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).

  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.