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Stars do not occur in space at completely arbitrary places. Some, such as the Sun, are single (field star), but others are members of pairs or form multiple-star systems. Still others form clusters of various types, and size. All of them are condensed from clouds of gas and dust. Two main types of star clusters occur: the small and sparse open (galactic) clusters containing tens to thousands of young stars, and the large and dense globular clusters containing up to several million stars. Large-scale groupings of some stellar types are called associations. | An OB Association is a group of stars of spectral types O and B that are close together, because they were all formed from a single cloud of gas. Since O and B stars are very bright their lives are short (up to about 20 million years), they do not have time to wander far away from their place of origin. Stars lighter than those of spectral types O and B are also formed from the same cloud of gas, and reside in the same region of space. When the O and B stars have all reached the ends of their lives, only the lesser stars are left and form an open cluster. T associations are groups of new-born stars before the ignition of nuclear burning. |
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Figure 06-01 shows a dense swarm of stars called Omega Centauri. It is located some 17,000 light-years from Earth. Omega Centauri is a massive globular star cluster of 150 light years in diameter, containing 10 million stars swirling in locked orbits around a common center of gravity. The stars in Omega Centauri are all very old, about 12 billion years. They are packed so densely in the cluster's core that it is difficult for ground-based telescopes to make out individual stars. Those in the core of Omega Centauri are so densely packed that occasionally one of them will actually collide with another one. Even in the dense center of Omega Centauri, stellar collisions will be infrequent. But the cluster is so old that many thousands of collisions must have occurred. When stars collide head-on, they probably just merge together and make one bigger star. But if the collision is a near miss, they may go into orbit around each other, forming a close binary star system. Omega Centauri is the most luminous and massive globular star cluster in the Milky Way. It is one of the few globular clusters that can be seen with the unaided eye. |
Figure 06-01 Globular Cluster, |
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only after cluster formation in the halo was completed. The globular clusters are more massive because they formed earlier in an environment of high density and low angular momentum. Apparently, the first systems to condense in the early universe had no angular momentum initially, since all the matter had emerged from the same infinitesimal volume of space; angular momentum was generated only later by tidal torques. Therefore, it seems to point to a close connection between globular clusters and cosmology, and suggests that the globular clusters are fossil remnants of the earliest condensed structures to form in the densest parts of the |
Figure 06-02a Star Clusters [view large image] |
universe. Observations outside the Milky Way also show the high frequency of globular clusters in some giant galaxies such as M87 and near the center of the Sagittarius dwarf galaxy where matter density should be much higher. |
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The left diagram in Figure 06-02b shows the distribution of globular clusters in the halo with low metal content (older); while the right one shows the distribution in the galactic disk with higher metal content (younger). Most of the younger globular clusters are concentrated toward the galactic nucleus where the matter density should be much higher. [Fe/H] is the ratio between abundances of iron and hydrogen in logarithmic scale, it is a measure of the metal content of the object. |
Figure 06-02 Globular Clusters in the Milky Way [view large image] |
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compact open cluster, NGC 2158, is visible in the same picture on the lower right. NGC 2158 is four times more distant that M35, over 10 times older (Figure 06-02a), and much more compact as it contains many more stars in roughly the same volume of space. NGC 2158's bright blue stars have self-destructed, leaving the cluster to be dominated by older and yellower stars. Figure 06-03b shows the open star cluster Pleiades, also known as the Seven Sisters and Messier 45. |
Figure 06-03a Open Cluster, M35 & NGC 2158 [view large image] |
Figure 06-03b Open Cluster, Pleiades |
It is a conspicuous object in the night sky with a prominent place in ancient mythology. The cluster contains thousands of stars, of which only a handful is commonly visible to the unaided eye. The stars in the Pleiades are thought to have |
