Nebula and Reflection Nebula with Open Star Cluster
Constellation: Sagittarius
Right Ascension: 18 : 02.6 (h:m)
Declination: -23 : 02 (deg:m)
Distance: 5.2 (kly)
Visual Brightness: 9.0 (mag)
Apparent Dimension: 28.0 (arc min)

Good luck and enjoy your observations!

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon 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.

Messier 19 (M19) – The NGC 6273 Globular Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 19 globular star cluster. Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier began noticing a series of “nebulous objects” in the night sky. Hoping to ensure that other astronomers did not make the same mistake, he began compiling a list of these objects,. Known to posterity as the Messier Catalog, this list has come to be one of the most important milestones in the research of Deep Sky objects.

One of these objects is Messier 19, a globular star cluster located in the constellation Ophiuchus. Of all the known globular clusters, M19 appears to be one of the most oblate (i.e. flattest) in the night sky. Discovered by William Herschel, this cluster is relatively difficult to spot with the naked eye, and appears as a fuzzy point of light with the help of magnification.

Description:

Speeding away from us at a rate of 146 kilometers per second, this gravitationally bound ball of stars measuring 140 light years in diameter, is one of the Messier globular clusters that has the distinction of being closest to the center of the Milky Way. At a little more than 5000 light-years from the intense gravitation of our own galactic core, it has played havoc on M19’s round shape.

In essence, Milky Way’s gravity has caused M19 to become one of the most oblate of all globular clusters, with twice as many stars along the major axis as along the minor. And, although it is 28,000 light-years from Earth, it’s actually on the opposite side of the galactic core. For all of its rich, dense mass, four RR Lyrae variable stars have been found in M19.

The constellation Ophiuchis. Credit: iau.org
The constellation Ophiuchis. Credit: iau.org

Is Messier 19 unique? It has some stellar branch properties that are difficult to pinpoint. And even its age (though estimated at around 11.9 billion years old) is indeterminate. Says F. Meissner and A. Weiss in their 2006 study, “Global fitting of globular cluster age indicators“:

“The determination of globular cluster (GC) ages rests on the fact that colour-magnitude diagrams (CMDs) of single-age single composition stellar populations exhibit specific time-dependent features. Most importantly, this is the location of the turn-off (TO), which – together with the cluster’s distance – serves as the most straightforward and widely used age indicator. However, there are other parts of the CMD that change their colour or brightness with age, too. Since the sensitivity to time is different for the various parts of the cluster CMD, it is possible to use either the various indicators independently, or the differences in colour and brightness between pairs of them; these latter methods have the advantage of being independent of distance.”

What’s occurring is a horizontal branch gap – an not-quite explainable difference in the way the stars inside M19 are aging. However, science is looking for the answer. As G. Busso et al. explained in their 2008 paper titled “The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441“:

“I show that a possible solution of the puzzle is to assume that a small fraction of the stellar population in the two clusters is strongly helium enriched. The presence of two distinct stellar populations characterized by two different initial He contents can help in explaining the brightness difference between the red portion of the HB and the blue component.”

The Messier 19 globular cluster, as viewed by the Two Micron All-Sky Survey (2MASS). Credit: 2MASS/ipac.caltech.edu
The Messier 19 globular cluster, as viewed by the Two Micron All-Sky Survey (2MASS). Credit: 2MASS/ipac.caltech.edu

Is helium the answer? Quite probably so. M. Salaris Astrophysics Research Institute and an international team of researchers explained in their 2004 study “The initial helium abundance of the Galactic globular cluster system“:

“Based on a recently updated set of stellar evolution models, we performed an accurate statistical analysis in order to assess whether GGCs show a statistically significant spread in their initial He abundances, and whether there is a correlation with the cluster metallicity. As in previous works on the subject, we do not find any significant dependence of the He abundance on the cluster metallicity; this provides an important constraint for models of Galaxy formation and evolution. Apart from GGCs with the bluest Horizontal Branch morphology, the observed spread in the individual helium abundances is statistically compatible with the individual errors. This means that either there is no intrinsic abundance spread among the GGCs, or that this is masked by the errors. In the latter case we have estimated a firm upper limit of 0.019 to the possible intrinsic spread. In case of the GGCs with the bluest Horizontal Branch morphology we detect a significant spread towards higher abundances inconsistent with the individual errors; this can be fully explained by additional effects not accounted for in our theoretical calibrations, which do not affect the abundances estimated for the clusters with redder Horizontal Branch morphology.”

