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Transcript
Salisbury Plain Observing Group—Deep Sky Objects : September Globular Clusters The spectacle of the September sky heralds the that that galaxy lying 1200 times further away return of the observing season and the delights of than M13 no doubt also has its own globular the coming dark nights. To start the new season’s clusters! articles, I thought it would be interesting to have a tour of some of September’s finest globular clusters from across the constellations. Whilst in Hercules, visit M92, which, were it not for M13, would have had the Great Globular title! M92 lies only a short star hop away from M13 to The obvious jewel of the northern skies is the north east and is easily found in binoculars if Messier 13, the Great Globular or Keystone Cluster you are careful– look for the non stellar fuzzy and in Hercules. Lying some 26,000 light years away, you have M92! This cluster lies another 1100 light M13 and years further than M13 and is 110 light years it is an across so is not much smaller than M13, but only is around 150 light comprises some 600,000 years across solar masses; easy binocular object (or in the darkest of skies, has around 400,000 solar masses. naked eye) and rewards magnification even in claim to fame of being a very old cluster estimated smaller apertures. to be 14 thousand million years old. Using my 6” reflector, I can begin to resolve stars at a magnification of around 60x and have gone up to 100x. Slightly larger apertures (8”+) can resolve the “propeller”; this is a shape reminiscent of the Mercedes Benz logo which can be seen towards the south east of the cluster. The propeller can be tricky to image but is a very worthwhile feature to sketch. Close by is NGC 6207, a galaxy lying 30 million light years away, which is visible in an 8”. Sobering to think It has the Slightly more challenging is NGC 6229, a smaller 9th magnitude cluster colloquially known as the overlooked globular cluster in Hercules. It is a small (1’) globular, but as it is condensed it is comparatively bright; I have spotted it from Kelling fairly easily. Our next stop is Sagitta which contains M71 lying 18,000 light years away. M71 was, for a long time, a matter of some debate over whether it was a loose globular cluster or older, compact open cluster. It can be spotted in binoculars, but does take magnification quite well; Luginbuhl & Skiff in their catalogue of Deep Sky Objects record seeing dark patches in the cluster giving the impression of “eyes” in 8” of aperture. Pressing onto Lyra now and M56; at only a third of the mass of M13, it is has been described as a good “non comet”. It is fairly easy to find, lying south west of an open chevron shaped asterism. It is around 55 light years across and is 27,000 light years away. Moderate powers used with a 6” start to resolve the cluster into its individual stars. Leaving Lyra, Delphinus is the next destination, an overlooked constellation lying south east of Cygnus. Despite it being small, it is conspicuous when you ignore Cygnus! Map of the Hercules Globular Clusters happy to hunt with For those who are telescope rather than binoculars, NGC 7006 is to the east of the Dolphins 41,000 light years and a diameter of 190 light head and is one of the furthest globulars lying years, comprising some 1,500,000 stars and is the some 185,000 light years away. It does need large most concentrated of all the Messier globulars. It aperture to obtain a sensible observation, but it can can be found on a straight line drawn down from be found in a 6” as a non star. To the South of the M15 to just inside the boundary of Aquarius and is tail is NGC 6934, again non resolvable visually, but easily spottable in binoculars - if you happen to be spottable in a 6”. in an area with clear mountain air you may just spot it naked eye!. M72 is even more distant than M2 and is 58,000 light years away. Like M71, this globular is classed as a very open globular and is spread across 98 light years of space. Locating it is not too tricky but it is farther down than M2. From M2 come down in a straight line to the 8 th magnitude star Sadalsuud then follow a straight line to beta Capricorni along which M72 is half way across. M72 is worth seeking as close by lies the Saturn Nebula, and M73 an open cluster. Of course, many other globular clusters lie in and around Saggitarius, which is an article in itself. If you’d like to learn more about clusters both Summer Trangle Globulars globular and open, then “Star Clusters” by Brent Pegasus holds M15, described by Messier as Archinell and Steven Hynes seems to be the standard “Nebula without star; it is round, the centre of it work, but I hope this article and accompanying maps will stimulate you to go and is brilliant, 3’ diameter” seek some of these celestial long term denizens this autumn, and get out under the night sky. And by John Herschel as “a magnificent globular cluster … comes up to a perfect blaze in the centre … it has straggling streams of stars drawing to a centre” M15 lies around 30,000 light years away, a diameter of 200 light years, and like M13 contains many red giant stars, but with its greater distance appears fainter and more compact. It’s also noted as one of the few globulars to contain a planetary nebula, Pease 1, which at magnitude 14, can be located with a appropriate filters. large aperture telescope and M15 itself is easily seen with binoculars with stellar resolution being achieved with apertures of around 4”. Comet Garrad (C/2009 P1) was in the vicinity of M15 during the 2nd and 3rd of August and made for a brilliant sight. Beneath the front legs of Pegasus lies Aquarius, containing 2 Messier globulars (2 and 72) and a faint NGC object. M2 sits at a distance of nearly Jonathan Gale