universe-review.cahtml> universe-review.caa href="index.htm" target="default.htm">universe-review.catd width="13%" align="center">universe-review.cab>Home Pageuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a> universe-review.caa href="overview.htm" target="default.htm">universe-review.catd width="10%" align="center">universe-review.cab>Overviewuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a> universe-review.caa href="F00-contents.htm" target="default.htm">universe-review.catd width="11%" align="center">universe-review.cab>Site Mapuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a> universe-review.caa href="F18-index.htm" target="default.htm">universe-review.catd width="10%" align="center">universe-review.cab>Indexuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a> universe-review.caa href="F17-glossary.htm" target="default.htm">universe-review.catd width="10%" align="center">universe-review.cab>Appendixuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a> universe-review.caa href="option2.htm" target="default.htm">universe-review.catd width="10%" align="center">universe-review.cab>Illustrationuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a> universe-review.caa href="FAQ.htm" target="default.htm">universe-review.catd width="10%" align="center">universe-review.cab>FAQuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a> universe-review.caa href="About.htm" target="default.htm">universe-review.catd width="10%" align="center">universe-review.cab>Aboutuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a> universe-review.caa href="form.htm" target="default.htm">universe-review.catd width="10%" align="center">universe-review.cab>Contactuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a>universe-review.ca/tr>universe-review.ca/table> universe-review.cabr />universe-review.cabr /> universe-review.cah2>universe-review.cafont color="blue">Star Clustersuniverse-review.ca/font>universe-review.ca/h2> universe-review.cahr align="left" width="20%" /> universe-review.cah3>Contentsuniverse-review.ca/h3> universe-review.caa href="universe-review.ca#cluster/default.htm#cluster/">Star Clustersuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#globular/default.htm#globular/">Globular Clustersuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#distribution/default.htm#distribution/">Distribution of Globular Clusters in the Milky Wayuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#open/default.htm#open/">Galactic (Open) Clustersuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#OB/default.htm#OB/">OB Associationsuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#protostar/default.htm#protostar/">T Associations and T Tauri Starsuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#cycle/default.htm#cycle/">A Sample of Stellar Life Cycleuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#mediumbh/default.htm#mediumbh/">Medium Black Holesuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#references/default.htm#references/">Referencesuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#Index/default.htm#Index/">Indexuniverse-review.ca/a>universe-review.cabr /> universe-review.cahr align="left" width="20%" /> universe-review.cah3 align="center">universe-review.caa name="cluster">Star Clustersuniverse-review.ca/a>universe-review.ca/h3> universe-review.catable border="0" width="100%">universe-review.catr>universe-review.catd width="49%"> universe-review.cap>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.universe-review.ca/p> universe-review.cap>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. universe-review.ca/p>universe-review.ca/td>universe-review.catd width="2%"> universe-review.ca/td>universe-review.catd width="49%">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.universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table> universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a> universe-review.cahr align="center" width="15%" /> universe-review.cah3 align="center">universe-review.caa name="globular">Globular Clustersuniverse-review.ca/a>universe-review.casup>universe-review.caa href="universe-review.ca#r01/default.htm#r01/">1universe-review.ca/a>,universe-review.caa href="universe-review.ca#r02/default.htm#r02/">2universe-review.ca/a>universe-review.ca/sup>universe-review.ca/h3> universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="30%">universe-review.caimg src="I06-01-OmegaCen.jpg" name="Globular Cluster" alt="Globular Cluster" align="left" width="210"/>universe-review.ca/td> universe-review.catd width="70%">Figure 06-01 shows a dense swarm of stars called universe-review.caa name="ome">Omegauniverse-review.ca/a> Centauri. It is located some 17,000 light-years from Earth. Omega Centauri is a massive globular star cluster, containing several 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. universe-review.ca/td>universe-review.catr> universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 06-01 Globular Cluster,universe-review.cabr />Omega Centauri (NGC 5139)universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="70%"> universe-review.ca/td> universe-review.ca/tr>universe-review.ca/table>universe-review.ca/table> universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a> universe-review.cahr