Messier 19 (M19) – The NGC 6273 Globular Cluster

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

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

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

Description:

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

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

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

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

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

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

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

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

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

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

History of Observation:

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

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

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

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

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

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

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

en:Messier 19 globular cluster by en:Hubble Space Telescope; 2.5? view en:NASA, en:STScI, en:WikiSky - en:WikiSky's
The Messier 19 globular cluster, as imaged by the Hubble Space Telescope. Credit:NASA/STSc /HST/WikiSky

Locating Messier 19:

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

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

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

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

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

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

Messier 14 (M14) – the NGC 6402 Globular Cluster

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

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

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

Description:

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

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

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

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

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

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

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

History of Observation:

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

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

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

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

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

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

Locating Messier 14:

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

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

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

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

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

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

Messier 12 (M12) – The NGC 6118 Globular Cluster

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

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

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

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