Categories: Astronomy

Astronomy Jargon 101: Globular Clusters

The NASA/ESA Hubble Space Telescope offers this delightful view of the crowded stellar encampment called Messier 68, a spherical, star-filled region of space known as a globular cluster. Mutual gravitational attraction amongst a cluster’s hundreds of thousands or even millions of stars keeps stellar members in check, allowing globular clusters to hang together for many billions of years. Astronomers can measure the ages of globular clusters by looking at the light of their constituent stars. The chemical elements leave signatures in this light, and the starlight reveals that globular clusters' stars typically contain fewer heavy elements, such as carbon, oxygen and iron, than stars like the Sun. Since successive generations of stars gradually create these elements through nuclear fusion, stars having fewer of them are relics of earlier epochs in the Universe. Indeed, the stars in globular clusters rank among the oldest on record, dating back more than 10 billion years. More than 150 of these objects surround our Milky Way galaxy. On a galactic scale, globular clusters are indeed not all that big. In Messier 68's case, its constituent stars span a volume of space with a diameter of little more than a hundred light-years. The disc of the Milky Way, on the other hand, extends over some 100 000 light-years or more. Messier 68 is located about 33 000 light-years from Earth in the constellation Hydra (The Female Water Snake). French astronomer Charles Messier notched the object as the sixty-eighth entry in his famous catalogue in 1780. Hubble added Messier 68 to its own impressive list of cosmic targets in this image using the Wide Field Camera of Hubble’s Advanced Camera for Surveys. The image, which combines visible and infrared light, has a field of view of approximately 3.4 by 3.4 arcminutes. Credit: Hubble/NASA/ESA

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll feel a little old and red with today’s topic: globular clusters!

In globular clusters you will find a lot of older, redder stars packed closely together. In many ways, they’re the retirement homes for the galaxy.

Globular clusters are relatively small, with most no more than a couple dozen parsecs across. But each one contains hundreds of thousands, and sometimes millions, of stars. That puts the average distance between stars at about 1 light-year, but in their cores the stars pack together over a thousand times more tightly than in our own neighborhood.

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The clusters are old. Indeed, they are some of the oldest objects in the universe. They have not seen new star formation for at least 8 billion years.

The Milky Way hosts about 150 known globular clusters, although astronomers suspect that the true number is closer to 200. Each galaxy has its own retinue of clusters, which larger galaxies hosting more and smaller galaxies hosting fewer. You can see some of them with the naked eye, like Omega Centauri, which is a massive cluster weighing 4 million solar masses sitting 17,000 light-years away from us.

Astronomers divide globular clusters into two rough categories. One population is relatively younger (around 8-10 billion years old), tend to have more heavier elements, and tend to be closer to the central core of the galaxy. The other population is older (10-12 billion years old), have more pristine elements, and are scattered around randomly.

While there is no universally accepted theory for the formation of globular clusters, astronomers suspect that the younger population formed alongside the galaxy, while the older clusters formed separated and were captured later.

Most interestingly, globular clusters lack any significant amounts of dark matter. Dark matter makes up most of the mass of normal galaxies, so this is a clue that clusters had a different history. Either clusters tried to grow into galaxies but failed, or they started out as smaller star clusters that got out of control.

Either way, astronomers are very interested in the nature of globular clusters, since they can tell us about the formation and evolution of galaxies.