History of Observation:

M19 was one of Charles Messier’s original discoveries, which he first observed on June 5th, 1764. In his notes, he wrote:

“I have discovered a nebula, situated on the parallel of Antares, between Scorpius and the right foot of Ophiuchus: that nebula is round & doesn’t contain any star; I have examined it with a Gregorian telescope which magnified 104 times, it is about 3 minutes of arc in diameter: one sees it very well with an ordinary refractor of 3 feet and a half. I have observed its passage of the Medirian, and compared it with that of the star Antares; I have determined the right ascension of that nebula of 252d 1′ 45″, and its declination of 25d 54′ 46″ south. The known star closest to that nebula is the 28th of the constellation Ophiuchus, after the catalog of Flamsteed, of sixth magnitude.”

Messier 19 and Antares. Credit: Wikisky
The Messier 19 globular cluster, relative to M4, M80 and Antares. Credit: Wikisky

While Charles didn’t resolve it, we must give him due credit for discovery, for its size wouldn’t make it a particularly easy object given his optics. Later, in 1784, William Herschel would become the first to open up its true identity:

“When the 19th of the Connoiss. is viewed with a magnifying power of 120, the stars are visible; the cluster is insulated; some of the small stars scattered in the neighborhood are near it; but they are larger than those belonging to the cluster. With 240 it is better resolved, and is much condensed in the centre. With 300 no nucleus or central body can be seen. The diameter with the 10 feet is 3’16”, and the stars in the centre are too accumulated to be separately seen. It will not be necessary to add that the two last mentioned globular clusters, viewed with more powerful instruments, are of equal beauty with the rest; and from what has been said it is obvious that here the exertion of a clustering power has brought the accumulation and artificial construction of these wonderful celestial objects to the highest degree of mysterious perfection.”

While you may – or may not – resolve Messier 19’s individual stars, even small telescopes can pick up on some of its ellipticity and larger telescopes will make out a definite blue tinge to its coloration. Before you yawn at viewing another globular cluster, remember that you are looking at the other side of our galactic center and think on the words about M19 from Admiral Symth.

“The whole vicinity,” he wrote, “afford a grand conception of the grandeur and richness even of the exterior creation; and indicate the beautious gradation and variety of the heaven of heavens. Truly has it been said, “Stars teach us as well as shine.” This is near the large opening or hole, about 4deg broad, in the Scorpion’s body, which WH [William Herschel] found almost destitute of stars.”

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The Messier 19 globular cluster, as imaged by the Hubble Space Telescope. Credit:NASA/STSc /HST/WikiSky

Locating Messier 19:

Finding M19’s location in binoculars is quite easy – it’s less than a fistwidth (8 degrees) east of Antares (Alpha Scorpi). However, ‘seeing’ M19 in binoculars (especially smaller ones) is a little more problematic. The steadier the binoculars are, the better your chances, since it will appear almost stellar at first glance. A good indicator is to have optical double 26 Ophiuchi in the field at the 2:00 position and look for the star that won’t quite come to focus in the 8:00 position.

Star 26 also makes for a great finderscope lead when locating M19 in a telescope as well. Even for aperture sizes as small as 114mm, this globular cluster will show quite easily in a telescope and reveal its oblate nature. When aperture size increase to the 8″ range, it will begin resolution and as it nears 12″ or more, you’ll pick up on blue stars.

And for your convenience, here are the quick facts of M19:

Object Name: Messier 19
Alternative Designations: M19, NGC 6273
Object Type: Class VIII Globular Star Cluster
Constellation: Ophiuchus
Right Ascension: 17 : 02.6 (h:m)
Declination: -26 : 16 (deg:m)
Distance: 28.0 (kly)
Visual Brightness: 6.8 (mag)
Apparent Dimension: 17.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon 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.