align="center" width="15%" /> universe-review.cah3 align="center">universe-review.caa name="distribution">Distribution of Globular Clusters in the Milky Wayuniverse-review.ca/a>universe-review.ca/h3> Nearly all stars are formed in groups or clusters of some kind. There is no real dichotomy between globular clusters and open clusters but rather a continuity of properties and perhaps of formation processes. Figure 06-02a shows the blurred dividing line between globular clusters (triangles) and open clusters (circles) in a plot with age against metal content. The halo of the Milky Way was built up over a period of at least a few billion years, perhaps by an universe-review.caa href="F05-galaxy.htm#formation">inside-out accretion processuniverse-review.ca/a>. This process continued at least until the youngest outer halo clusters were formed about 10 billion years ago. The fact that the oldest stars and clusters in the galactic disk are about as old as the youngest halo clusters suggests that the galactic disk may have formed universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="20%">universe-review.caa href="I06-02-clusters.jpg">universe-review.caimg src="I06-02-clusters.jpg" name="Distribution of Globular Clusters" alt="Distribution of Globular Clusters" align="left" width="180"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="80%">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 universe-review.ca/td>universe-review.ca/tr> universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 06-02a Star Clusters universe-review.caa href="I06-02-clusters.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="80%">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.universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table> universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="30%">universe-review.caa href="I06-06-gcdistribution.jpg">universe-review.caimg src="I06-06-gcdistribution.jpg" name="Distribution of Globular Clusters" alt="Distribution of Globular Clusters" align="left" width="280"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="70%"> 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 universe-review.caa href="R05-05-metal.htm">metal contentuniverse-review.ca/a> of the object. universe-review.ca/td>universe-review.ca/tr> universe-review.catr>universe-review.catd>universe-review.cah4>Figure 06-02 Globular Clusters in the Milky Way universe-review.caa href="I06-06-gcdistribution.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table> universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a> universe-review.cahr align="center" width="15%" /> universe-review.cah3 align="center">universe-review.caa name="open">Galactic (Open) Clustersuniverse-review.ca/a>universe-review.casup>universe-review.caa href="universe-review.ca#r03/default.htm#r03/">3universe-review.ca/a>universe-review.ca/sup>universe-review.ca/h3> Open clusters of stars can be near or far, young or old, and diffuse or compact. Open clusters may contain from 100 to 10,000 stars, all of which formed at nearly the same time. Bright blue stars frequently distinguish younger open clusters. M35, pictured in Figure 06-03a on the upper left, is relatively nearby at 2800 light years distant, relatively young at 150 million years old, and relatively diffuse, with about 2500 stars spread out over a volume 30 light years across. An older and more universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="20%">universe-review.caa href="I06-07-openclusters.jpg">universe-review.caimg src="I06-07-openclusters.jpg" name="Open Clusters" alt="Open Clusters" align="left" width="180"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="20%">universe-review.caa href="I06-02-Pleiades.jpg">universe-review.caimg src="I06-02-Pleiades.jpg" name="Pleiades" alt="Pleiades" align="left" width="185"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="60%">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 universe-review.caa name="seven">Pleiadesuniverse-review.ca/a>, also known as the Seven Sisters and Messier 45. universe-review.ca/td>universe-review.ca/tr> universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 06-03a Open Cluster, M35 & NGC 2158 universe-review.caa href="I06-07-openclusters.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="20%">universe-review.cah4>Figure 06-03b Open Cluster, Pleiades universe-review.cabr />universe-review.caa href="I06-02-Pleiades.