Lecture 6: Globular clusters
Globular clusters
• Large star clusters of roughly spherical shape
• Each cluster contains 105 to 106 stars
• Found in galactic halo and bulge (Population II)
• ~125 known but as many as 500 may exist
• Centre of distribution defines galactic centre (Shapley, 1918)
The two brightest globular clusters, 47 Tuc (left) and ω Cen (above)
The galactic distribution of globular clusters
Some well-known globular clusters
Name V Distance (pc) Diameter
47 Tuc 4.0 5100 10
ω Cen 3.6 5000 20
M3 6.4 13000 13
M5 5.9 8500 12
M13 5.9 7700 11
M22 5.1 3000 9
M5 colour-magnitude diagram
M3 colour-magnitude diagram
Colour-magnitude diagram for the globular cluster M13
HR diagrams of globular clusters
• Red giant branch, nearly vertical, to MV = –3
• Horizontal branch, mainly A stars, MV = +0.6
• Subgiant branch, mainly F and G stars, covering wide range in luminosity
• Main sequence stars: cool red dwarfs (G, K, M)
• Asymptotic giant branch: luminous cool red giants, evolving off the horizontal branch, with C or O cores and a He-burning shell
Distances to globular clusters
Distances can best be obtained by fitting the horizontal branch stars to MV = +0.6.
This absolute magnitude is known because globular HB contain a pulsating star called RR Lyrae stars. These are also found in the field near Sun, and their distances measured by a variety of methods.
Ages of globular clusters
• Ages are derived from the theory of stellar evolution, based on the predicted rate of nuclear reactions in stellar cores.
• In practice, shape of HR diagram, especially
location of the cluster turn-off point, is fitted to theoretical isochrones.
• Result: globular clusters are all 12 to 15 × 109 years old
Conclusion
• Globular clusters are among the first objects to form in the Galaxy
• They often have very low metal abundances, with M/H in range 10-1 to 10-3 of the solar value
• They formed at the time the Galaxy comprised a large mass of H and He gas that was under-going a rapid initial gravitational collapse
