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  Cluster and Nebula: Astronomy: Cluster and Nebula - CITIES OF STARS
Posted on Monday, November 29 @ 02:54:51 CST by astronomy
 
 
  Cluster and Nebula

CITIES OF STARS

In contrast to open clusters, globular clusters are tight, spherical aggregations of tens of thousands to hundreds of thousands of stars. They contain very old stars and formed before the disk of our galaxy took shape. Globular clusters are actually distributed in a spherical halo around the Milky Way, but from our perspective in the disk of the galaxy, they seem concentrate around the galactic center, in the constellations of Sagittarius and Ophiuchus.



Of the 150 globular clusters that have been cataloged, only a few can be glimpsed with the naked eye. In binoculars, most appear as mere smudges of light. Through a 4 to 6 inch (100 to 150 mm) telescope, they take on more definition, looking like cottony globes with a sprinkling of outlying stars; with high magnification, you may even be able to resolve individual stars within the core. A 10 inch (250 scope aimed at a dark sky will reveal a spectacular granular sphere with strands of stars radiating out from the center. A broadband light pollution reduction filter can help you locate faint globulars, as will averted vision.

When you find a globular, notice its shape. Some will look irregular, like a freeze-frame of bees swarming chaotically around a hive. Rich, rapidly rotating clusters can exhibit a distinct flattening. Examine the globular's disk for subtleties such as star chains, voids (apparent holes), or threads of dark dust.

One of the brightest globulars is Ml 3 in Hercules, which is high overhead in Northern Hemisphere skies in late July. The grandest globular cluster of them all, however, is Omega Centaun (NGC5139),best seen from the Southern Hemisphere around mid-year. It contains about a million stars packed into a region that measures about 150 light-years across.

 
 
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