A guide to globular clusters
BY MARK ARMSTRONG
Posted: 14 May 2013
With observing opportunities starting to dwindle as the nights get shorter and lighter, globular clusters make for good targets in the less than ideal observing circumstances, with their generally high-surface brightness.
Globular clusters are spectacular, densely-packed near spherical collections of ancient stars that populate mainly the extended outer halo of our galaxy. They are thought to have formed in the very early life of our galaxy, over 11 billions years ago, making them much older than open clusters. Our Galaxy has 150 to 200 members, a drop in the ocean when compared with numbers linked to some other galaxies; the giant elliptical M87 in Virgo has an incredible 16,000 globulars and M31 has over 300 confirmed globulars, with other candidates pushing the total beyond 1000. Galactic globulars come in a range of diameters and masses; the smallest can be only just more massive than the largest open clusters but the largest such as M19 and M54, weighing in at 1.5 million solar masses and containing several million stars, rival some dwarf galaxies. Indeed, it's possible that the brightest and most spectacular of them all, mighty Omega Centauri, is the striped-down core of a small galaxy that suffered as a result of a close encounter with our galaxy eons ago.
How easy a globular is to see is not just down to its brightness; its degree of condensation (how densely packed it is) is a critical factor too. A dense, compact, star rich globular will show a greater contrast with the background sky and therefore be easier to see than one that is diffuse, that tends to get lost easier in the background. Experienced amateurs use a 12-point scale which indicates the degree of condensation, ranging from I (very dense and compact) to XII (extremely diffuse with no central concentration). M13 is rated V, as is M5, although many would reckon it is less dense that M13 so perhaps it should be VI. M3 is rated V1, with Omega Centauri more diffuse at VII. M92 is rated as IV so is more compact than most, so should be easier to pick up; indeed it is an easy object in binoculars, clearly non-stellar. For a view of a really dense globular, observe M2 (class II) in Aquarius, this autumn.
One of the best in the entire sky, Messier 3 (NGC 5272), eclipsed only by M13 in the northern sky, is still well on show in the great constellation of Canes Venatici. M3 was Messier's first true discovery in May 1764 and is big, bright and easy to spot in binoculars, as well as being visible to the naked-eye in dark, transparent skies. M3 is one of the largest and most massive clusters, weighing in at 800,000 solar masses and is thought to contain about 500,000 stars in a sphere some 190 light-years across. It has a highly elliptical galactic orbit and at present is thought to lie 34,000 light years away.
It doesn't require much aperture to start to recognise M3's true nature, as even a 80-mm 'scope at low power shows it as grainy and apertures of 100mm and above will start to resolve the cluster, making it a harder proposition in this respect than its principle rivals in the northern sky, M13 and M5. M3 is rated as class V1 in the 12-point scale of degree of condensation (with I being very dense and compact to XII being extremely diffuse), making it slightly more diffuse than its aforementioned rivals, both rated V. Its apparent size through the eyepiece depends on aperture, appearing around seven arcminutes in a 100mm increasing to 15 arcminutes in large amateur 'scopes. Spectacular images are possible through even modest 'scopes with deep LRGB CCD images revealing M3's breathtakingly beautiful form extending to almost 20 arcminutes.
Despite being in a relatively barren part of the sky M3 is quite easy to find; just look over half way between brilliant Arcturus (alpha Boötis) and Cor Caroli (alpha CVn), a little closer to the former. At the moment M3 culminates due south just after 11pm BST at a very healthy 65 degrees or so and there's a five-hour observing window until the morning twilight intervenes around 4am.
Scanning the sky with binoculars in the direction of Serpens, close to the border with Virgo, you may come across a hazy spot, like an out of focus star. This is the big, bright and beautiful globular cluster Messier 5 (NGC 5904), so good in fact that the great observer, Edward Emerson Barnard thought it ‘much more beautiful than M13.
