Editing Star cluster (section)

==Globular cluster==
[[File:New_Hubble_image_of_star_cluster_Messier_15.jpg|thumb|The [[globular cluster]] [[Messier 15]] photographed by [[Hubble Space Telescope|HST]]]]
{{Main|Globular cluster}}

Globular clusters are roughly spherical groupings of from 10&nbsp;thousand to several million stars packed into regions of from 10 to 30&nbsp;[[light-year]]s across. They commonly consist of very old [[stellar populations|Population&nbsp;II]] stars – just a few hundred million years younger than the universe itself – which are mostly yellow and red, with masses less than two [[solar mass]]es.<ref name=snp>{{cite book |first1=Robert |last1=Dinwiddie |first2=Will |last2=Gater |first3=Giles |last3=Sparrow |first4=Carole |last4=Stott |year=2012 |series=Nature Guide |title=Stars and Planets |publisher=DK |isbn=978-0-7566-9040-3 |pages=14, 134–137}}</ref> Such stars predominate within clusters because hotter and more massive stars have exploded as [[supernova]]e, or evolved through [[planetary nebula]] phases to end as [[white dwarf]]s. Yet a few rare blue stars exist in globulars, thought to be formed by stellar mergers in their dense inner regions; these stars are known as [[blue straggler]]s.

In the Milky Way galaxy, globular clusters are distributed roughly spherically in the [[galactic halo]], around the [[Galactic Center]], orbiting the center in highly elliptical [[orbit]]s. In 1917, the astronomer [[Harlow Shapley]] made the first respectable estimate of the Sun's distance from the Galactic Center, based on the distribution of globular clusters.

Until the mid-1990s, globular clusters were the cause of a great mystery in astronomy, as theories of [[stellar evolution]] gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However, greatly improved distance measurements to globular clusters using the [[Hipparcos]] satellite and increasingly accurate measurements of the [[Hubble constant]] resolved the paradox, giving an age for the universe of about 13&nbsp;billion years and an age for the oldest stars of a few hundred million years less.

Our Galaxy has about 150&nbsp;globular clusters,<ref name=snp/> some of which may have been captured cores of small galaxies stripped of stars previously in their outer margins by the tides of the [[Milky Way]], as seems to be the case for the globular cluster [[Messier 79|M79]]. Some galaxies are much richer in globulars than the Milky Way: The giant [[elliptical galaxy]] [[Messier 87|M87]] contains over a thousand.

A few of the brightest globular clusters are visible to the [[naked eye]]; the brightest, [[Omega Centauri]], was observed in antiquity and catalogued as a star, before the telescopic age. The brightest globular cluster in the northern hemisphere is [[Messier 13|M13]] in the constellation of [[Hercules (constellation)|Hercules]].