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="60%">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 universe-review.ca/td> universe-review.ca/tr>universe-review.ca/table>universe-review.ca/table> formed together around 100 million years ago, making them 1/50th the age of our sun, and they lie some 130 parsecs (425 light years) away. The blue haze around the hot stars is the reflection nebula produced by dust cloud behind the stars. For northern hemisphere viewers, the cluster is above and to the right of Orion the Hunter as one faces south, and it transits -- reaches its highest point in the sky -- around 4am in September, midnight in November, and 8pm in January. universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a> universe-review.cahr align="center" width="15%" /> universe-review.cah3 align="center">universe-review.caa name="OB">OB Associationsuniverse-review.ca/a>universe-review.casup>universe-review.caa href="universe-review.ca#r04/default.htm#r04/">4universe-review.ca/a>universe-review.ca/sup>universe-review.ca/h3> OB associations are sparsely populated groupings of stars, typically between a few tens and a few hundreds of light-years across. They consist mainly of very young stars that have formed in the relatively recent past -- a few million or tens of millions of years ago -- from the same large interstellar cloud. Their mutual gravitational attraction is insufficient to bind them permanently together, however, and eventually the grouping breaks up and the individual stars go their separate ways. Two distinct varieties of stellar association are recognized: T associations, which consist of numerous, low-mass T Tauri stars; and OB associations, which are loosely grouping of between 10 and 100 O stars and B stars, scattered across a region up to universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="20%">universe-review.caa href="I06-03-Scorpius.jpg">universe-review.caimg src="I06-03-Scorpius.jpg" name="OB Association" alt="OB Association" align="left" width="150"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="20%">universe-review.caa href="I06-08-Antares.jpg">universe-review.caimg src="I06-08-Antares.jpg" name="Antares" alt="Antares" align="left" width="260"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="60%">several hundred light-years across. The universe-review.caa name="Scorpius">Scorpiusuniverse-review.ca/a> 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, bloated universe-review.ca/td>universe-review.ca/tr> universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 06-04a Scorpius universe-review.caa href="I06-03-Scorpius.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="20%">universe-review.cah4>Figure 06-04b Dying Star, Antares universe-review.caa href="I06-08-Antares.jpg">[view normal image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="60%">Antares (arrow) in Figure 06-04b (yellow star in Figure 06-04a) is the next star likely to explode. universe-review.cabr />universe-review.cabr />universe-review.ca/td> universe-review.ca/tr>universe-review.ca/table>universe-review.ca/table> universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a> universe-review.cahr align="center" width="15%" /> universe-review.cah3 align="center">universe-review.caa name="protostar">T Associations and T Tauri Starsuniverse-review.ca/a>universe-review.casup>universe-review.caa href="universe-review.ca#r05/default.htm#r05/">5universe-review.ca/a>universe-review.ca/sup>universe-review.ca/h3> Interstellar dust clouds actually come as an enigma to scientists, because of the near vacuum medium the particles must travel through to combine and form clouds. Strong forces must play an essential role in their formation. The stellar winds from dying red giant stars are one example of the type of force needed. As these heavier elements cool off, they recombine as molecular dust particles. Over time they become increasingly dense and ionized by the starlight from nearby stars. Gravitational forces continually pull the dust and gas toward the central part of the nebulae. As more particles combine, the more friction is created and thermal heating would begin. This thermal heating represents the beginning of the star formation process. Protostars begin very large and condense through time. As they condense, they give off energy. They have not yet begun to create thermonuclear energy, which stars like our sun generate. These protostars are called T Tauri stars. Since they tend to occur in groups, which are always associated with regions full of interstellar nebulosity and dust, they are often referred to as T associations. A star forming nebula is rather large and can churn out many of these protostars. This is how an open cluster forms. The temperature in protostars keep increasing, until they reach main sequence temperatures, when they illuminate their surface through the process of nuclear fusion. The light from the older stars within the cluster can affect the formation of the protostars. It blasts away dark nebulae, which are the seed of stellar formation. This can cause a smaller star to form. universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="20%">universe-review.caa href="I06-05-TTauri.jpg">universe-review.caimg src="I06-05-TTauri.jpg" name="T Tauri Star" alt="T Tauri Star" align="left" width="170"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="20%">universe-review.caa href="I06-05-TTauri2.jpg">universe-review.caimg src="I06-05-TTauri2.jpg" name="T Tauri Star" alt="T Tauri Star" align="left" width="200"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="60%"> 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. universe-review.ca/td>universe-review.ca/tr> universe-review.catr>universe-review.catd>universe-review.cah4>Figure 06-05a T Tauri Star universe-review.caa href="I06-05-TTauri.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd>universe-review.cah4>Figure 06-05b Star Formation universe-review.caa href="I06-05-TTauri2.