globular cluster

An astronomical term that refers to a roughly spherical cluster of gravitationally bound stars. A globular cluster has anything from tens of thousands to a million individual stars and can be ten to hundreds of light years across.

The Milky Way has around 200 globular clusters which orbit the Galactic Core with a highly elliptical path that is not confined to the Galactic Disk. Stars in globular clusters tend to have low concentrations of heavy elements, implying that the stars in globular clusters are older than the stars in the Galactic Disk.

It was from the observation of globular clusters that Harlow Shapely was able to determine the Sun's position in the Milky Way.

A list of local globular clusters (local meaning within our galaxy :) is at:

  http://physun.physics.mcmaster.ca/~harris/mwgc.dat

The Large and Small Magellanic Clouds are not globular clusters; they are irregular galaxies, and in fact they have globular clusters of their own. Similarly, the Andromeda Galaxy is not our nearest galactic neighbor. That distinction goes to the Sagittarius Dwarf Galaxy, SagDEG, which is currently being devoured by our own galaxy, in a process known as Galactic Cannibalism.

Globular clusters are important to the study of cosmology because they are the oldest objects that we know how to measure the age of. As such, they place a lower bound on the age of the universe.

Globular clusters are roughly spherical collections of stars bound together by their common gravity. They are found throughout the Milky Way and generally around all galaxies. Their distribution is roughly spherical throughout the Galaxy.

Distribution of globular star clusters in the Milky Way

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The Milky Way's globular clusters are very interesting astronomical objects, and have been observed extensively for a number of reasons. Firstly they are easily discernable from the general star field because of their shape. They are also fairly bright objects making many of them visible with small telescopes. Charles Messier catalogued 29 of them in his collection of (stupid fuzzy)¹ objects that are not comets. William Herscel was able to resolve individual stars in the clusters for the first time, thereby giving birth to the idea that many of the diffuse nebulas in the sky might actually be dense clusters of stars. Later Harlow Shapley used the distribution of globular clusters on the sky, to discover the Sun's position in the Milky Way. This was possible because of the fact that star clusters are distributed spherically around the Galactic center. This makes the on sky distribution as seen from earth highly assymmetric and one can use this to discern rather accurately the direction of the Milky Way center. Also by estimating the distances to some of the clusters it is possible to find the approximate distance from the sun to the center. (Because of varius misassumptions the distance found at that time was very much smaller that the distance accepted today).

Today the known globular clusters number 150. If you add a few for clusters that are too faint to discover or are cloaked by the immense dust clouds in the disc of the Milky Way you might be able to bring that number as high as 180-200. The stars in globular clusters are almost spherically distributed, only around 5% of all known globular clusters have an on sky long to short axis ratio of under 0.8. The elongation is caused by a small amount of ordered circular motion in the otherwise random motion of the cluster stars. Though there are quite a few very bright globular clusters, they really come in brightness ranges from the extremely faint and loosely populated, to the very bright and highly dense clusters. Another remarkable feature of globular clusters are that they are almost entirely devoid of gas. This is very unlike the galactic disc and core, open clusters and the small satelite galaxies of the Milky Way which has a virtual abundance of gas and dust.

All the stars in a particular globular cluster were formed in almost the exact same epoch, and they are all very old. This can be seen by plotting the individual stars in a so called color-magnitude diagram (also known as a Hertzsprung-Russell diagram however color-magnitude is more frequently used in present litterature). In such a diagram a regular collection of stars from the neighbourhood of our sun would be seen as a main sequence of stars which stretch from the upper left to the lower right of the diagram, a branch of stars starting roughly in the middle of the main sequence and stretching out to upper right, a big lump of stars in the middle of this branch (stretching out to the left a bit) and a few white dwarf stars in the bottom left half. This configuration is typical for a distribution of stars with a wide range of ages and metalicities (to an astronomer metal means anything other than hydrogen or helium).

A color-magnitude diagram for a globular cluster looks very different however. Here the main sequence starts down in the lower right like above, but stops quite abrubtly at the point where the branch which stretches upwards to the right starts. The point where the the main sequence stops is called the main sequence turnoff (MSTO) or just turnoff. Here the curve turns up and to the right in a short tight sequence called the subgiant branch (SGB). At the right end of this sequence the curve bends up like in the regular CM diagram, the rest of the stretch up towards the upper right is the red giant branch. To the left of the RGB there will be a slim horisontal patch called (you guessed it!) the horisontal branch (HB). Now the reason that this form of the CM diagram clearly show to astronomers that all the stars are equally old, is that the theories of star evolution quite accurately predict that if you have a collection of stars with a wide range of masses but who all formed at the same time, then after about 10 billion years the distribution will turn out exactly as seen in the CM diagrams. Actually the form of the CM diagram can be so accurately predicted that it is in fact possible to pinpoint the time where the stars were born within a couple of billion years. This and various simmilar techniques of CM analysis all point towards that almost all clusters formed some 13 to 15 billion years ago, thereby making them the oldest known objects in the universe.

Source: Galactic astronomy by James Binney and Michael Merrifield.

1) The story goes that Messier was always primarily interested in studying comets, and in looking for those all too often he found these annoying (well he thought so) fuzzy patches of sky which was very similar to comets except they never moved around. He started the catalogue so he that that he would not waste time looking at objects which were known from previous observations not to be comets.