Star Clusters
and Their Evolution
Are clusters common? Most stars form in clusters from fragments of large clouds. (Many wander away from their siblings later in life.)
Why are clusters interesting? All stars have (approximately) the same age.

The Quintuplet open cluster
  • Examples: The Pleiades (7 Sisters), 440 ly away in Taurus; M44, the Beehive; NGC 4755, the Jewel Box.
  • Interesting cases: The Double Cluster in Perseus.

  Open Clusters

  • What are they? Small systems (a few pc across), with hundreds or thousands of stars; Most crowded groups of blue stars in the galactic disk are open clusters, although some star patterns (asterisms) fool us into thinking they are clusters [like NGC 6997].
  • Surrounding nebulae: Many open clusters have them, because they are young (like NGC 6188, or NGC 2244).
  • How do we know their age? Look at their HR diagrams, and remember that more massive stars burn out faster; For a rough estimate, or for very distant clusters, just look at the color.
  • What ages do we find? For the Pleiades, around 60 Myr; besides, they have heavy elements...

Globular Clusters

  • What are they? Spherical groups of hundreds of thousands of stars, at most about 50 pc across!
  • Where are they? Distributed in a sphere around the disk of our galaxy; About 200 are known in our galaxy (there may be more near the galactic center); We also see them in other galaxies (of all types; a famous one is G1 in M31; the LMC has 13 known ones, the Sombrero Galaxy M104 about 2000 of them), and a few are intergalactic.
  • Examples: The largest ones are Omega Centauri (M10 or NGC5139) and 47 Tucanae; M22, spanning an area of the sky as large as the Moon.
  • How do we know their age? From their color or HR diagram.
  • Main Sequence cutoff/turnoff: Compare different ages (for example, B stars: 10 Myr; G stars: 10 billion yr).
  • And what ages do we find? In our galaxy at least 10 billion yr, they were formed very early on; In the LMC they are either about 13 Gyr old or about 3Gyr old! Some galaxies (such as Stephan's Quintet) have young, blue globular clusters.
  • What do they tell us? Their age gives constraints on the age of the universe; Their age and distribution in a galaxy may show what happened to a galaxy since the time it formed (galaxy collisions, mergers, etc).

The Omega Centauri
globular cluster, the brightest in our sky and the largest known one in our galaxy [so big it may actually be a stripped-down dwarf galaxy!].

  Interesting Issues

  • Evolution of clusters: Star clusters can lose stars (by ejecting them after close encounters) or gain stars; How does this work exactly? How common is it?
  • Relationships: Can globular clusters (like NGC 6397) trigger the formation of open clusters (like NGC 6231)?
  • Formation of globular clusters: Why and how did they form? The first ones might have formed nearly simultaneously, and be remnants of dwarf galaxies, while later ones are results of galaxy collisions and mergers; In our galaxy there might have been many more in the past.
  • Centers of globular clusters: How many of them have a black hole at their center? We now have possible evidence that M15 and G1 (in LMC) do.
  • Stellar interactions in globular clusters: Densities are so high that stars can go through several binary relationships or groups of more stars in their lifetimes, and often merge into more massive ones; This explains the existence of "blue stragglers" in old clusters.
  • Are there other types of clusters? Not in our galaxy and in many others, but some do have a new type of faint extended cluster, for example NGC 1023.

page by luca bombelli <bombelli at>, modified 29 sep 2012