Last week we talked about several star systems, but what if we added thousands or even millions of stars to the mix? A star cluster. There are different types of clusters, though. Open clusters contain hundreds or thousands of stars held together by gravity. They are young and evaporate over time, their stars running wild to roam freely through space. Globular clusters, on the other hand, are larger, contain hundreds of thousands of stars, and are more spherical. They are very old, a significant fraction of the age of the Universe itself, which means their stars contain fewer heavy elements, are redder, and probably don’t have planets (although 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
Notice 9:25

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PHOTOS/VIDEOS
Magellan's gemstone in the southern sky [NGC 290] http://www.spacetelescope.org/images/heic0603c/ [credit: European Space Agency and NASA]
An extreme star cluster comes to life in a new Hubble image http://www.spacetelescope.org/images/heic0715a/ [credit: NASA, ESA and Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]
View of a Sun-like star within an open cluster (artist's impression) http://www.spacetelescope.org/videos/heic1321d/ [credit: NASA, ESA and M. Kornmesser]
Star motion in Omega Centauri http://www.spacetelescope.org/videos/heic1017a/ [credit: NASA, ESA, J. Anderson and R. van der Marel (STScI)]
47 Tucanae: Probing Extreme Matter Through Neutron Star Observations http://chandra.harvard.edu/photo/2013/47tuc/ [credit: NASA/CXC/Michigan State/A.Steiner et al]
Hubble refines distance to Pleiades star cluster http://hubblesite.org/newscenter/archive/releases/2004/20/image/a/ [credits: NASA, ESA and AURA/Caltech]
M45 Pleiades https://www.noao.edu/image_gallery/html/im1116.html [credit: TA Rector (University of Alaska Anchorage), Richard Cool (University of Arizona) and WIYN]
From the Pleiades to the Hyades http://www.deepskycolors.com/archive/2011/11/06/from-the-Pleiades-to-the-Hyades.html [credit: Rogelio Bernal Andreo]
Image from Messier Atlas 035 https://commons.wikimedia.org/wiki/File:Messier_035_2MASS.jpg [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/heic1510a/ [credits: NASA, ESA and Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]
The oldest cluster in its cloud http://www.spacetelescope.org/images/potw1428a/ [credit: ESA/Hubble & NASA]
An unexpected population of young-looking stars http://www.spacetelescope.org/images/potw1244a/ [credit: ESA/Hubble & NASA]
View of a globular cluster (artist's impression) http://www.spacetelescope.org/videos/heic1321c/ [credit: NASA, ESA and M. Kornmesser]
All that glitters http://www.spacetelescope.org/images/potw1449a/ [credit: ESA/Hubble & NASA]

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