Messier 16 (M16) – The Eagle Nebula

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 16 open star cluster – aka. The Eagle Nebula (and a slew of other names). Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier began noticing a series of “nebulous objects” in the night sky. Hoping to ensure that other astronomers did not make the same mistake, he began compiling a list of these objects,. Known to posterity as the Messier Catalog, this list has come to be one of the most important milestones in the research of Deep Sky objects.

One of these objects it he Eagle Nebula (aka. NGC 661. The Star Queen Nebula and The Spire), a young open cluster of stars located in the Serpens constellation. The names “Eagle” and “Star Queen” refer to visual impressions of the dark silhouette near the center of the nebula. The nebula contains several active star-forming gas and dust regions, which includes the now-famous “Pillars of Creation“.

Description:

Located some 7,000 light years away in the next inner spiral arm of the Milky Way galaxy, the Eagle Nebula spans some 70 by 50 light years across. Born around 5.5 million years ago, this glittering swarm marks an area about 15 light years wide, and within the heart of this nebula is a cluster of stars and a region that has captured our imaginations like nothing else – the “Pillars of Creation”.

Here, star formation is going on. The dust clouds are illuminated by emission light, where high-energy radiation from its massive and hot young stars excited the particles of gas and makes them glow. Inside the pillars are Evaporating Gaseous Globules (EGGs), concentrations of gas that are emerging from the “womb” that about to become stars.

M16 Stars, Pillars, and the Eagle's EGGs
Wide-field IR view of the Eagle Nebula, showing its Stars, the Pillars, and the Eagle’s EGGs. Credit: ESO

These pockets of interstellar gas are dense enough to collapse under their own weight, forming young stars that continue to grow as they accumulate more and more mass from their surroundings. As their place of birth contracts gravitationally, the interior gas reaches its end and the intense radiation of bright young stars causes low density material to boil away.

These regions were first photographed by the Hubble Space Telescope in 1995. As Jeff Hester – a professor at Arizona State University and an investigator with the Hubble’s Wide Field and Planetary Camera 2 (WFPC2) – said of the discovery:

“For a long time astronomers have speculated about what processes control the sizes of stars – about why stars are the sizes that they are. Now in M16 we seem to be watching at least one such process at work right in front of our eyes.”

The Hubble has shown us what happens when all the gas boils away and only the EGGs are left. “It’s a bit like a wind storm in the desert,” said Hester. “As the wind blows away the lighter sand, heavier rocks buried in the sand are uncovered. But in M16, instead of rocks, the ultraviolet light is uncovering the denser egg-like globules of gas that surround stars that were forming inside the gigantic gas columns.”

The Eagle Nebula's pillars of creation taken in 1995 (right) and 2015. The new image was obtained with the Wide Field Camera 3, installed by astronauts in 2009. Credit: Left: NASA, ESA/Hubble and the Hubble Heritage Team. Right: NASA, ESA/Hubble, STScI, J. Hester and P. Scowen (Arizona State University)
The Eagle Nebula’s pillars of creation taken in 1995 (right) and 2015. The new image was obtained with the Wide Field Camera 3, installed by astronauts in 2009. Credit: Left: NASA, ESA/HST/Hubble Heritage Team/STScI, J. Hester and P. Scowen (Arizona State University).

And some of these EGGs are nothing more than what would appear to be tiny bumps and teardrops in space – but at least we are looking back in time to see what stars look like when they were first born. “This is the first time that we have actually seen the process of forming stars being uncovered by photoevaporation,” Hester emphasized. “In some ways it seems more like archaeology than astronomy. The ultraviolet light from nearby stars does the digging for us, and we study what is unearthed.”

