Globular Clusters
Overview
Globular star clusters are gravitationally bound groups of roughly 100,000 to a few million stars. They are compact—stars near their centers are within a light year of one another—roughly spherical clusters that range from 30 to 100 light years in diameter.
Globular clusters are marked with yellow indicators. In reality, the cluster is much smaller than these markers, so we need a way to see them from the night sky or from outside the galaxy. Here, we are looking toward the center of the Milky Way Galaxy with Scorpius and Sagittarius in view. Larger markers indicate closer clusters.
Globular clusters are not thought to be fertile environments for planet formation. However, imagine being on a planet located inside one of these clusters—its sky would be illuminated by an abundance of stars brighter than the Sun.
Messier 2, a globular cluster in the constellation Aquarius. Credit: NASA, ESA, STScI, and A. Sarajedini (University of Florida)
Understanding Our Place in the Galaxy
Globular clusters were instrumental in our understanding of the size and nature of the Galaxy and universe. The story began in 1912, when Henrietta Leavitt (1868–1921), a “computer” for astronomers at the Harvard College Observatory, though an accomplished astronomer in her own right, discovered a relationship between the period of Cepheid variable stars and their intrinsic luminosity, allowing her to determine their distance.
In 1918, the astronomer Harlow Shapley (1885–1972) noted that the open clusters were mainly in the plane of the Milky Way, while more than half the globular clusters were in or near the constellation Sagittarius. He deduced the center of our star system must be far off in the direction of Sagittarius.
A view from outside the Milky Way Galaxy showing the distribution of globular clusters in yellow. The clusters are distributed around the center of the Milky Way Galaxy, and revolve around the Galaxy’s center in elliptical orbits.
Star Cluster or Dwarf Galaxy?
The line between globular cluster and dwarf galaxy is not always clear. Dwarf galaxies orbit the Milky Way in a similar fashion—long elliptical orbits around the Milky Way’s center. Many dwarf galaxies have been stripped of gas and dust via collisions with the Milky Way. So, where does a globular cluster end and a dwarf galaxy begin? Astronomers continue to seek the answer to that question.
For more on dwarf galaxies, see Local Group.
Looking back at the Milky Way from extragalactic space. We see the Milky Way Galaxy and the yellow points indicating the location of the globular clusters, huddled around the Galaxy. Co-mingling with the globular clusters are the dwarf galaxies of the Local Group in aqua. These dwarf galaxies also revolve around the Milky Way.
Cosmically Old
Globular clusters are among the oldest objects in the Galaxy. They were around when the Galaxy formed 13 billion years ago, perhaps even before the Galactic disk evolved into the shape it is today. Some of the oldest stars in the entire Galaxy are found in these clusters.
Completeness
The globular clusters form one of the most complete data sets in the atlas. These represent almost all the clusters we see in our Milky Way Galaxy. Our blind spot results from the Milky Way itself—that band of light across the night sky (and the gas and dust within it) prohibits us from seeing what lies within it and beyond it.
Profiles
Dossier
Census: |
161 clusters |
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Asset File: |
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OpenSpace Version: |
3 |
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Prepared by: |
Brian Abbott, Zack Reeves (AMNH) |
Source Version: |
4.07 |
License: |