Messier 54 – the NGC 6715 Globular Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at globular cluster known as Messier 54!

During 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 others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is the globular cluster known as Messier 54. Located in the direction of the Sagittarius constellation, this cluster was once thought to be part of the Milky Way, located about 50,000 light years from Earth, In recent decades, astronomers have come to realize that it is actually part of the Sagittarius Dwarf Galaxy, located some 87,000 light-years away.

What You Are Looking At:

Running away from us at a speed of 142 kilometers per second, this compact globe of stars could be as wide as 150 light years in diameter and as far away as 87,400 light years. Wait… Hold the press… Almost 90 thousand light years? Yeah. Messier 54 isn’t part of our own Milky Way Galaxy!

In 1994 astronomers made a rather shocking discovery… this tough to resolve globular was actually part of the Sagittarius Dwarf Elliptical Galaxy. As Michael H. Siegal (et al) said in their study:

“As part of the ACS Survey of Galactic Globular Clusters, we present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy. Our deep (F606W ~ 26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4 to -0.6. We also clearly show, for the first time, a prominent, ~2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (~0.1-0.8 Gyr old), more metal-rich ([Fe/H]~0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.”

Inside its compact depths lurk at least 82 known variable stars – 55 of which are the RR Lyrae type. But astronomers using the Hubble Space telescope have have also discovered there are two semi-regular red variables with periods of 77 and 101 days. Kevin Charles Schlaufman and Kenneth John Mighell of the National Optical Astronomy Observatory explained in their study:

“Most of our candidate variable stars are found on the PC1 images of the cluster center – a region where no variables have been reported by previous ground-based studies of variables in M54. These observations cannot be done from the ground, even with AO as there are far too many stars per resolution element in ground-based observations.”

The globular cluster Messier 54. Credit: NASA

But what other kinds of unusual stars could be discovered inside such distant cosmic stellar evolutionary laboratory? Try a phenomena known as blue hook stars! As Alfred Rosenberg (et al) said in their study:

“We present BV photometry centered on the globular cluster M54 (NGC 6715). The color-magnitude diagram clearly shows a blue horizontal branch extending anomalously beyond the zero-age horizontal-branch theoretical models. These kinds of horizontal-branch stars (also called “blue hook” stars), which go beyond the lower limit of the envelope mass of canonical horizontal-branch hot stars, have so far been known to exist in only a few globular clusters: NGC 2808, Omega Centauri (NGC 5139), NGC 6273, and NGC 6388. Those clusters, like M54, are among the most luminous in our Galaxy, indicating a possible correlation between the existence of these types of horizontal-branch stars and the total mass of the cluster. A gap in the observed horizontal branch of M54 around Teff = 27,000 K could be interpreted within the late helium flash theoretical scenario, which is a possible explanation for the origin of blue hook stars.”

But with the stars packaged together so tightly, even more has been bound to occur inside of Messier 54. As Tim Adams (et al) indicated in their study:

“We investigate a means of explaining the apparent paucity of red giant stars within post-core-collapse globular clusters. We propose that collisions between the red giants and binary systems can lead to the destruction of some proportion of the red giant population, by either knocking out the core of the red giant or by forming a common envelope system which will lead to the dissipation of the red giant envelope. Treating the red giant as two point masses, one for the core and another for the envelope (with an appropriate force law to take account of the distribution of mass), and the components of the binary system also treated as point masses, we utilize a four-body code to calculate the time-scales on which the collisions will occur. We then perform a series of smooth particle hydrodynamics runs to examine the details of mass transfer within the system. In addition, we show that collisions between single stars and red giants lead to the formation of a common envelope system which will destroy the red giant star. We find that low-velocity collision between binary systems and red giants can lead to the destruction of up to 13 per cent of the red giant population. This could help to explain the colour gradients observed in PCC globular clusters. We also find that there is the possibility that binary systems formed through both sorts of collision could eventually come into contact perhaps producing a population of cataclysmic variables.”

Messier 54, as imaged by the VLT Survey Telescope at ESO’s Paranal Observatory in northern Chile. Credit: ESO

But the discoveries haven’t ended yet…. Because 2009 studies have revealed evidence for an intermediate mass black hole inside Messier 54 – the first known to have ever been discovered in a globular cluster.

