Globular cluster
From South Dublin Astronomical Society
| Globular Cluster M15 |
|---|
 Image by Dave Grennan
|
Globular clusters, for example M13 in Hercules are unmistakable in telescopes as a 'ball' of stars, in binoculars individular stars are not resolveable, and they appear more like a 'fuzzy' star.
Contents |
Cluster Composition
Globular clusters are tightly knit groups of stars all gravitationally bound to each other. These groups may consist of anything from ten thousand to millions of stars and are usually all of approximately of the same age. They can vary in size from tens of light years across to hundreds of light years in diameter.
If we assume that all the stars in a cluster are of about equal distance from us we can plot them on a HR diagram using apparent magnitude instead of absolute Magnitude to get a relative HR diagram. Globular clusters have much fewer stars on the main sequence and also show horizontal branches. The globular clusters have a clear point where their stars break away from the main sequence to become red giants, this is called the Main Sequence Turnoff Point (MSTP). Since the position on the main sequence is dependent on Mass, we can calculate the Mass of stars that are currently leaving the main sequence. This turns out to be about 0.8 solar masse. Current stellar evolution models suggest that it take about 12billion years for a 0.8 solar mass star to leave the main sequence so the globular clusters must have formed very early in the universes history.
Some stars appear to linger on the main sequence. These are known as 'Blue stragglers'. In order for these stars to linger on the main sequence they must be getting additional mass. Current studies suggest that these stars are in binary systems and the other stars in the binary have seeded mass to the blue stragglers.
The horizontal branch stars in the HR diagrams of globular clusters represents the stage in a stars evolution after it experiences helium flash. These are among the larger stars in the cluster and have begun helium burning in their cores.
The most massive, and hence most evolved stars in globular clusters are on the asymptotic branch. These stars have burned all the helium in their cores and are on the final phase to becoming planetary nebulae, though few planetary nebulae are seen in globular clusters.
With the possible exception of blue stragglers, none of these star types are seen in open clusters as open clusters a simply too young to have these kinds of stars.
Stellar Compositions
Using spectroscopy we can examine the properties of the stars within the clusters. The globular clusters show strong absorption lines for Hydrogen and Helium, but very little traces of heavier elements are present. These are classed as Population II stars. The lack of metals in the globular clusters also confirms their age as very anchent stars which must have formed before there was much metal around, ie before supernova activity had started among the early massive stars.
Location, distribution, and size
Globular clusters occur thorough the sky, with the greatest concentration towards the Sagittarius region of the sky containing the galactic bulge (of the 150 known clusters in our Galaxy, 91.3% are located in the hemisphere of Sagittarius). Harlow Shapley used this observation in 1917 to show that the solar system lies “about half way from the centre to the edge of the galaxy”.
Measurements of the radial velocities of globular clusters tell us that many move in highly elliptic orbits about the centre of the Galaxy. Their orbits have been shown to form a roughly spherical halo around the Galaxy, which can extend to 100,000 light years, significantly bigger than the diameter of the galactic plane. Their orbits also show that they do not all follow the same rotation as the galactic plane. Some orbits also imply that the clusters must pass through the galactic plane on their orbits. Within the globular clusters the stars orbit about a common centre of mass, this can allow some stars to reach high enough velocities to escape from the cluster which causes the cluster to shrink and as a consequence stars get more kinetic energy and more are able to escape. This process is known as core collapse. Core collapse and tidal forces acting on globular clusters when they move through the plane of the galaxy are believed to be responsible for a large percentage of stars in the galactic halo.
Summary
Globular clusters formed early in the universe. There is no evidence to suggest that globular clusters are still forming in our galaxy so we must assume that there were special circumstances early in the Milky Ways history to facilitate the creation of these clusters. The details of the origins of globular clusters remains a mystery, though they have been detected in other galaxies, sometimes as in the case of M31 in great detail, so they are not unique to the milky way. The future of globular clusters is hard to predict as they continually cause surprises for astronomers, for example a gas giant planet has recently been discovered in the globular cluster M4. It seems likely however that over the coming billions of years the globular clusters will continue to erode though core collapse and passing through the galactic plane and it is unlikely that any new clusters will be formed.