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.ca/tr>universe-review.ca/table> Figure 06-05a and 06-05b are schematic diagrams showing the evolutionary sequence of a T Tauri star. When a sufficiently dense core forms inside a molecular cloud, it will begin to collapse. Conservation of angular momentum will ensure that the collapsing cloud forms a flattened, spinning nebula. The central part of the star-forming clound, which collapses faster than its outer parts, froms the protostar. It continues to accumulate mass by accreting material from the outer parts of the cloud. Some of the remaining gas is expelled in two oppositely directed jets, perpendicular to the plane of the disk. It will become a full-blown star once the pressure at the center is high enough to ignite nuclear fusion, the surrounding gas in the disk will be blown away leaving a planetary system around the star.universe-review.cabr />universe-review.cabr /> universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="25%">universe-review.caa href="I06-11-highmassstar.jpg">universe-review.caimg src="I06-11-highmassstar.jpg" name="Massive Star" alt="Massive Star" align="left" width="230"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="75%"> universe-review.caa name="massive">universe-review.ca/a>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. universe-review.ca/td>universe-review.ca/tr> universe-review.catr>universe-review.catd>universe-review.cah4>Figure 06-06a Massive Protostar universe-review.caa href="I06-11-highmassstar.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.ca/tr>universe-review.ca/table> universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="15%">universe-review.caa href="I06-12-McNeil.jpg">universe-review.caimg src="I06-12-McNeil.jpg" name="McNeil's Nebula" alt="McNeil's Nebula" align="left" width="120"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="85%"> 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 universe-review.caa href="F08-star.htm#variables">Table 08-02universe-review.ca/a>.universe-review.cabr /> universe-review.cah4>universe-review.caa href="I06-12-McNeil.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4> universe-review.ca/td>universe-review.ca/tr> universe-review.catr>universe-review.catd>universe-review.cah4>Figure 06-06b McNeil's Nebula universe-review.ca/h4>universe-review.ca/td> universe-review.ca/tr>universe-review.ca/table> universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a> universe-review.cahr align="center" width="15%" /> universe-review.cah3 align="center">universe-review.caa name="cycle">A Sample of Stellar Life Cycleuniverse-review.ca/a>universe-review.casup>universe-review.caa href="universe-review.ca#r06/default.htm#r06/">6universe-review.ca/a>universe-review.ca/sup>universe-review.ca/h3> universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="30%">universe-review.caa href="I06-04-opencluster.jpg">universe-review.caimg src="I06-04-opencluster.jpg" name="Open Cluster Formation" alt="Open Cluster Formation" align="left" width="300"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="70%"> universe-review.cap>In Figure 06-07, different stages of a star's life cycle appear in this Hubble Space Telescope picture of the environs of galactic emission nebula universe-review.caa name="ngc3603">NGC3603universe-review.ca/a>. 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 universe-review.caa href="F07-planets.htm#birth">Figure 07-01universe-review.ca/a>.universe-review.ca/p> universe-review.ca/td>universe-review.ca/tr> universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 06-07 NGC 3603, Stellar Life Cycleuniverse-review.ca/h4>universe-review.ca/td> universe-review.catd width="30%">universe-review.cah4>universe-review.caa href="I06-04-opencluster.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.ca/tr>universe-review.ca/table>universe-review.ca/table> universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a> universe-review.cahr align="center" width="15%" /> universe-review.cah3 align="center">universe-review.caa name="mediumbh">Medium Black Holesuniverse-review.ca/a>universe-review.ca/h3> universe-review.catable border="0" width="100%">universe-review.catr> universe-review.catd width="20%">universe-review.caa href="I06-09-M82.jpg">universe-review.caimg src="I06-09-M82.jpg" name="M82" alt="M82" align="left" width="190"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="20%">universe-review.caa href="I06-10-mediumbh.jpg">universe-review.caimg src="I06-10-mediumbh.jpg" name="Medium Black Hole" alt="Medium Black Hole" align="left" width="190"/>universe-review.ca/a>universe-review.ca/td> universe-review.catd width="60%"> Evidence indicates that the bright and peculiar galaxy M82 had collided with its neighboring galaxy universe-review.caa href="I06-09-M81,82.jpg">M81universe-review.ca/a>. 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 universe-review.ca/td>universe-review.ca/tr> universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 06-08 M82 universe-review.cabr />universe-review.caa href="I06-09-M82.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="20%">universe-review.cah4>Figure 06-09 Medium Black- hole universe-review.caa href="I06-10-mediumbh.