History of Observation:

The star cluster associated with M16 (NGC 6611) was first discovered by Philippe Loys de Chéseaux in 1745-6. However, it was Charles Messier who was the very first to see the nebulosity associated with it. As he recorded in his notes:

“In the same night of June 3 to 4, 1764, I have discovered a cluster of small stars, mixed with a faint light, near the tail of Serpens, at little distance from the parallel of the star Zeta of that constellation: this cluster may have 8 minutes of arc in extension: with a weak refractor, these stars appear in the form of a nebula; but when employing a good instrument one distinguishes these stars, and one remarks in addition a nebulosity which contains three of these stars. I have determined the position of the middle of this cluster; its right ascension was 271d 15′ 3″, and its declination 13d 51′ 44″ south.”

A new look at M16, the Eagle Nebula in this composite from the Herschel telescope in far-infrared and XMM-Newton in X-ray. Credits: far-infrared: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium; X-ray: ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger
Composite image of M16 from the Herschel telescope in far-infrared and XMM-Newton in X-ray. Credits: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger

Oddly enough, Sir William Herschel, who was famous for elaborating on Messier’s observations, didn’t seem to notice the nebula at all (according to his notes). And Admiral Smyth, who could always be counted on for flowery prose about stellar objects, just barely saw it as well:

“A scattered but fine large stellar cluster, on the nombril of Sobieski’s shield, in the Galaxy, discovered by Messier in 1764, and registered as a mass of small stars in the midst of a faint light. As the stars are disposed in numerous pairs among the evanescent points of more minute components, it forms a very pretty object in a telescope of tolerable capacity.”

But of course, the nebula isn’t an easy object to spot and its visibility on any given night depends greatly on sky conditions. As historical evidence suggest, only one of the two masters (Messier) caught it. So take a lesson from history and return to the sky many times. One day you’ll be rewarded!

Locating Messier 16:

One of the easiest ways to find M16 is to identify the constellation of Aquila and begin tracing the stars down the eagle’s back to Lambda. When you reach that point, continue to extend the line through to Alpha Scuti, then southwards towards Gamma Scuti. Aim your binoculars or image correct finderscope at Gamma and put it in the 7:00 position.

The location of M16 in the Serpens constellation. Credit: constellation-guide.com
The location of M16, relative to the “Teapot” asterism in the Sagittarius constellation. Credit: constellation-guide.com

For those using a finderscope, M16 will easily show up as a faint haze. Even those using binoculars won’t miss it. If Gamma is in the lower left hand corner of your vision – then M16 is in the upper right hand. For all optics, you won’t be able to miss the open star cluster and the faint nebulosity of IC 4703 can be seen from dark sky locations.

Another way to find M16 is by first locating the “Teapot” asterism in Sagittarius constellation (see above), and then by following the line from the star Kaus Australis (Epsilon Sagittarii) – the brightest star in Sagittarius – to just east of Kaus Media (Delta Sagittarii). Another way to find the nebula is by extending a line from Lambda Scuti in Scutum constellation to Alpha Scuti, and then to the south to Gamma Scuti.

Those using large aperture telescopes will be able to see the nebula well, but sky conditions are everything when it comes to this one. The star cluster which is truly M16 will always be easy, but the nebula is a challenge.

And as always, here are the quick facts on M16 to help you get started:

Object Name: Messier 16
Alternative Designations: M16, NGC 6611, Eagle Nebula (IC 4703)
Object Type: Open Star Cluster and Emission Nebula
Constellation: Serpens (Cauda)
Right Ascension: 18 : 18.8 (h:m)
Declination: -13 : 47 (deg:m)
Distance: 7.0 (kly)
Visual Brightness: 6.4 (mag)
Apparent Dimension: 7.0 (arc min)

And be sure to enjoy this video of the Eagle Nebula and the amazing photographs of the “Pillar of Creation”:

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon 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.

Messier 15 (M15) – The Great Pegasus Cluster

Welcome back to Messier Monday! Today, in our ongoing tribute to Tammy Plotner, we take a look at the M15 globular cluster, one of the oldest and best known star clusters in the night sky. Enjoy!

In the 18th century, French astronomer Charles Messier began noticing a series of “nebulous objects” in the night sky while looking for comets. Not wanting other astronomers to make the same mistake, he began compiling a list of these objects into a catalog. In time, this list would include 100 objects, and came to be known by future astronomers as the Messier Catalog.