“We report the detection of a stellar density cusp and a velocity dispersion increase in the center of the globular cluster M54, located at the center of the Sagittarius dwarf galaxy (Sgr). The central line-of-sight velocity dispersion is 20.2 ± 0.7 km s-1, decreasing to 16.4 ± 0.4 km s-1 at 2farcs5 (0.3 pc). Modeling the kinematics and surface density profiles as the sum of a King model and a point-mass yields a black hole mass of ~9400 M sun.” says R. Ibata (et al), “However, the observations can alternatively be explained if the cusp stars possess moderate radial anisotropy. A Jeans analysis of the Sgr nucleus reveals a strong tangential anisotropy, probably a relic from the formation of the system.”

History of Observation:

On July 24, 1778 when Charles Messier first laid eyes on this faint fuzzy, he had no clue that he was about to discover the very first extra-galactic globular cluster. In his notes he writes: “Very faint nebula, discovered in Sagittarius; its center is brilliant and it contains no star, seen with an achromatic telescope of 3.5 feet. Its position has been determined from Zeta Sagittarii, of 3rd magnitude.”

Years later Sir William Herschel would also study M54, and in his private notes he writes: “A round, resolvable nebula. Very bright in the middle and the brightness diminishing gradually, about 2 1/2′ or 3′ in diameter. 240 shews too pretty large stars in the faint part of the nebulosity, but I rather suppose them to have no connection with the nebula. I believe it to be no other than a miniature cluster of very compressed stars.”

Countless other observations would follow as the M54 became cataloged by other astronomers and each would in turn describe it only as having a much brighter core and some resolution around the edges. Have fun trying to crack this one!

Locating Messier 54:

M54 isn’t hard to find… Just skip down to Zeta Sagittarii, the southwestern-most star of Sagittarius “teapot” and hop a half degree south and a finger width (1.5 degrees) west. The problem is seeing it! In small optics, such as binoculars or a finder scope, it will appear almost stellar because of its small size. However, if you just look for what appears like a larger, dim star that won’t quite come into perfect focus, then you’ve found it.

In smaller telescopes, you’ll get no resolution on this class III globular cluster because it is so dense. Large aperture doesn’t fare much better either, with only some individual stars making their appearance at the outer perimeters. Because of magnitude and size, Messier 54 is better suited to dark sky conditions.

The location of Messier 54 in the Sagittarius 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 54
Alternative Designations: M54, NGC 6715
Object Type: Class III Extragalactic Globular Cluster
Constellation: Sagittarius
Right Ascension: 18 : 55.1 (h:m)
Declination: -30 : 29 (deg:m)
Distance: 87.4 (kly)
Visual Brightness: 7.6 (mag)
Apparent Dimension: 12.0 (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 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 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 18 (M18) – The NGC 6613 Star Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 18 open 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 was Messier 18 (aka. NGC 6613), a relatively dim open star cluster located in the constellation Sagittarius. Located in close proximity to Messier 17 (the Omega Nebula), it is possible that these two clusters formed together.

Description:

Located about 4,900 light years from Earth, and spread over an expanse measuring 17 light-years across, this group of around 20 stars is only about 32 million years old. Its hottest members are spectral type B3, yet you will also see many yellow and orange stars as well. But as already noted, M18 may not be alone in space.

According to research done by R. and C. R. de la Fuente Marcos, M18 may very well be a binary cluster, paired with the open cluster – NGC 6618 – which is harbored inside M17:

“We have shown that binary open clusters appear to constitute a statistically significant sample and that the fraction of possible binary clusters in the Galactic disk is comparable to that in the Magellanic Clouds. The spatial proximity of two almost coeval open clusters, compared to the large distances which typically separate these objects, suggests that both objects were formed together. In starforming complexes, one star cluster might capture another to form a bound state in the presence of a third body or of energy dissipation. This mechanism may also be at work within orbital resonances for non-coeval clusters.”

Messier 18 location. Image: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
The location of Messier 18 in the Sagittarius constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

History of Observation:

M18 was one of Charles Messier’s original discoveries, which took place in 1764. As he wrote in his notes upon observing the cluster:

“In the same night [June 3 to 4, 1764], I have discovered a bit below the nebula reported here above, a cluster of small stars, environed in a thin nebulosity; its extension may be 5 minutes of arc: its appearances are less sensible in an ordinary refractor of 3 feet and a half [FL] than that of the two preceding [M16 and M17]: with a modest refractor, this star cluster appears in the form of a nebula; but when employing a good instrument, as I have done, one sees well many of the small stars: after my observations I have determined its position: its right ascension is 271d 34′ 3″, and its declination 17d 13′ 14″ south.”

