Ancient Open Clusters

[Eroica]

Globular clusters are usually very old, whereas open clusters are generally quite young. Well, that's as you would expect. In globular clusters the stars are quite densely packed together, so their gravity holds them together for billions of years. In open clusters, on the other hand, the stars are further apart and so gradually drift apart over the eons. Our own Sun was probably formed in a stellar nursery like M42 (Great Orion Nebula), but it has long since lost contact with its sister stars.

So how is it that M67 in Cancer, NGC 188 in Cepheus and NGC 6791 in Lyra are still around after about 4, 5 and 8 billion years respectively?

(Wouldn't it be cool if the Sun was still part of a cluster? Imagine what the night sky would look like!)

[ngc3314]

Globular clusters are usually very old, whereas open clusters are generally quite young. Well, that's as you would expect. In globular clusters the stars are quite densely packed together, so their gravity holds them together for billions of years. In open clusters, on the other hand, the stars are further apart and so gradually drift apart over the eons. Our own Sun was probably formed in a stellar nursery like M42 (Great Orion Nebula), but it has long since lost contact with its sister stars.

So how is it that M67 in Cancer, NGC 188 in Cepheus and NGC 6791 in Lyra are still around after about 4, 5 and 8 billion years respectively?

(Wouldn't it be cool if the Sun was still part of a cluster? Imagine what the night sky would look like!)

The very oldest open clusters are almost certainly the few lucky survivors of vast numbers that have been disrupted, still around through some combination of initial richness, orbits tilted a little more than usual to the galactic plane, and sheer happenstance of not having encountered some disruptive mass like a giant molecular cloud. Even among globular clusters, those whose orbits never come too close to the galactic center include less dense and populous clusters, suggesting that some of the globulars formed over time haven't survived to be recognizable as such by now.

[Romanus]

From what little reading I've done on the subject, I've learned that open clusters can last longer than usual in several instances:

1.) If they are very massive.
2.) If they are farther from the galactic center.
3.) If they are far from the galactic plane.

Messier 67 satisfies the last two requirements; it's farther from the galactic center, and far from the galactic plane, so it rarely encounters the large molecular clouds that frequently tidally disperse open clusters.

[Crimson]

Here's a link that backs up Romanus's point: M67: The Ultimate Survivor (http://KenCroswell.com/M67.html).

[Eroica]

Thanks for the link.


As M67's stars orbit the cluster's center, they exchange energy with one another in a way that equalizes each star's energy of movement, or kinetic energy. Because this energy depends on a star's mass and velocity, the more massive stars slow down and sink toward M67's center. The less massive ones can speed up so much they escape from the cluster.
Interesting. I never knew that!

[Maddad]

It's why we have such a dense concentration of massive objects in the galatic core. Now the other day some source was saying there's 10,000 stellar mass black holes there.

[Crimson]

Thanks for the link.


As M67's stars orbit the cluster's center, they exchange energy with one another in a way that equalizes each star's energy of movement, or kinetic energy. Because this energy depends on a star's mass and velocity, the more massive stars slow down and sink toward M67's center. The less massive ones can speed up so much they escape from the cluster.
Interesting. I never knew that!

You're welcome. The exchange of energy is called "equipartition of energy" and the escape of stars from the cluster is called "evaporation."