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="60%">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. universe-review.ca/td> universe-review.ca/tr>universe-review.ca/table>universe-review.ca/table> Recent observation in 2004 suggests that these star clusters may harbour blackholes of intermediate-mass in the 100 - 1000 solar-mass range. This kind of medium blackhole with mass in between the universe-review.caa href="F05-galaxy.htm#blackhole">supermassive blackholeuniverse-review.ca/a> at the center of galaxy and the universe-review.caa href="F08-star.htm#microblackholes">stellar blackholeuniverse-review.ca/a> with a few solar mass is expected but has never been detected until observations from NASA's Chandra space observatory captured the X-ray image as shown in Figure 06-09. Analysis shows that one cluster, MGG 11, coincides spatially with the brightest ULX (ultraluminous X-ray source) discovered so far, the luminosity of which corresponds to an intermediate-mass blackhole of 300 - 900 solar masses. Numerical simulations indicate that collisions between stars could have created a "runaway" star that ultimately became an intermediate-mass blackhole. On the other hand, another star cluster MGG 9 in the same image fails to develop a blackhole because the larger cluster radius leads to a timescale of five times longer than for MGG 11. universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a> universe-review.cahr align="center" width="15%" /> universe-review.caol>universe-review.cah3>universe-review.caa name="references">References:universe-review.ca/a>universe-review.ca/h3> universe-review.cali>universe-review.caa name="r01">universe-review.ca/a>Globular Cluster -- universe-review.caa href="http://www.seds.org/messier/glob.html">http://www.seds.org/messier/glob.htmluniverse-review.ca/a>universe-review.ca/li> universe-review.cali>universe-review.caa name="r02">universe-review.ca/a>Globular Cluster, universe-review.caa href="R06-01-globularclusters.htm">Science Magazine, News (January, 2003) universe-review.ca/a>universe-review.ca/li> universe-review.cali>universe-review.caa name="r03">universe-review.ca/a>Open (Galactic) Cluster -- universe-review.caa href="http://www.seds.org/messier/open.html">http://www.seds.org/messier/open.html/a>universe-review.ca/li> universe-review.cali>universe-review.caa name="r04">universe-review.ca/a>OB Association, Scorpius -- universe-review.caa href="http://www.mpifr-bonn.mpg.de/staff/tpreibis/usco.html">http://www.mpifr-bonn.mpg.de/staff/tpreibis/usco.htmluniverse-review.ca/a>universe-review.ca/li> universe-review.cali>universe-review.caa name="r05">universe-review.ca/a>T Tauri Star -- universe-review.caa href="http://etacha.as.arizona.edu/~eem/ttau/index.html">http://etacha.as.arizona.edu/~eem/ttau/index.htmluniverse-review.ca/a>universe-review.ca/li> universe-review.cali>universe-review.caa name="r06">universe-review.ca/a>Stellar Life Cycle, in pictures -- universe-review.caa href="http://astron.berkeley.edu/~bmendez/ay10/2000/cycle/cycle.html">http://astron.berkeley.edu/~bmendez/ay10/2000/cycle/cycle.htmluniverse-review.ca/a>universe-review.ca/li> universe-review.ca/ol> universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a> universe-review.cahr align="center" width="15%" /> universe-review.cah3>universe-review.caa name="Index">Indexuniverse-review.ca/a>universe-review.ca/h3> universe-review.catable border="0" width="100%"> universe-review.catr> universe-review.catd width="49%"> universe-review.caa href="universe-review.ca#distribution/default.htm#distribution/">Distribution of Globular Clusters in the Milky Wayuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#ngc3603/default.htm#ngc3603/">Emission nebula NGC 3603universe-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#open/default.htm#open/">Galactic (Open) clusteruniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#globular/default.htm#globular/">Globular clusteruniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#massive/default.htm#massive/">Massive protostaruniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#mediumbh/default.htm#mediumbh/">Medium blackholesuniverse-review.ca/a>universe-review.cabr /> universe-review.ca/td>universe-review.catd width="2%"> universe-review.ca/td>universe-review.catd width="49%"> universe-review.caa href="universe-review.ca#OB/default.htm#OB/">OB associationuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#ome/default.htm#ome/">Omega Centauriuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#seven/default.htm#seven/">Pleiades (Seven Sisters)universe-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#protostar/default.htm#protostar/">Protostars (T Tauri stars)universe-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#Scorpius/default.htm#Scorpius/">Scorpiusuniverse-review.ca/a>universe-review.cabr /> universe-review.caa href="universe-review.ca#cycle/default.htm#cycle/">Stellar life cycleuniverse-review.ca/a>universe-review.cabr /> universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table> universe-review.cahr align="center" width="15%" /> universe-review.cah4>universe-review.caa href="index.htm">[Home Page]universe-review.ca/a>universe-review.ca/h4> universe-review.ca/body> universe-review.ca/html>