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several hundred light-years across. The Scorpius OB association is a loose group of stars as shown in Figure 06-04a. This group contains many hot, extremely luminous OB-type stars. It is the site of recent star formation. The stars in such groups are mostly not gravitationally bound but are expanding away from some common center, which presumably marks their birthplace. A study indicates that the Scorpius association has had 20 supernova explosions over the past 11 million years. The bright, |
Figure 06-04a Scorpius [view large image] |
Figure 06-04b Dying Star, Antares [view normal image] |
bloated Antares in Figure 06-04b (yellow star in Figure 06-04a) is the next star likely to explode. |
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Usually, the stars within a cluster have barely enough gravitational pull to keep them together. This can create a rather large spread of open cluster members. The average cluster exists for approximately 100 million years, because as stars are created they drift further and further from each other. These stars continue drifting until they are no longer within the grasp of the gravitational forces of the clusters. The star constellation Ursa Major is actually part of a vastly spread out (23 degrees) open star cluster. |
Figure 06-05a Birth of A Star [view large image] |
Figure 06-05b Pre-main Seq. Evolution [view large image] |
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The formation of low-mass stars like the Sun can be explained by the gravitational collapse of a molecular cloud fragment into a protostellar core and the subsequent accretion of gas and dust from the surrounding interstellar medium. Theoretical considerations suggest that the radiation pressure from the protostar on the in-falling material may prevent the formation of stars above 10 solar masses through this mechanism, although some calculations have claimed that stars up to 40 solar masses can in principle be formed via the accretion through a disk. Observation in 2004 show a massive star (in the Omega nebula, Figure 06-06a) being born from a large rotating accretion disk. The protostar has already assembled about 20 solar masses, and accretion process is still going on. The gas reservoir of the circumstellar disk contains at least 100 solar masses of additional gas, providing sufficient fuel for substantial further growth of the forming star. |
Figure 06-06a Massive Protostar [view large image] |
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In January 2004, a new object (now known as McNeil's nebula) was discovered in Orion. A look back at images of this part of the sky revealed that in November 2003, a young low-mass star had brightened suddenly. There was a 50-fold increase in X-ray emissions before the optical outburst. The X-ray emission probably stems from the high-energy accretion. McNeil's nebula is not present on images taken in the 1990s and as early as 1951, but it has been found in mid-1960s, so it may be a variable T Tauri star as indicated in Table 08-02.[view large image] |
Figure 06-06b McNeil's Nebula |
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In Figure 06-07a, different stages of a star's life cycle appear in this Hubble Space Telescope picture of the environs of galactic emission nebula NGC3603. For the beginning, eye-catching "pillars" of glowing hydrogen at the right signal newborn stars emerging from their dense, gaseous, nurseries. Less noticeable, dark clouds or "Bok globules" at the top right corner are likely part of a still earlier stage, prior to their collapse to form stars. At picture center lies a cluster of bright hot blue stars whose strong winds and ultraviolet radiation have cleared away nearby material. Massive and young, they will soon exhaust their nuclear fuel. Nearing the end of its life, the bright supergiant star Sher 25 is seen above and left of the cluster, surrounded by a glowing ring and flanked by ejected blobs of gas. The ring structure is reminiscent of Supernova 1987a and Sher 25 itself may be only a few thousand years from its own devastating finale. The two teardrop-shaped objects below the cluster toward the bottom of the picture are similar to suspected protoplanetary disks encompassing stars in the Orion Nebula as shown in Figure 07-01. |
Figure 06-07a NGC 3603, Stellar Life Cycle |
[view large image] |
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Figure 06-07b NGC 346, Star Nursery [view large image] |
years ago. Further analysis shows that this bubble is located within a large expanding gaseous shell, possibly powered by the explosion and the winds of other bright stars in its vicinity. |
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Evidence indicates that the bright and peculiar galaxy M82 had collided with its neighboring galaxy M81. Inspection of the image in Figure 06-08 and other Hubble Space Telescope images now indicate massive young globular star clusters were formed during the encounter. Stars in these clusters that are 600 million years old are just now exhausting their central hydrogen fuel, indicating that M82 brightening occurred just that long ago. M82 is located |
Figure 06-08 M82 |
Figure 06-09 Medium Black- hole [view large image] |
about 12 million light years away and visible with binoculars towards the constellation of Ursa Major. The star-field shown in the figure spans about 10,000 light years. |