In this circumstance, we must give Messier great credit considering his observations were performd long before the nature of open clusters and stellar movement were understood. While Messier seems to have spotted some nebulosity around the cluster (which may have belonged to M17), he takes a later historic cut from Smyth:

“A neat double star, in a long and straggling assemblage of stars,below the Polish shield. A 9 and B 11 [mag], both blueish. This cluster was discovered by Messier in 1764, and registered as a mass of small stars appearing like a nebula in a 3 1/2-foot telescope; which affords another instance that the means of that very zealous observer did not quadrate with his diligence.”

What a shame Smyth wasn’t around to later know that M18 could be paired with its nebulous neighbor!

Credit: 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.
The open cluster Messier 18 (NGC 6613), as observed by the Two Micron All Sky Survey (2MASS). Credit: University of Massachusetts/IPAC/Caltech/NASA/NSF

Locating Messier 18:

Because Messier 18 is nothing more than a small collection of stars which are slightly brighter than the background Milky Way stars, it isn’t easy to distinguish it using binoculars or a finderscope if you’ve never seen it before. One of the most sure ways of locating it is to become familiar with Messier 17 and simply aim a couple of degrees (about a field of view) south.

While it won’t strike you as a grand object, you will notice that the stars are compressed in this area and that there are several dozen of them which appear brighter than the rest. In a telescope, use your lowest magnification. Since this is a very well spread cluster, it is easily resolved in even modest instruments.

And here are the quick facts on M18 to get you started:

Object Name: Messier 18
Alternative Designations: M18, NGC 6613
Object Type: Open Star Cluster
Constellation: Sagittarius
Right Ascension: 18 : 19.9 (h:m)
Declination: -17 : 08 (deg:m)
Distance: 4.9 (kly)
Visual Brightness: 7.5 (mag)
Apparent Dimension: 9.0 (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 17 (M17) – the Omega Nebula

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 17 nebula – aka. The Omega Nebula (and a few other names).

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 is the star-forming nebula known as Messier 17 – or as it’s more famously known, the Omega Nebula (or Swan Nebula, Checkmark Nebula, and Horseshoe Nebula). Located in the Sagittarius constellation, this beautiful nebula is considered one of the brightest and most massive star-forming regions in our galaxy.

Description:

From its position in space some 5,000 to 6,000 light years from Earth, the “Omega” nebula occupies a region as large as 40 light years across, with its brightest porition covering a 15 light year expanse. Like many nebulae, this giant cosmic cloud of interstellar matter is a starforming region in the Sagittarius or Sagittarius-Carina arm of our Milky Way galaxy.

What you see is the hot hydrogen gas that is illuminated when its particles are excited by the hottest of the stars that have just formed within the nebula. Also, some of the light is being reflected by the nebula’s own dust. These remain hidden by dark obscuring material, and we know their presence only through the detection of their infrared radiation.

Credit: NASA/Ignacio de la Cueva Torregrosa
Image of M17 showing specific elements based on their color, including sulfur (red), hydrogen (green), oxygen (blue). Credit: NASA/Ignacio de la Cueva Torregrosa

In an study titled “Interstellar Weather Vanes: GLIMPSE Mid-Infrared Stellar-Wind Bowshocks in M17 and RCW49“, astronomer Matthew S. Povich (et al.) of the University of Wisconsin-Madison said of M17:

“We report the discovery of six infrared stellar-wind bowshocks in the Galactic massive star formation regions M17 and RCW49 from Spitzer GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) images. The InfraRed Array Camera (IRAC) on the Spitzer Space Telescope clearly resolves the arc-shaped emission produced by the bowshocks. We use the stellar SEDs to estimate the spectral types of the three newly-identified O stars in RCW49 and one previously undiscovered O star in M17. One of the bowshocks in RCW49 reveals the presence of a large-scale flow of gas escaping the HII region. Radiation-transfer modeling of the steep rise in the SED of this bowshock toward longer mid-infrared wavelengths indicates that the emission is coming principally from dust heated by the star driving the shock. The other 5 bowshocks occur where the stellar winds of O stars sweep up dust in the expanding HII regions.”