One of these objects is the globular cluster known as M15. Located in the northern constellation Pegasus, it is one of the brightest clusters in the night sky (with a visual brightness that is roughly 360,000 times that of our Sun). It is also one of the finest globular clusters in the northern section of the sky, the best deep-sky object in the constellation of Pegasus, and one of the oldest and best known globular clusters.

Description:

Messier 15 is probably the most dense globular cluster in our entire Milky Way galaxy – having already undergone a process of contraction. What does that mean to what you’re seeing? This ball of stars measures about 210 light years across, yet more than half of the stars you see are packed into the central area in a space just slightly more than ten light years in size.

By looking for single stars within globular clusters, the Hubble Space Telescope was either looking for a massive black hole or evidence of a “core collapse” – the intense gravity of so many stars so close together. Although it was peeking nearly 37,000 light-years away, the Hubble was able to resolve hundreds of stars converging on M15’s core. Like magnetism, their gravity would either cause them to attract or repel one another – and a black hole may have formed at some point in the cluster’s 12-billion-year life.

The globular cluster known as Messier 15, located some 35 000 light-years away in the Pegasus constellation. Credit: Mount Lemmon SkyCenter/University of Arizona
The globular cluster known as Messier 15, located some 35 000 light-years away in the Pegasus constellation. Credit: Mount Lemmon SkyCenter/University of Arizona

The study which addressed this data – which appeared in the January 1996 issue of the Astronomical Journal, was led by Puragra Guhathakurta of UCO/Lick Observatory, UC Santa Cruz – asked the question of whether or not the speed of the cluster’s stars could tell us if M15’s dense core was caused by a single huge object, or just mutual attraction. As Guhathakurta stated in the study:

“It is very likely that M15’s stars have concentrated because of their mutual gravity. The stars could be under the influence of one giant central object, although a black hole is not necessarily the best explanation for what we see. But if any globular cluster has a black hole at its center, M15 is the most likely candidate.”

John Bahcall and astrophysicist Jeremiah Ostriker of Princeton University were the first to forward the idea that Messier 15 might be hiding a black hole. While it is distinct from many other globular clusters by having such a dense core, it really isn’t that much different than all the rest of the globular clusters we see. Yet, no where else in our galaxy, except at its core, are the stars that dense!

It is estimated that 30,000 distinct stars exist in the inner 22 light-years of the cluster alone. The closer the Hubble telescope looked, the more stars it found. This increase in stellar density continued all the way to within 0.06 light-years of the center – about 100 times the distance between our Sun and Pluto. “Detecting separate stars that close to the core was at the limit of Hubble’s powers,” says Brian Yanny of the Fermi National Accelerator Laboratory.

The location of M15, within the Pegasus Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of M15, within the Pegasus Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

At this point, even the great Hubble could not distinguish individual stars, or locate the exact position of the core. Guhathakurta and is colleagues theorized that the stars crowd even closer inside the radius, so they plotted the distribution of the stars as a function of distance from the core. When the results came back, they had two answers – either a black hole was responsible, or a gravothermal catastrophe called core collapse was the culprit.

“It’s a catastrophe in the sense that once it starts, this process can run away very quickly,” said Guhathakurta. “But other processes could cause the core to bounce back before it collapses all the way.”

At an estimated 13.2 billion years old, it is one of the oldest known globular clusters, but it isn’t done throwing some surprises at us. M15 was the first globular cluster in which a planetary nebula, Pease 1 or K 648 (“K” for “Kuster”), could be identified – and can be seen with larger aperture amateur telescopes. Even stranger is the fact that Messier 15 contains 112 variable stars, and 9 known pulsars – neutron stars which are the leftovers of ancient supernovae. And one of these is a double neutron star system – M15 C.

