Compared to open clusters globular clusters are old and regularly shaped We find them outside the galactic plane in something called the galactic halo.
First of all, we know they are old because they lack metals. They also are typically red meaning that they are not newly formed as blue stars are typically young. They also are large in size and contain hundred of thousands of star in a sphere hundreds or thousands of light years in diameter.
They are spherical because of the multitude of stars which allow gravity to pull them into a sphere. Unlike open clusters with relatively few stars, which do not have a define shape. Globular clusters also formed near the time when our galaxy formed.
They were first discovered by Charles Messier in the 1700's while he was searching for comets. Many are found in the Messier catalogue along with open clusters and nebulae. All of the globular clusters are outside the plane of the Milky Way. They have also been found orbiting other galaxies similar to ours.
M13 (Messier Catalog, 13th item), also known as the Hercules Cluster (in the constellation Hercules)
Image Credit:
Martin Pugh
M72
Image Credit:
Globular clusters were also important in determining the size of the Milky Way. Back in the 1920's, there was a Great Debate on how big the Milky Way was The determining factor was to look at how the globular clusters orbited. the center of the galaxy, and astronomers could use that to figure out out that our galaxy was about 100,000 light-years in size.
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