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Astrophysics 102: Extragalactic Globular Clusters

February 19, 2011

Christine over at Cosmic Rays has a very interesting post on new research into the globular cluster system of one of our nearest neighbouring galaxies, M31, the Andromeda Galaxy:

M31 (Image: Tony Hallas)

In our own Milky Way, globular clusters are found in a halo surrounding our galaxy:

Globular Clusters in our Galaxy (Image: Atlas of the Universe)

And unsurprisingly, as Catherine reports, the same situation exists in other galaxies, although the numbers of globular clusters per galaxy varies widely (and appears to be a function of the physical laws of the universe, galaxy mass and galactic evolution).

M13, a galactic Globular Cluster (Image: Yuugi Kitahara)

Leaving aside their obvious beauty and their impressive physical properties, unlike a lot of the objects I’ve been blogging about, globular clusters can be seen in instruments as small as binoculars, and even in small telescopes they put on a phenomenal show. Furthermore, some of M31’s globular clusters can also (just) be viewed in amateur telescopes.

But did you know that some of our own galaxy’s globular clusters are actually invaders from another galaxy? Studies of cluster kinematics & metallicity have found that the galactic globular cluster M54 actually belongs to the Sagittarius dwarf galaxy:

The Sagittarius dwarf galaxy (Image: NASA)

The Sagittarius dwarf galaxy is one of the closest (and faintest – it wasn’t discovered until 1994!) satellite galaxies of our own, and M54 is thought to lie in the nucleus of said galaxy; there are also 3 other “galactic” globular clusters associated with the Sagittarius dwarf galaxy(Layden & Sarajedini 1997, ADS/arXiv); indeed it has also been suggested (e.g. Caretta et al. 2010, ADS/arXiv) that certain other galactic globulars are the “stripped” remains of former galaxies devoured by the Milky Way in the past, although the jury is still out on their claims, to say the least.

But in the meantime, in a few months time (if you’re in the Northern Hemisphere), if you have binoculars or a small telescope, and a good southern horizon, take a look out for M54, sitting just inside the handle of the “teapot” of Sagittarius and remember that the stars therein originated in another galaxy altogether.

The location of M54 within Sagittarius (Image: Torsten Bronger)


Layden, A., & Sarajedini, A. (1997). The Globular Cluster M54 and the Star Formation History of the Sagittarius Dwarf Galaxy The Astrophysical Journal, 486 (2) DOI: 10.1086/310848

Carretta, E., Bragaglia, A., Gratton, R., Lucatello, S., Bellazzini, M., Catanzaro, G., Leone, F., Momany, Y., Piotto, G., & D’Orazi, V. (2010). Detailed abundances of a large sample of giant stars in M 54 and in the Sagittarius nucleus Astronomy and Astrophysics, 520 DOI: 10.1051/0004-6361/201014924

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