Last week we covered multiple star systems, but what if we added thousands or even millions of stars to the mix? A star cluster. There are different kinds of clusters, though. Open clusters contain hundreds or thousands of stars held together by gravity. They’re young and evaporate over time, their stars let loose to roam space freely. Globular clusters, on the other hand, are larger, have hundreds of thousands of stars, and are more spherical. They’re very old, a significant fraction of the age of the Universe itself, and that means their stars have less heavy elements in them, are redder, and probably don’t have planets (though we’re not really sure).

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Chapters:
Introduction: Star Clusters 00:00
Determining the Age of Star Clusters 2:04
Open Clusters Evaporate 3:23
The Pleiades Star Cluster 4:27
Globular Clusters 5:50
Review 9:25



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PHOTOS/VIDEOS
Magellanic gemstone in the southern sky [NGC 290] http://www.spacetelescope.org/images/... [credit: European Space Agency & NASA]
Extreme star cluster bursts into life in new Hubble image http://www.spacetelescope.org/images/... [credit: NASA, ESA and the Hubble Heritage (STScI/AURA)ESA/Hubble Collaboration]
View of a Sunlike star within an open cluster (artist’s impression) http://www.spacetelescope.org/videos/... [credit: NASA, ESA, and M. Kornmesser]
Motion of stars in Omega Centauri http://www.spacetelescope.org/videos/... [credit: NASA, ESA, J. Anderson and R. van der Marel (STScI)]
47 Tucanae: Probing Extreme Matter Through Observations of Neutron Stars http://chandra.harvard.edu/photo/2013... [credit: NASA/CXC/Michigan State/A.Steiner et al]
Hubble Refines Distance to Pleiades Star Cluster http://hubblesite.org/newscenter/arch... [credit: NASA, ESA and AURA/Caltech]
M45 Pleiades https://www.noao.edu/image_gallery/ht... [credit: T.A. Rector (University of Alaska Anchorage), Richard Cool (University of Arizona) and WIYN]
From the Pleiades to the Hyades http://www.deepskycolors.com/archive/... [credit: Rogelio Bernal Andreo]
Messier 035 Atlas Image https://commons.wikimedia.org/wiki/Fi... [credit: Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation]
Globular cluster 47 Tucanae http://www.spacetelescope.org/images/... [credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)ESA/Hubble Collaboration]
The oldest cluster in its cloud http://www.spacetelescope.org/images/... [credit: ESA/Hubble & NASA]
An unexpected population of younglooking stars http://www.spacetelescope.org/images/... [credit: ESA/Hubble & NASA]
View of a globular cluster (artist’s impression) http://www.spacetelescope.org/videos/... [credit: NASA, ESA, and M. Kornmesser]
All that glitters http://www.spacetelescope.org/images/... [credit: ESA/Hubble & NASA]