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  Globular Clusters.jpg -  Globular clusters contain a spherical collection of stars held together by gravity all born at roughly the same time. They are found in the halo of galaxies and are generally older and contain more stars than open clusters.  Exactly how they form is not known.  They may contain hundreds of thousands or even millions of stars.  It's suspected that the largest may contain a black hole at their center.  The Milky Way contains an estimated 160 globular clusters while the giant elliptical M87 may contain as many as 13,000.  The first globular cluster discovered was M22 in 1665, although individual stars within a cluster were not resolved for another hundred years. In 1908 it was discovered that the absolute brightness of a certain kind of variable star, known as a cepheid variable, could be determined by measuring it's period.  Starting in 1914, Harlow Shapley identified what he thought were cepheid variable stars within several globular clusters within the Milky Way (of course at that time it was believed that all objects were within the Milky Way).  He measured their period and apparent brightness and computed their distances. He assumed globular clusters were spherically distributed and used his calculations to estimate the dimensions of the Milky Way.  Unfortunately the stars he measured were actually the then not understood RR Lyrae variables.  These are similar to cepheid's but have a slightly different brightness - period relationship.  This caused Shapley to overestimate the distances.  Globular clusters are also not so nicely distributed over the Milky Way.  In spite of these errors he did show that the Milky Way was much larger than previously believed.  He also showed that the Sun was relatively far from the galaxy's center also contrary to the then popular belief. Plotting the distribution of stars within a globular cluster on a Hertzsprung-Russell diagram can show both its distance and age.  Such a diagram normally plots absolute brightness on the vertical axis and color  (or the equivalent temperature or spectral type or color index) on the horizontal axis.  Since all the stars are at approximately the same distance measuring their apparent brightness is equivalent to measuring their absolute brightness.  The vertical axis of the resulting plot can be converted to absolute brightness by recognizing that the stars on the cluster's apparent main sequence should have the same absolute brightness as nearby main sequence stars.  This relationship can be verified if there are any cephid variables or RR Lyrae stars within the cluster allowing a more direct determination of absolute brightness.  Distance to the cluster is a simple function of apparent and absolute brightness.  All the stars within the cluster have about the same age differing only in their mass.  More massive, brighter, hotter, stars burn through their hydrogen core leaving the Main Sequence before less massive, dimmer, cooler, stars.  This means that stars will leave the Main Sequence over time from left to right.  This leaves a gap at the left end of the main sequence line.  The size of this gap identifies the age of the cluster. A blue straggler is a bright, bluish, main sequence star within a globular cluster that, due to its mass, should have already evolved out of the main sequence.  A few dozen have been observed. There have been several theories developed to explain their existence, the most widely accepted being that they are the result of smaller stars merging.  If true, this would be the first evidence that stars can merge. Because globular clusters appear to be among the oldest objects in the universe, astronomers for many years used them to compute a lower bound on the age of the universe. This presented a problem until 1990 for advocates of the Big Bang theory since it placed the age of the universe at values less than that of several globular clusters. This impossibility, in spite of the discovery of the Cosmic Microwave Background, CMB, in 1965,  kept the Steady Statevs Big Bang debate alive until the COBE satillite was used to study the CMB. COBE and its successor, WMAP, provided sufficiently accurate measurements of cosmic parameters to place the age of the universe at 13.75 billion years with an uncertainty of only 0.11 billion years. The Plank satillite has refined this estimate to 13.82 billion years. Current estimates place the oldest globular clusters (M15, M68, M92) at no more than 13 billion years.                                  
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M13 Hercules

Globular clusters contain a spherical collection of stars held together by gravity all born at roughly the same time. They are found in the halo of galaxies and are generally older and contain more stars than open clusters. Exactly how they form is not known. They may contain hundreds of thousands or even millions of stars. It's suspected that the largest may contain a black hole at their center. The Milky Way contains an estimated 160 globular clusters while the giant elliptical M87 may contain as many as 13,000. The first globular cluster discovered was M22 in 1665, although individual stars within a cluster were not resolved for another hundred years.

In 1908 it was discovered that the absolute brightness of a certain kind of variable star, known as a cepheid variable, could be determined by measuring it's period. Starting in 1914, Harlow Shapley identified what he thought were cepheid variable stars within several globular clusters within the Milky Way (of course at that time it was believed that all objects were within the Milky Way). He measured their period and apparent brightness and computed their distances. He assumed globular clusters were spherically distributed and used his calculations to estimate the dimensions of the Milky Way. Unfortunately the stars he measured were actually the then not understood RR Lyrae variables. These are similar to cepheid's but have a slightly different brightness - period relationship. This caused Shapley to overestimate the distances. Globular clusters are also not so nicely distributed over the Milky Way. In spite of these errors he did show that the Milky Way was much larger than previously believed. He also showed that the Sun was relatively far from the galaxy's center also contrary to the then popular belief.

Plotting the distribution of stars within a globular cluster on a Hertzsprung-Russell diagram can show both its distance and age. Such a diagram normally plots absolute brightness on the vertical axis and color (or the equivalent temperature or spectral type or color index) on the horizontal axis. Since all the stars are at approximately the same distance measuring their apparent brightness is equivalent to measuring their absolute brightness. The vertical axis of the resulting plot can be converted to absolute brightness by recognizing that the stars on the cluster's apparent main sequence should have the same absolute brightness as nearby main sequence stars. This relationship can be verified if there are any cephid variables or RR Lyrae stars within the cluster allowing a more direct determination of absolute brightness. Distance to the cluster is a simple function of apparent and absolute brightness. All the stars within the cluster have about the same age differing only in their mass. More massive, brighter, hotter, stars burn through their hydrogen core leaving the Main Sequence before less massive, dimmer, cooler, stars. This means that stars will leave the Main Sequence over time from left to right. This leaves a gap at the left end of the main sequence line. The size of this gap identifies the age of the cluster.

A blue straggler is a bright, bluish, main sequence star within a globular cluster that, due to its mass, should have already evolved out of the main sequence. A few dozen have been observed. There have been several theories developed to explain their existence, the most widely accepted being that they are the result of smaller stars merging. If true, this would be the first evidence that stars can merge.

Because globular clusters appear to be among the oldest objects in the universe, astronomers for many years used them to compute a lower bound on the age of the universe. This presented a problem until 1990 for advocates of the Big Bang theory since it placed the age of the universe at values less than that of several globular clusters. This impossibility, in spite of the discovery of the Cosmic Microwave Background, CMB, in 1965, kept the Steady State vs Big Bang debate alive until the COBE satillite was used to study the CMB. COBE and its successor, WMAP, provided sufficiently accurate measurements of cosmic parameters to place the age of the universe at 13.75 billion years with an uncertainty of only 0.11 billion years. The Plank satillite has refined this estimate to 13.82 billion years. Current estimates place the oldest globular clusters (M15, M68, M92) at no more than 13 billion years.

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