History of Observation:

M15 was discovered by Jean-Dominique Maraldi on September 7, 1746 while he was looking for a comet. Says he:

“On September 7 I noticed between the stars Epsilon Pegasi and Beta Equulei, a fairly bright nebulous star, which is composed of many stars, of which I have determined the right ascension of 319d 27′ 6″, and its northern declination of 11d 2′ 22”. About 25 years later, Charles Messier would independently rediscover it to add to his own catalog, describing it as: “In the night of June 3 to 4, 1764, I have discovered a nebula between the head of Pegasus and that of Equuleus it is round, its diameter is about 3 minutes of arc, the center is brilliant, I have not distinguished any star; having examined it with a Gregorian telescope which magnifies 104 times, it had little elevated over the horizon, and maybe that observed at a greater elevation one can perceive stars.”

Camera SBIG STX16803 CCD Camera Filters Astrodon Gen II Dates December 2015 Location Mount Lemmon SkyCenter Exposure RGB = 2 : 2 : 2 Hours Acquisition Astronomer Control Panel (ACP), Maxim DL/CCD (Cyanogen), FlatMan XL (Alnitak) Processing CCDStack, Photoshop, PixInsight Credit Line & Copyright Adam Block/Mount Lemmon SkyCenter/University of Arizona
Deep Broadband (RGB) image of M15, taken from the Mount Lemmon SkyCenter. Credit and Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Sir William Herschel would be the first to resolve some of its stars, but not the core. It would be his son John who would later pick up structure. However, like the dutiful and colorful observer that he was, Admiral Smyth will leave us with this lasting impression:

“Although this noble cluster is rated as globular, it is not exactly round, and under the best circumstances is seen as in the diagram, with stragglers branching from a central blaze. Under a moderate magnifying power, there are many telescopic and several brightish stars in the field; but the accumulated mass is completely insulated, and forcibly strikes the senses as being almost infinitely beyond those apparent comets. Indeed, it may be said to appear evidently aggregated by mutual laws, and part of some stupendous and inscrutable scheme of involution; for there is nothing quiescent throughout the immensity of the vast creation.”

Considering Smyth’s observations were made nearly two centuries before we really began to understand what was going on inside Messier 15, you’ll have to admit he was a very good observer!

Locating Messier 15:

Surprisingly enough, globular cluster M15 is easy to find. Once you’ve located the “Great Square” of Pegasus, simply choose its brightest and southwesternmost star – Alpha. Now identify the small, kite shape of the constellation of Delphinus. Roughly halfway between these two (and slightly south), you’ll spy a slightly reddish star – Epsilon Peg (Enif).

By placing Enif in your binoculars or image correct finderscope at the 7:00 position, you can’t miss this bright, compact beauty. Even the smallest of optics will reveal the round glow and telescopes starting at 4″ will begin resolution – while large telescopes will simply amaze you. However, don’t expect to open this globular up to the core region. As already noted, its pretty dense in there!

And here are the quick facts for Messier 15, for your convenience:

Object Name: Messier 15
Alternative Designations: M15, NGC 7078
Object Type: Class IV Globular Cluster
Constellation: Pegasus
Right Ascension: 21 : 30.0 (h:m)
Declination: +12 : 10 (deg:m)
Distance: 33.6 (kly)
Visual Brightness: 6.2 (mag)
Apparent Dimension: 18.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the

Messier 14 (M14) – the NGC 6402 Globular Cluster

Welcome back to Messier Monday! Today, in our ongoing tribute to Tammy Plotner, we take a look at the M14 globular cluster!

In the 18th century, French astronomer Charles Messier began cataloging all the “nebulous objects” he had come to find while searching the night sky. Having originally mistook these for comets, he compiled a list these objects in the hopes of preventing future astronomers from making the same mistake. In time, the list would include 100 objects, and would come to be known as the Messier Catalog to posterity.

One of these objects was the globular cluster which he would designate as M14. Located in the southern constellation Ophiuchus, this slightly elliptically-shaped stellar swarm contains several hundred thousand stars, a surprising number of which are variables. Despite these stars not being densely concentrated in the central region, this object is not hard to spot for amateur astronomers that are dedicated to their craft!

Description:

Located some 30,000 light years from Earth and measuring 100 light years in diameter, this globular cluster can be found in the southern Ophiuchus constellation, along with several other Messier Objects. Although it began its life some 13.5 billion years ago, it is far from being done changing. It is still shaking intracluster dust from its shoes.

