Deep Sky Objects Archives

September 12, 2016November 21, 2016 by Tammy Plotner

Messier 23 – The NGC 6494 Open Star Cluster

Messier 23, Messier 21, Trifid Nebula (M20) and Omega Nebula (M17). Credit: Wikisky

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.

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:

August 8, 2016August 8, 2016 by Tammy Plotner

Messier 22 – The Sagittarius Nebula

The Sagittarius Cluster, aka. Messier 22. Credit: Wikipedia Commons/Hewholooks

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

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*

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!

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

Sources:

August 1, 2016August 1, 2016 by Tammy Plotner

Messier 21 (M21) – The NGC 6531 Open Star Cluster

The Messier 21 open star cluster and the Trifid Nebula. Credit: Wikisky

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!

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.”

*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)

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

Sources:

July 11, 2016July 11, 2016 by Tammy Plotner

Messier 18 (M18) – The NGC 6613 Star Cluster

Messier 18, shown in proximity to M17 (Omega Nebula), and Messier 24 (Sagittarius Star Cloud). Credit: Wikisky

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)

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

June 27, 2016June 27, 2016 by Tammy Plotner

Messier 16 (M16) – The Eagle Nebula

Composite image of the Eagle Nebula (Messier 16, or NGC 6611), based on images obtained with the Wide-Field Imager camera on the MPG/ESO 2.2-metre telescope at the La Silla Observatory. Credit: ESO

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!

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”:

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

April 29, 2016April 29, 2016 by Tammy Plotner

The Constellation Aries

The Aries constellation and nearby Deep Sky Objects. Credit: thinglink.com

Welcome back to constellation Friday! Today, in honor of our dear friend and contributor, Tammy Plotner, we examine the Aries constellation. Enjoy!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. His treatise, known as the Almagest, would serve as the authoritative source of astronomy for over a thousand years to come. Since the development of modern telescopes and astronomy, this list has come to be expanded to include the 88 constellation that are recognized by the International Astronomical Union (IAU) today.

Of these constellations, Aries – named in honor of the Ram from classical Greek mythology – is featured rather prominently. This faint constellation has deep roots, and is believed to date all the way back to the astrological systems of the ancient Babylonians. Positioned on the ecliptic plane, it is bordered by constellations of Perseus, Triangulum, Pisces, Cetus and Taurus, and is also the traditional home of the vernal equinox.

Continue reading “The Constellation Aries”

February 12, 2016April 26, 2016 by Tammy Plotner

The Andromeda Constellation

A photo of the constellation Andromeda with all Bayer-designated stars marked and the IAU figure drawn in. Credit: Roberto Mura/Wikipedia Commons

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. His treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come. Thanks to the development of modern telescopes and astronomy, this list was amended by the early 20th century to include the 88 constellation that are recognized by the International Astronomical Union (IAU) today.

Of these, Andromeda is one of the oldest and most widely recognized. Located north of the celestial equator, this constellation is part of the family of Perseus, Cassiopeia, and Cepheus. Like many constellation that have come down to us from classical antiquity, the Andromeda constellation has deep roots, which may go all the way back to ancient Babylonian astronomy.

Continue reading “The Andromeda Constellation”