Tuesday, May 17, 2016

Messier 14 (M14) – the NGC 6402 Globular Cluster

Messier 14 (M14) – the NGC 6402 Globular Cluster:

Messier 14 with amateur telescope. Credit: Wikipedia Commons/Hewholooks

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!


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.

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."
Messier 14 was also the site of a nova that appeared in 1938. However, it was not registered until 1964, when Amelia Wehlau of the University of Western Ontario surveyed a collection of photographic plates taken by Helen Sawyer Hogg between 1932 and 1963. NASA's Hubble Space Telescope (HST) took a look for the nova' remnants, too, planning for more than a decade to obtain images and spectra of this region.

The Hubble team, which was led by Bruce Margon of the University of Washington, used the European Space Agency's Faint Object Camera (FOC) onboard HST to observe space in the vicinity of the nova. As the indicated in the research paper - titled "Faint Camera Observations Of A Globular Cluster Nova Field":

"The results are tremendously encouraging not only with respect to locating the quiescent nova, but also as a preview of the use of HST cameras in crowded, faint regions such as globular clusters. It is already clear even from this preliminary stage of the analysis that we have learned much. The brightness today of the nova remnant must be considerably less than suggested by the ground-based data, which we now know to have summed at least five separate stars."

History of Observation:

M14 is one of the original discoveries of Charles Messier, who cataloged it on June 1st, 1764. In his notes, he wrote of the object:

"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 and a half [FL]; it is round, and its diameter can be 2 minutes of arc; above it and 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", and 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]."
But as usual, it was Admiral Smyth who historically recorded it best when he said:

"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 Rasalague [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 [M10], which probably would put on the same appearance as this, were it removed half its distance farther from us."

Locating Messier 14:

Because M14 is rather small and on the faint side for small optics, it isn't easy to find in binoculars or a finderscope. The best way to start is to identify Delta Ophiuchi and begin about a handspan east. If you have difficulty, try about one third the distance between Beta and Eta Ophiuchi. Because of its relative size, it will appear almost stellar - but if you look closely, you'll notice that it's a "star" that won't quite come to a sharp focus.

With a minimum of 10X magnification, you can easily see that Messier 14 is a deep sky object and it will appear "fuzzy" to smaller telescopes and begin resolution with aperture of around 6". Large telescopes can fully resolve this loosely structured globular and can even distinguish some ellipticity in its general shape.

Here are the quick facts to help you get started. And as always, we hope that you enjoy your observations!

Object Name: Messier 14

Alternative Designations: M14, NGC 6402

Object Type: Class VIII Globular Cluster

Constellation: Ophiuchus

Right Ascension: 17 : 37.6 (h:m)

Declination: -03 : 15 (deg:m)

Distance: 30.3 (kly)

Visual Brightness: 7.6 (mag)

Apparent Dimension: 11.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier 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.

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