Is Messier 17 still actively producing stars? You bet. Even protostars have been discovered hiding in its folds. As M. Nielbock (et al), wrote in 2008:

“For the first time, we resolve the elongated central infrared emission of the large accretion disk in M 17 into a point-source and a jet-like feature that extends to the northeast. We regard the unresolved emission as to originate from an accreting intermediate to high-mass protostar. In addition, our images reveal a weak and curved southwestern lobe whose morphology resembles that of the previously detected northeastern one. We interpret these lobes as the working surfaces of a recently detected jet interacting with the ambient medium at a distance of 1700 AU from the disk centre. The accreting protostar is embedded inside a circumstellar disk and an envelope causing a visual extinction. This and its K-band magnitude argue in favour of an intermediate to high-mass object, equivalent to a spectral type of at least B4. For a main-sequence star, this would correspond to a stellar mass of 4 M.”

Omega Nebula location. Image: Wikisky
The location of the Omega Nebula, with other Messier objects and major stars shown. Image: Wikisky

How many new stars lay hidden inside? Far more than the famous Orion nebula may contain. So says a 2013 study produced by L. Eisa (et al):

“The complex resembles the Orion Nebula/KL region seen nearly edge-on: the bowl-shaped ionization blister is eroding the edge of the clumpy molecular cloud and triggering massive star formation, as evidenced by an ultra-compact HII region and luminous protostars. Only the most massive members of the young NGC 6618 stellar cluster exciting the nebula have been characterized, due to the comparatively high extinction. Near-infrared imagery and spectroscopy reveal an embedded cluster of about 100 stars earlier than B9. These studies did not cover the entire cluster, so even more early stars may be present. This is substantially richer than the Orion Nebula Cluster which has only 8 stars between O6 and B9.”

History of Observation:

The Omega Nebula was first discovered by Philippe Loys de Cheseaux and is just one of the six nebulae in his documents. As he wrote of his discovery:

“Finally, another nebula, which has never been observed. It is of a completely different shape than the others: It has perfectly the form of a ray, or of the tail of a comet, of 7′ length and 2′ broadth; its sides are exactly parallel and rather well terminated, as are its two ends. Its middle is whiter than the border.” Because De Cheseaux’s work wasn’t widely read, Charles Messier independently rediscovered it on June 3, 1764 and cataloged it in his own way: “In the same night, I have discovered at little distance of the cluster of stars of which I just have told, a train of light of five or six minutes of arc in extension, in the shape of a spindle, and in almost the same as that in the girdle of Andromeda; but of a very faint light, not containing any star; one can see two of them nearby which are telescopic and placed parallel to the Equator: in a good sky one perceives very well that nebula with an ordinary refractor of 3 feet and a half. I have determined its position in right ascension of 271d 45′ 48″, and its declination of 16d 14′ 44” south.

Omega Nebula sketch by John Herschel, 1833. Credit: messier-objects.com
Omega Nebula sketch by John Herschel, 1833. Credit: messier-objects.com

By historical accounts, it was Sir William Herschel who may have truly had a little bit of insight on what this object might one day mean when he observed it on his own and reported:

“1783, July 31. A very singular nebula; it seems to be the link to join the nebula in Orion to others, for this is not without a possibility of being stars. I think a great deal more of light and a much higher power would be of service. 1784, June 22 (Sw. 231). A wonderful nebula. Very much extended, with a hook on the preceding [Western] side; the nebulosity of the milky kind; several stars visible in it, but they seem to have no connection with the nebula, which is far more distant. I saw it only through short intervals of flying clouds and haziness; but the extent of the light including the hook is above 10′. I suspect besides, that on the following [Eastern] side it goes on much farther and diffuses itself towards the north and south. It is not of equal brightness throughout and has one or more places where the milky nebulosity seems to degenerate into the resolvable [mottled] kind; such a one is that just following the hook towards the north. Should this be confirmed on a very fine night, it would bring on the step between these two nebulosities which is at present wanting, and would lead us to surmise that this nebula is a stupendous stratum of immensely distant fixed stars, some of whose branches come near enough to us to be visible as a resolvable nebulosity, while the rest runs on to so great a distance as only to appear under the milky form.”

