M2, a globular cluster in Aquarius. The second object on Charles Messier's eighteenth century list of diffuse objects which could be confused with comets, the cluster contains more than a hundred thousand stars, crammed into a region a little over 150 light years in diameter. Almost two hundred such clusters are scattered around the periphery of our galaxy, remnants of the earliest stages of our galaxy's formation. This cluster, one of the more distant globulars in our galaxy, lies fifty thousand light years below the Galactic South Pole, more than twice as far from the center of the galaxy as our Sun and Solar System. (D. Williams, N. A. Sharp, AURA, NOAO, NSF apod010611)

     M3, a globular cluster 35,000 light years from the Sun, in Canes Venatici. M3 contains more than half a million stars, packed into a region a little over 150 light years in diameter, or about five times as many stars per cubic light year as M2, and nearly a hundred times more stars per cubic light year as in the region near the Sun. Globular clusters such as M2 and M3 are so old that only stars cooler, smaller and fainter than the Sun still remain on the Main Sequence. In such clusters, stars which were originally like the Sun have grown old, died, and collapsed to become white dwarfs. (S. Kafka & K. Honeycutt (Indiana University), WIYN, NOAO, NSF, apod060312)

     Shown below are three images of the globular cluster in Hercules, M13, one of the best known globular clusters, because its position places it high in northern skies during late spring and summer evenings. Approximately 25,000 light years from our Sun, more than a hundred thousand stars are packed into its fairly typical 150 light year diameter.

     An image showing the appearance of the cluster in a moderate size telescope, under reasonably dark sky conditions. (Michael Richmann, apod980819)

     A larger image, showing the more spectacular appearance observed with larger instruments. (Yuugi Kitahara, apod000301)

     A similar image, but with software manipulation of the electronically digitized image, to avoid the overexposure of the central regions visible in the second image. (Eddie Guscott, apod040511)

Because of its brightness, visible without optical aid in a dark sky, and therefore labeled as a starlike object, 47 Tucanae. It is the second largest and brightest globular cluster, exceeded only by Omega Centauri (NGC 5139). Even at a distance of 16 to 18 thousand light years, its 120 light-year diameter is as large as the full moon, and contains several million stars.
     NGC 104 was recently used to prove a theory of stellar mass sorting by globular clusters. More massive stars tend to settle to the center of the cluster, while less massive ones spread out over a larger region. To prove that, 130 thousand stars near the center of the cluster were studied for seven years, using techniques that allowed movements as small as 1/100th of an HST image pixel to be detected. "Blue stragglers", Main Sequence stars located well above the cluster's turnoff point, were shown to be moving more slowly than stars of more normal mass. (The "stragglers" are believed to be produced by collisions between normal mass stars, which are virtually impossible in normal regions of stellar space, but fairly common in the densely packed cores of globular clusters.)
     Directly below is a "normal" image of the entire cluster. Below that is an illustration of the region studied in detail by the HST, to measure the motions of stars in the cluster's core. Upper image, ESO

Lower image, ground-based image on the left, VLT, R. Kotak & H. Boffin, ESO; HST closeup, ESA, G. Meylan (Ecole Polytechnique Federale de Lausanne) NASA

     Shown below are three images of the most massive globular cluster in our galaxy, Omega Centauri, or NGC 5139. Over ten million stars are packed into its 150 light year diameter, making it nearly a hundred times as massive as a typical globular cluster. Unfortunately, its far southern position, in the constellation of Centaurus, makes it impossible to see from most parts of North America; but it is a spectacular object for southern observers.

     An exceptionally detailed view of Omega Centauri, giving a good impression of the incredible number of stars packed into its relatively small diameter. Even though fifteen thousand light years from Earth, the cluster is easily visible without optical aid in a dark southern sky. (Loke Kun Tan (StarryScapes), apod020416)

     A wide-field view of Omega Centauri and the surrounding star fields of the southern Milky Way emphasizes the extreme compactness of the globular cluster. The stars apparently surrounding the cluster actually lie much closer to us, in the disk of our galaxy, a region where stars are generally several light years apart, while the cluster, like most of the 150 globular clusters scattered around our galaxy, lies in the halo, a region where stars are generally a few light decades apart. In the cluster itself, however, stars are only light weeks to light months apart. As a result, particularly in the dense core of the cluster, stellar collisions, which are virtually impossible in the more widely scattered disk and halo, occur on a very infrequent but occasional basis, as discussed below. (Steve Crouch, apod060526)

     In the center of Omega Centauri, stars are packed ten thousand times more densely than in the Solar neighborhood. Above, a Hubble Space Telescope image resolves the core of Omega Centauri into individual stars. (Adrienne Cool (SFSU) et al., Hubble Heritage Team (STScI/AURA), NASA, apod011010)
     Like all the globular clusters which rim our Milky Way, Omega Centauri was formed well over ten billion years ago, at about the same time the galaxy itself first began to form. Since this is longer than the lifetime of the Sun, virtually all the stars still visibly shining in the cluster are smaller, fainter, redder stars than the Sun, which have Main Sequence lifetimes longer than the current age of the Universe, while all the stars of mass similar to or greater than the Sun, which have Main Sequence lifetimes shorter than the age of the cluster, have already died, and are now essentially unobservable white dwarfs, neutron stars and black holes. However, aside from the faint yellowish stars which make up the majority of those imaged here, there are also a number of much brighter yellowish-orange "Red Giants", which are in the process of dying, and rather strangely, a few relatively hot, bright blue stars, which, if they had originally been that hot and bright, would have died long ago, and are almost certainly the result of collisions of smaller stars, merging into more massive stars. Near collisions can also result in the formation of close binary systems, in which gas flows from one star to another, producing interesting mass-transfer effects.