M5 can be found with the naked eye by eagle-eyed observers at dark sites some 25 degrees south-east of Arcturus and eight degrees west of alpha Serpentis. Its impact is somewhat diminished by its low northerly declination and this is where M3 and M13 score heavily. On the other hand, it does mean M5 is visible from both hemispheres. Small scopes reveal a distinctly elliptical shape with a bright core and resolution of the outlying stars. Moving up to 'scopes in the 150-200-mm class gives magnificent views, with resolution more or less down to the core at moderate magnifications. M5 can be observed as soon as it gets dark and is at its best at 12.45am BST when it's 40 degrees up. Pick up a copy of the May issue of Astronomy Now for an in-depth, exhaustive observing, sketching and imaging guide to M5.
Unquestionably the finest globular cluster in the Northern Hemisphere, M13 (NGC 6205) is many observers' first experience of a globular cluster and from then on are hooked. M13 is only eclipsed by the great southern globulars Omega Centauri and 47 Tucanae, with M22 in Sagittarius providing very stiff competition. M13 is very easy to find on the west side of the Keystone asterism of Hercules, lying a third of the way down from eta to zeta Herculis and it's just within naked-eye range too from the darkest of sites, glowing at an integrated magnitude +5.7.
Binoculars will snare M13 easily, flanked by two seventh-magnitude stars, but only show an extended fuzzy spot with a brighter core. Small telescopes of good quality in the 80-100-mm class should show an apparent diameter of eight to ten arcminutes and start to resolve some of the outlying stars in this giant ball of suns. Upgrading to a 150-mm and employing magnification of 200x quite simply gives stunning views.
M13 was discovered by Edmond Halley in 1714 and Messier added it to his list of comet-like nebulous objects in 1764. The indefatigable and brilliant William Herschel was the first to recognise its true nature 20 years later. M13 has an eccentric, 500 million year orbit around the galactic centre and it can be as remote as 80,000 light years from us but at present it lies much closer at 26,000 light years. M13 is one of the larger clusters with a physical diameter of 160 light years, equating to an apparent diameter on the celestial sphere of 21 arcminutes. Astronomers believe M13 contains no more than one million stars with a total mass of 600,000 solar masses.
M92 (NGC 6341) is a very good globular cluster residing in Hercules but is often overshadowed by the great globular cluster M13, only ten degrees to its south-west. M92 is smaller and fainter (mag. +6.5 and 14') than M13 (+5.7 and 21') because it is physically smaller and further away. M92 weighs in at 400,000 solar masses, crammed into a 110 light year diameter lying 27,000 light years away.
Try switching the view between M13 and M92 and see how you rate them? Vary the magnification and take plenty of time observing each cluster. M92 appears asymmetric even in scopes as small as 80-mm, with the central condensation offset to the north-east and there are no obvious star chains, unlike those in M5 and M13.
Both M92 and M13 are superbly placed on May evenings, basically observable all night, culminating in the early hours. M92 is circumpolar from the UK (never sets).
Messier 10 & 12
Ophiuchus is home to no less than seven Messier globular cluster with the M10 (NGC6254) and M12 (NGC6218) pair (only slightly over three degrees apart) being the most accessible to Northern Hemisphere observer and happen to be two of the finest anywhere in the heavens. M10 is slightly superior to its neighbour in all categories, shining at mag. +6.6 and extending out to almost 20 arcminutes in apparent diameter.
M12 is a noticeably looser globular even through small apertures (IX classification) and if you have a 100-150-mm 'scope then you should be able to fully resolve this cluster to its core if the seeing permits a high enough power. M12 is smaller and fainter than M10; although there's not much in it in brightness terms, a mere two tenths at +6.8, in size it loses out by five arcminutes despite lying 4,000 light years closer than its neighbour at 20,760 light years. This is down to its true physical size of about 85 light years, much smaller than M10. By virtue of being south of the celestial equator (albeit marginally) both M10 and M12 don't pull well clear of the southeastern horizon until midnight and culminate around 2am, when they are both comfortably over 30 degrees above the southern horizon.
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