So where did the name “Omega Nebula” come from? That credit goes to John Herschel, who stated in his observing notes:

“The figure of this nebula is nearly that of the Greek capital Omega, somewhat distorted and very unequally bright. It is remarkable that this is the form usually attributed to the great nebula in Orion, though in that nebula I confess I can discern no resemblence whatever to the Greek letter. Messier perceived only the bright preceding branch of the nebula now in question, without any of the attached convolutions which were first noticed by my Father. The chief peculiarities which I have observed in it are, 1st, the resolvable knot in the following portion of the bright branch, which is in a considerable degree insulated from the surrounding nebula; strongly suggesting the idea of an absorption of nebulous matter; and 2ndly, the much feebler and smaller knot in the north preceding end of the same branch, where the nebula makes a sudden bend at an acute angle. With a view to a more exact representation of this curious nebula, I have at different times taken micrometrical measures of the relative places of the stars in and near it, by which, when laid down on the chart, its limits may be traced and identified, as I hope soon to have better opportunity to do than its low situation in this latitudes will permit.”

Credit: NASA/JPL-Caltech/M. Povich (Univ. of Wisconsin)
Infrared images of M17, taken by the Spitzer Space Telescope. Credit: NASA/JPL-Caltech/M. Povich (Univ. of Wisconsin)

Locating Messier 17:

Because M17 is both large and quite bright, its distinctive “2” shape isn’t hard to make out in optics of any size. For binoculars and image correct finderscopes, try starting with 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. M16 is slightly more than 2 degrees (about a fingerwidth) southwest of this star.

If you are in a dark sky location, you can also identify it easily in binoculars by starting at the M24 “Star Cloud”, north of Lambda Sagittari (the teapot lid star), and simply scanning north. This nebula is bright enough to even cut through moderately light polluted skies with ease, but don’t expect to see it when the Moon is nearby. You’ll enjoy the rich starfields combined with an interesting nebula in binoculars, while telescopes will easily begin resolving the interior stars.

And here are the quick facts on M17 for your convenience:

Object Name: Messier 17
Alternative Designations: M17, NGC 6618, Omega, Swan, Horseshoe, or Lobster Nebula
Object Type: Open Star Cluster with Emission Nebula
Constellation: Sagittarius
Right Ascension: 18 : 20.8 (h:m)
Declination: -16 : 11 (deg:m)
Distance: 5.0 (kly)
Visual Brightness: 6.0 (mag)
Apparent Dimension: 11.0 (arc min)

And be sure to enjoy this video from the European Southern Observatory (ESO) that shows this nebula in all its glory:

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 8 (M8) – The Lagoon Nebula

Welcome to another Messier Monday. In our ongoing tribute to the great Tammy Plotner, we bring you another item from the Messier Catalog!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function, acting as a milestone in the history of the study of Deep Sky Objects.

However, not all objects in the catalog were first discovered by Charles Messier himself. Some, like the Lagoon Nebula, were observed sooner, owing to the fact that they are visible to the naked eye. This interstellar cloud, which is located in the Sagittarius constellation, has been known of since the late 17th century, and is one of only two star-forming nebulae that is visible to the naked eye from mid-northern latitudes.

Continue reading “Messier 8 (M8) – The Lagoon Nebula”

Messier 6 – The Butterfly Cluster

Welcome back to Messier Monday! We continue our tribute to our dear friend, Tammy Plotner, by looking at Messier 6, otherwise known as NGC 6405 and the Butterfly Cluster. Enjoy!

In the late 18th century, Charles Messier was busy hunting for comets in the night sky, and noticed several “nebulous” objects. After initially mistaking them for the comets he was seeking, he began to compile a list of these objects so other astronomers would not make the same mistake. Known as the Messier Catalog, this list consists of 100 objects, consisting of distant galaxies, nebulae, and star clusters.

This Catalog would go on to become a major milestone in the history of astronomy, as well as the study of Deep Sky Objects. Among the many famous objects in this catalog is M6 (aka. NGC 6405), an open cluster of stars in the constellation of Scorpius. Because of its vague resemblance to a butterfly, it is known as the Butterfly Cluster.

Continue reading “Messier 6 – The Butterfly Cluster”