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Object Name: Messier 13
Alternative Designations: M13, NGC 6205, the “Great Hercules Cluster”
Object Type: Class V Globular Cluster
Right Ascension: 16 : 41.7 (h:m)
Declination: +36 : 28 (deg:m)
Distance: 25.1 (kly)
Visual Brightness: 5.8 (mag)
Apparent Dimension: 20.0 (arc min)
Locating Messier 13: To locate M13, all one needs to know is the “Keystone” asterism of Hercules. While this lopsided rectangle isn’t particularly bright, once you understand where to find it, you’ll be able to spot it even under relatively light polluted skies. Both Vega (in the constellation of Lyra) and Arcturus (in Bootes) are very bright stars and the keystone is about 1/3 the distance between them. Once you locate it, always remember that Messier 13 is on the leading western side – no matter what position Hercules may be in. By just generally aiming your binoculars in the center of the two stars on the western side, you can’t miss this BIG, bright globular cluster! When using a finderscope, aim slightly north of the center point and you’ll easily spot it as well. From a dark sky location, M13 can often be seen unaided as a small, fuzzy spot on the sky.
What You Are Looking At: Hanging out in space at a distance of 25,100 light years, this 24 million year old beauty is one of the most impressive globular clusters for the northern hemisphere. Containing over a million stars packed into a 145 light year sphere, the center of this glorious object is 500X more concentrated than its outer perimeters. And out of all of those stars there stands one stranger… Barnard Number 29. What is it? In a world where all the stars should be the same age, there is a spectral type B2 – a young, blue star. Where did it come from? Apparently on one of M13′s journeys around our galaxy, it collected a field star, for radial velocity measurements have proved that it does belong to the globular cluster!
Are there other surprises inside? When the Hubble Space Telescope was pointed towards Messier 13 they found 15 blue straggler star candidates and 10 other possibles. The stars in the blue horizontal branch of M13 appeared to be centrally depleted relative to other stellar types and the blue stragglers in the combined sample are centrally concentrated relative to the older red giant stars. However, the Stromgren photometry work done Frank Grundah (et al) suggests this is a normal occurrence in evolution. “We also note the existence of what appears to be two separate stellar populations on the horizontal branch of M13. Among other possibilities, it could arise as the result of differences in the extent to which deep mixing occurs in the precursor red giants.”
According to Yoon and Lee, “One of the long-standing problems in modern astronomy is the curious division of Galactic globular clusters, the “Oosterhoff dichotomy,” according to the properties of their RR Lyrae stars. Here, we find that most of the lowest metallicity clusters, which are essential to an understanding of this phenomenon, display a planar alignment in the outer halo. This alignment, combined with evidence from kinematics and stellar population, indicates a captured origin from a satellite galaxy. We show that, together with the horizontal-branch evolutionary effect, the factor producing the dichotomy could be a small time gap between the cluster-formation epochs in the Milky Way and the satellite. The results oppose the traditional view that the metal-poorest clusters represent the indigenous and oldest population of the Galaxy.”
But how old are its stars… really? There could be more than one answer. According the work of R. Glebocki (et al), stellar rotation within Messier 13 can also play a role in how the stars age. “Much theoretical and observational work about the role that rotation plays in stellar evolution has been done. Angular momentum is one of the fundamental parameters in the process of star formation as well as in early life of a star. A considerable amount of research has been done on the stellar axial rotational velocities. Clusters present unique possibility of determination of age of stars.”
History: M13 was discovered by Edmond Halley in 1714 and whose notes said: “This is but a little Patch, but it shews it self to the naked Eye, when the Sky is serene and the Moon absent.” And it was later catalogued by Charles Messier on June 1, 1764: “In the night of June 1 to 2, 1764, I have discovered a nebula in the girdle of Hercules, of which I am sure it doesn’t contain any star; having examined it with a Newtonian telescope of four feet and a half [FL], which magnified 60 times, it is round, beautiful & brilliant, the center brighter than the borders: One perceives it with an ordinary [non-achromatic] refractor of one foot [FL], it may have a diameter of three minutes of arc: It is accompanied by two stars, the one and the other of the ninth magnitude, situated, the one above and the other below the nebula, & little distant. I have determined its position at its passage of the Meridian, and compared with the star Epsilon Herculis; its right ascension has been concluded to be 248d 18′ 48″, and its declination 36d 54′ 44″ north. It is reported in the Philosophical Transactions, no. 347, page 390, that Mr. Halley discovered by hazard that nebula in 1714: it is, he says, almost on a straight line with Zeta and Eta according to Bayer, a bit closer to the star Zeta than to Eta, & when comparing its situation between the stars, its place is rather close to Scorpius 26d 1/2 with 57 degrees Northern [ecliptic] latitude, it is nothing but a small patch; but one sees it well without a telescope when the weather is fine, and if there is no light of the moon.”
Although Sir William Herschel would soon enough resolve it into stars and again by his son and many others, no one describes its history more eloquently than Admiral Smyth: “A large cluster, or rather ball of stars, on the left buttock of Hercules, between Zeta and Eta; the place of which is differentiated from Eta Herculis, from which it lies south, a little westly, and 3deg 1/2 distant. This superb object blazes up in the centre, and has numerous outliers around its attenuated disc. It was accidentally hit upon by Halley, who says, “This is but a little patch, but it shows itself to the naked eye, when the sky is serene, and the moon absent.” The same paper, in describing this as the sixth and last of the nebulae known in 1716, wisely admits, “there are undoubtedly more of these which have not yet come to our knowledge:” ere half a century passed, Messier contributed his 80 or 90 in the Catalogue of 103; and before the close of that century WH [William Herschel] alone had added to the above 6, no fewer than 2500; and his son, in re-examining these, added 520 more! In my own refractor its appearance was something like the annexed diagram; but I agree with Dr. Nichol, that no plate can give a fitting representation of this magnificent cluster. It is indeed truly glorious, and enlarges on the eye by studying gazing. “Perhaps,” adds the Doctor, “no one ever saw it for the first time through a telescope, without uttering a shout of wonder.” This brilliant cluster was discovered by Halley in 1714; and fifty years afterwards it was examined by M. Messier, with his 4-foot Newtonian, under a power of 60, and described as round, beautiful, and brilliant; but, “ferret” as he was in these matters, he adds, “Je me suis assuré qu’elle ne contient aucune étoile.” This is rather startling, since the slightest optical aid enables the eye to resolve it into an extensive and magnificent mass of stars, with the most compressed part densely compacted and wedged together under unknown laws of aggregation. In 1787, Sir William Herschel pronounced it “a most beautiful cluster of stars, exceedingly compressed in the middle, and very rich.” It has been recently viewed in the Earl of Rosse’s new and powerful telescope, when the components were more distinctly separated, and brighter, than had been anticipated; and there were singular fringed appendages to the globular figure, branching out into the surrounding space, so as to form distinct marks among the general outliers.”
And so Messier 13 has been part of our imaginations for many years – and in 1974 a message was sent from Aricebo designed to communicate the existence of human life to hypothetical extraterrestrials. Why? It was though perhaps that amongst all those stars that there was a much greater chance of finding a planet with intelligent life…
Or is there?
B&W image thanks to Palomar Observatory, courtesy of Caltech, M13 core region by NASA, ESA, and the Hubble Heritage Team (STScI/AURA) and color image thanks to N.A.Sharp, REU program/NOAO/AURA/NSF.