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# 14.15 Open Star Clusters

Throughout the life of a spiral galaxy, pockets of gas — molecular clouds — collapse one by one into teeming caldrons of star formation. The majority of star formation is found in spiral arms themselves, where these gas clouds are shocked or compressed. Over time, as the star formation eats through all the gas and dust, what is left behind is a cluster of stars. To differentiate these clusters from the globular clusters in the galactic halo, star clusters made of young stars are called open clusters. This name also describes on many of them are shaped like a large splatter of stars, without a the central, globular, concentration seen in globular clusters. Over time, these open clusters will be shredded by the galaxies differential rotation. Most open clusters break apart into individual stars within only a few hundred million years because of dynamic forces acting on them. The individual stars have space velocities that take them far from their birthplace. As they disperse, they no longer light up the residue of gas left over from their birth. In comparison with most cosmic lifetimes, open clusters are short-lived, but during their brief lives they standout as gems in the sky; a favorite target for amateur astronomers with backyard telescopes.

Open cluster NGC 290 in the Small Magellanic Cloud. Click here for original source URL.

Although some clusters, such as the Pleiades (or Seven Sisters), are easy to recognize with the unaided eye, others are so far away that they require large telescopes to detect. Still others are so close that they cover much of our sky and were not even recognized until recent years. Clusters have been of major importance because they have revealed the shape, size, and age of the vast assembly of stars in which we live.

The study of open clusters is relatively new. Writing about clusters in 1930, Harvard astronomer Harlow Shapley pointed out that "their problems are intimately interwoven with the most significant questions of stellar organization and galactic evolution." Yet at the same time Shapely noted that scientific study of them had hardly begun. Nobody had even known how to measure their distances or plot their distribution in space until the 1920s.

Open star clusters are moderately close-knit, irregularly shaped groupings of stars. They usually contain 100 to 1000 members and are about 4 to 20 pc in diameter. Our Sun is possibly inside or on the edge of an open cluster centered only about 22 pc away toward the constellation Ursa Major, many of whose stars belong to this cluster. The best-known clusters, the Hyades and the Pleiades, lie 12 degrees apart in our winter evening sky, about 42 and 127 pc away, respectively. About 900 open clusters are concentrated along the Milky Way band, indicating that they lie in a flattened structure or plane.

Stellar associations are cousins of open clusters. They often have fewer stars but are larger in size and have a looser structure. Some large associations include an open star cluster within them. They may have 10 to a few hundred members and diameters of about 10 to 100 pc. They are rich in very young stars, such as O and B stars (which burn their fuel too fast to last long), or T Tauri stars (which evolve toward the main sequence too fast to last long). Several hundred associations have been cataloged. The smallest associations grade into small, multiple-star-like groups, such as the Trapezium in the Orion Nebula, which might be a link between multiple stars and small clusters. Like open clusters, associations are involved with regions of recent star formation and are short-lived.

The Pleiades open star cluster. Click here for original source URL.

", irregularly shaped groupings of stars. They usually contain 100 to 1000 members and are about 4 to 20 pc in diameter. Our Sun is possibly inside or on the edge of an open cluster centered only about 22 pc away toward the constellation Ursa Major, many of whose stars belong to this cluster. The best-known clusters, the Hyades and the Pleiades, lie 12º apart in our winter evening sky, about 42 and 127 pc away, respectively. About 900 open clusters are concentrated along the Milky Way band, indicating that they lie in a flattened structure or plane.

Stellar associations are cousins of open clusters. They often have fewer stars but are larger in size and have a looser structure. Some large associations include an open star cluster within them. They may have 10 to a few hundred members and diameters of about 10 to 100 pc. They are rich in very young stars, such as O and B stars (which burn their fuel too fast to last long), or T Tauri stars (which evolve toward the main sequence too fast to last long). Several hundred associations have been cataloged. The smallest associations grade into small, multiple-star-like groups, such as the Trapezium in the Orion Nebula, which might be a link between multiple stars and small clusters. Like open clusters," associations are involved with regions of recent star formation and are short-lived.

The Hyades open star cluster. Click here for original source URL