Globular Star Clusters
Globular Star Clusters
| Group Name | gc |
| Reference | Catalog of Parameters for Milky Way Globular Clusters (Harris 1997) |
| Prepared by | Brian Abbott (AMNH/Hayden) |
| Labels | Yes |
| Files | gc.speck, gc.label |
| Dependencies | none |
| Census | 145 clusters and labels |
Globular star clusters are gravitationally bound groups of 100,000 to 1 million stars. They are compact, spherical “balls” of stars with very high stellar densities in their centers (stars near their center are spaced a few light-months from each other). These clusters are typically 30 to 100 light-years in diameter. If Earth were located inside one of these clusters, our sky would be lit by thousands of stars brighter than the brightest stars we currently see.
Size of Our Star System
Globular clusters were paramount to our understanding of the structure of our Galaxy. The story began in 1912, when Henrietta Leavitt (1868-1921), a “computer” for astronomers at the Harvard College Observatory, discovered a relationship between the period of Cepheid variable stars and their intrinsic luminosity (absolute magnitude). She found that the longer the period of variability, the more luminous the star. By observing the period of variation, Leavitt then knew the star's absolute magnitude, or intrinsic luminosity, and with the observed apparent brightness, she was able to find the distance to these stars.
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 that these clusters must be distributed around the center of our star system, the Milky Way, and that we were viewing that point from afar. If he found the distances to these clusters, he would find the distance to the center of our Galaxy, overthrowing the long-held belief that Earth was at the center of the Universe.
Shapley observed the presence of RR Lyrae stars in these clusters. RR Lyrae stars vary in brightness over periods of less than a day, so they are easy to observe provided they are bright enough. While the intrinsic brightness of Cepheids was known, the period-luminosity relationship had not yet been established for RR Lyrae stars. Shapley was able to calibrate these variable stars to the intrinsic brightness scale and was then able to find the distances to the clusters. Jan Oort, a Dutch astronomer, confirmed this result by studying the motions of stars, showing that they are all in orbit about a distant center.
We know today that Shapley overestimated their distances by about a factor of 3, making the Galaxy about 300,000 light-years in diameter. Shapley was a proponent of the Milky Way Universe cosmology, believing that all that we see is part of our Galaxy and that our Galaxy is the entire Universe. In April 1920, Shapley and the astronomer H. D. Curtis met at the National Academy of Sciences to debate this cosmology in what is now called The Great Debate. This question would be answered within five years by Edwin Hubble (1889-1953) when, in 1923, he discovered Cepheid variables in the Andromeda Nebula. He found the distance to the Andromeda and M33 to be about 300 kiloparsecs (just less than 1 million light-years). An underestimation, but these results set the scale. Now the Andromeda Nebula could be considered a galaxy in its own right and the Universe was now known to be far larger than the Milky Way.
| Data Variables for Globular Star Clusters | |||
| Number | Name | Description | Units |
| 0 | metal | Log of the metallicity | suns |
| 1 | rcore | Core radius | arcmin |
| 2 | distly | Distance | light-years |
Exploring the Catalog
The globular clusters form one of the most complete data sets in the Atlas. Data for the 145 clusters in the catalog were compiled by William Harris (McMaster University, Canada) and represent almost all the clusters in our Galaxy (a few on the opposite side of Galactic center may be invisible to us).
| Selection Expressions for the Globular Clusters | ||
| Alias | Partiview Command | Description |
| oldgc | thresh metal -2.0 -1.5 | Show the metal-deficient, older clusters |
| younggc | thresh metal -1.0 1.0 | Display the metal-rich, younger clusters |
In Partiview, it's easy to see what Shapley observed 80 years ago: that most of the clusters are located in Sagittarius near the Galactic center as seen from Earth. The closer the marker, the closer the cluster. One of the nearest clusters to us is Messier 4, in the constellation Scorpius. It is about 7,200 light-years away. Some of the farthest clusters lie at the edge of the Galactic halo and perhaps even beyond it [more on this in the “The Galactic Outskirts” tutorial].
Cluster Ages
The age of a globular cluster is directly related to something called the metallicity. (In astronomy, all atomic elements heavier than helium are called metals, and the metallicity is the fractional abundance of metals in an object.) Clusters with stellar populations that have higher metallicities are typically found near the Galactic center, while clusters that are deficient in metals are found in the Galactic halo. Use the following Partiview commands to invoke preset selection expressions to see these data subsets. (Remember see all returns all data to view.)
see oldgc
see younggc
Having formed about 10 billion to 12 billion years ago, globular clusters are among the oldest objects in the Galaxy. They were around when the Galaxy formed, perhaps even before the disk evolved to the shape it is today. Some of the oldest stars in the entire Galaxy are found in these clusters.
The metal-rich clusters are several billion years younger than the metal-poor clusters. This may be a reflection of galaxy interaction and tidal captures from other small galaxies. In fact, astronomers are now studying whether some of the globular clusters previously thought to belong to the Milky Way are bound to other small satellite galaxies.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott