- What makes a cluster different from a crowd of stars?
- A shared origin. Every star in one condensed out of the same cloud at about the same time and still travels on a common orbit, so they share an age, a distance and a chemical composition — which makes a cluster the closest thing astronomy has to a controlled experiment.
- How many stars, and how crowded?
- An open cluster holds tens to a few thousand spread over tens of light years. A globular holds a hundred thousand to a million inside a few tens of light years, so its core is ten thousand to a million times more crowded than the Sun's neighbourhood — though even there the stars average about a tenth of a light year apart.
- Why do the two kinds sit in different places?
- Open clusters lie in the disc, where gas is still collapsing into new stars. Globulars sit in a roughly spherical halo on orbits that predate the disc: they are around twelve billion years old, and nothing in the Milky Way has made a new one in a very long time.
- What are they used for?
- Dating and distance. Because every star in a cluster is the same age, the point where the brightest have started to leave the main sequence gives the cluster's age directly — and a cluster is a whole population at one distance, which is what anchors the distance scale.
- Do they last forever?
- No. An open cluster is loosely bound and the galaxy pulls it apart within a few hundred million years, scattering its stars through the disc. Globulars are dense enough to survive, but they too shed stars steadily, leaving streams along their orbits.
- Which clusters can I see without a telescope?
- The Pleiades are the obvious one — six or seven stars to most eyes, and a few dozen in binoculars. The Hyades form the V of Taurus, the Beehive is a naked-eye smudge in Cancer from a dark site, and Omega Centauri is a fuzzy star to the southern naked eye. All but the last are open clusters.
- How many stars are in a globular cluster?
- Usually a hundred thousand to a million, in a ball a few tens of light-years across. Nobody counts them individually; the total comes from the cluster's brightness and its measured motions. Hubble can pick out single stars even in the crowded centres — the counting problem is the faint end, where most of the members are too dim to tally one by one.
- Are the stars in a cluster actually close together?
- By galactic standards, extremely — but not touching. In the core of a globular the typical separation is a fraction of a light-year, against four light-years between the Sun and its nearest neighbour. A planet there would have a night sky with thousands of stars brighter than Venus, and collisions would still be rare.