The constellation Ophiuchis. Credit: iau.org
The constellation Ophiuchis. Credit: iau.org

What this means is that M14, like many globular clusters, contains a good deal of matter that it picked up during its many times orbiting the center of our Galaxy. According to studies done by N. Matsunaga (et al):

“Our goal is to search for emission from the cold dust within clusters. We detect diffuse emissions toward NGC 6402 and 2808, but the IRAS 100-micron maps show the presence of strong background radiation. They are likely emitted from the galactic cirrus, while we cannot rule out the possible association of a bump of emission with the cluster in the case of NGC 6402. Such short lifetime indicates some mechanism(s) are at work to remove the intracluster dust… (and) its impact on the chemical evolution of globular clusters.”

Another thing that makes Messier 14 unusual is the presence of CH stars, such as the one that was discovered in 1997. CH stars are a very specific type of Population II carbon stars that can be identified by CH absorption bands in the spectra. Middle aged and metal poor, these underluminous suns are known to be binaries. Patrick Cote, the chief author of the research team that discovered the star, wrote in their research report to the American Astronomical Society:

“We report the discovery of a probable CH star in the core of the Galactic globular cluster M14 (=NGC 6402 = C1735-032), identified from an integrated-light spectrum of the cluster obtained with the MOS spectrograph on the Canada-France-Hawaii telescope. Both the star’s location near the tip of the red giant branch in the cluster color-magnitude diagram and its radial velocity therefore argue for membership in M14. Since the intermediate-resolution MOS spectrum shows not only enhanced CH absorption but also strong Swan bands of C2, M14 joins Centaurus as the only globular clusters known to contain “classical” CH stars. Although evidence for its duplicity must await additional radial velocity measurements, the CH star in M14 is probably, like all field CH stars, a spectroscopic binary with a degenerate (white dwarf) secondary.”

M14 Globular Cluster. Credit: tcaa.us
M14 Globular Cluster. Credit: tcaa.us

History of Observation:

The first recorded observations of the cluster were made by Charles Messier, who described it as a nebula without stars and catalogued it on June 1st, 1764. As he noted in his catalog:

“In the same night of June 1 to 2, 1764, I have discovered a new nebula in the garb which dresses the right arm of Ophiuchus; on the charts of Flamsteed it is situated on the parallel of the star Zeta Serpentis: that nebula is not considerable, its light is faint, yet it is seen well with an ordinary [non-achromatic] refractor of 3 feet & a half [FL]; it is round, & its diameter can be 2 minutes of arc; above it & very close to it is a small star of the nineth magnitude. I have employed for seeing this nebula nothing but the ordinary refractor of 3 feet & a half with which I have not noticed any star; maybe with a larger instrumentone could perceive one. I have determined the position of that nebula by its passage of the Meridian, comparing it with Gamma Ophiuchi, it has resulted for its right ascension 261d 18? 29?, & for its declination 3d 5? 45? south. I have marked that nebula on the chart of the apparent path of the Comet which I have observed last year [the comet of 1769].”

In 1783, William Herschel observed the cluster and was the first to resolve it into individual stars. As he noted, “With a power of 200, I see it consists of stars. They are better visible with 300. With 600, they are too obscure to be distinguished, though the appearance of stars is still preserved. This seems to be one of the most difficult objects to be resolved. With me, there is not a doubt remaining; but another person, in order to form a judgement, ought previously to go through all the several gradations of nebulae which I have resolved into stars.“

As always, it was Admiral William Henry Smyth who provided the most lengthy and detailed description, which he did in July of 1835:

“A large globular cluster of compressed minute stars, on the Serpent-bearer’s left arm. This fine object is of a lucid white colour, and very nebulous in aspect; which may be partly owing to its being situated in a splendid field of stars, the lustre of which interferes with it. By diminishing the field under high powers, some of the brightest of these attendants are excluded, but the cluster loses its definition. It was discovered by Messier in 1764, and thus described: “A small nebula, no star; light faint; form round; and may be seen with a telescope 3 1/2 feet long.” The mean apparent place is obtained by differentiation from Gamma Ophiuchi, from which it is south-by-west about 6deg 1/2, being nearly midway between Beta Scorpii and the tail of Aquila, and 16deg due south of Rasalhague [Alpha Ophiuchi]. Sir William Herschel resolved this object in 1783, with his 20-foot reflector, and he thus entered it: “Extremely bright, round, easily resolvable; with [magnification] 300 I can see the stars. The heavens are pretty rich in stars of a certain size [magnitude, brightness], but they are larger [brighter] than those in the cluster, and easily to be distinguished from them. This cluster is considerably behind the scattered stars, as some of them are projected upon it.” He afterwards added: “From the observations with the 20-foot telescope, which in 1791 and 1799 had the power of discering stars 75-80 times as far as the eye, the profundity of this cluster must be of the 900th order.” “It resembles the 10th Connoissance des temps [Messier 10], which probably would put on the same appearance as this, were it removed half its distance farther from us.”

Finder Chart for M14 (also shown M10 and M12). Credit: freestarcharts.com
Finder Chart for Messier 14 (also showing M10 and M12). Credit: freestarcharts.com

Locating Messier 14:

Messier 14 can be found by first locating Delta Ophiuchi, which M14 is located at about 21 degrees east and 0.4 degrees north from. It can also be found about one-third of the way from Beta to Eta Ophiuchi. If you know where Messier 10 is, take a look 0.8 degrees north and 10 degrees east of it to find M14. The cluster can also be located along the imaginary line from Cebalrai, an orange giant with an apparent magnitude of 2.76 and the fifth brightest star in Ophiuchus, to Antares, the bright red supergiant located in Scorpius.

With an apparent magnitude of +7.6, M14 can be easily observed with binoculars. For those using small telescopes, the bright center and faint halo can be viewed, whereas 8-inch instruments will reveal the cluster’s elliptical shape. To resolve individual stars, you will need a 12-inch telescope or larger. The best time of year to observe the cluster is in the months of May, June and July.

And here are the quick facts for Messier 15, for your convenience:

Object Name: Messier 14
Alternative Designations: M14, NGC 6402
Object Type: Globular Cluster
Constellation: Ophiuchus
Right Ascension: 17 : 37.6 (h:m)
Declination: -03 : 14 (deg: m)
Distance: 30.3 (kly)
Visual Brightness: 7.6 (mag)
Apparent Dimension: 11.0 (arc minutes)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon 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.

Messier 12 (M12) – The NGC 6118 Globular Cluster

Welcome back to another edition of Messier Monday! Today, we continue in our tribute to Tammy Plotner with a look at the M12 globular cluster!

In the 18th century, French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky which he originally mistook for comets. After realizing his mistake, he began compiling a list of these objects in order to ensure that other astronomers did not make the same error. In time, this list would include 100 objects, and would come to be known as the Messier Catalog to posterity.

One the many objects included in this is Messier 12 (aka. M12 or NGC 6218), a globular cluster located in the Ophiuchus constellation some 15,700 light-years from Earth.  M12 is positioned just 3° from the cluster M10, and the two are among the brightest of the seven Messier globulars located in Ophiuchus. It is also interesting to note that M12 is approaching our Solar System at a velocity of 16 km/s.

Continue reading “Messier 12 (M12) – The NGC 6118 Globular Cluster”

Messier 10 (M10) – The NGC 6254 Globular Cluster

Welcome to another installment of Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by taking a look at Messier Object 10.

In the 18th century, French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky while searching for comets. Hoping to ensure that other astronomers did not make the same mistake, he began compiling a list of 1oo  of them. This list came to be known as the Messier Catalog, and would have far-reaching consequences.

In addition to being as a major milestone in the history of astronomy and the study of Deep Sky Objects. One of these objects is known as Messier 10 (aka. NGC 6254), a globular cluster that is located in the equatorial constellation of Ophiuchus. Of the many globular clusters that appear in this constellation (seven of which were cataloged by Messier himself) M10 is the brightest, and can be spotted with little more than a pair of binoculars. Continue reading “Messier 10 (M10) – The NGC 6254 Globular Cluster”