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"HUBBLE SCIENCE"

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"Globular Clusters / Stellar Pockets"

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A globular cluster is a big sphere of stars 
that have collected together they're  

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orbiting around a Common Center but it's 
very spherical very organized looks like  

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a huge basketball contains hundreds of 
thousands of stars these clusters have  

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formed early on in the formation of a 
galaxy when the cluster starts forming  

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it's a very diffuse and over time things get 
concentrated more and more toward the center.  

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Stars are actually moving all around and if 
they're sort of orbiting around the center  

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it's not a static cluster even though a single 
picture makes everything look like it's in place.

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One of the more famous ones is Messier 13.  

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Omega Centauri is a prototypical 
one in the southern hemisphere  

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globular clusters are very old there seem to be 
some of the first stars formed in the Galaxy as  

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the material that came together in the past one 
of the pockets of condensed material that occurred  

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around the center of the Galaxy drew together into 
the selections of stars so they're mostly very old  

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stars, 10 or 12 billion years old although we do 
see a second generation of newer younger stars  

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in globular clusters still you know relatively 
old billions of years ago but it's not like it  

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had one burst of star formation and that was it. It
did have a chance to form a second wave later on.  

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Westerlund 1 still in one is what's called an open cluster 
it's a very loose collection of stars over a broad  

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area it's more sort of a cloud of stars as more 
stars gather in an open cluster like that and more  

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material comes together it's going to naturally 
pull things in toward its center. Something like  

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Westerlund 1 over many millions of years could evolve 
into something that looks more like a globular  

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and we do think that most globulars have one or 
more black holes at the center which is a source  

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of immense gravity to help hold these clusters 
together Hubble saw in the globular cluster NGC  

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6397 the first clear evidence for multiple black 
holes at the center we had previously assumed that  

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most globulars would have a single black hole 
at the center in this case we found a number  

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of separate black holes they're moving around 
each other at high speeds around a common center.  

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If you have a number of black holes in 
tight orbits around each other at the  

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center that works as well as a single black 
hole for holding together the overall cluster

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