# Is every star formed in an open star cluster?

As far as I understand, an open cluster is formed from a single molecular cloud, with each star in the cluster having roughly similar age and properties.

Our Sun is not part of any star cluster, but it could have been formed in one. Is every star formed in an open star cluster, or can they develop independently?

• Hi, can you let us know if you know any of the theory about how stars form? Basically, why would you expect that star formation depends on the simultaneous formation of other stars several to hundreds of light-years distant? – Carl Witthoft Sep 11 at 14:58
• As far as I understand an open cluster is formed from the same molecular cloud, with each star in it having roughly similar age and properties. – Radium Sep 11 at 15:29
• Yes normally a gas cluster should lead to several stars. The question should rather be if all stars are originally gravitationally bound to a cluster or not. The first seems the case, as for the original gas cluster is surely a collapsing one. The second night be possible as for the dynamics of the system could somehow eject the forming or new born star. Let us wait for an answer to this. – Alchimista Sep 12 at 10:03
• Have you heard of Bok globules? They're smaller than GMCs (giant molecular clouds), and as Wikipedia says, they most commonly result in the formation of double- or multiple-star systems, not open clusters. I suspect that the isolated formation of single stars is very rare, but I'm interested in what our professional astrophysicists have to say on this topic. – PM 2Ring Sep 14 at 6:19

The question is still an open matter of current research.

It seems to be true that the vast majority of star formation takes place in groups and aggregates of various sizes - from a few stars to millions of stars in "super" star clusters. This is likely because collapsing clouds of gas are normally much more massive than a star and the collapse process reduces the Jeans mass and renders the cloud unstable to fragmentation into smaller cloud cores.

However, it seems that the vast majority ($$>90$$%) of star clusters/associations are either born in a gravitationally unbound state or become gravitationally unbound within a few million years. The gravitationally bound open clusters, who's exemplars include the Pleiades, are comparatively rare survivors (or at least partly survived) of this "infant mortality". So, in that sense we can say no, most stars are not born in open clusters, but the likelihood is that most were born in aggregates with near neighbours that went their separate ways shortly after birth.

Current thinking is that our Sun was born in a cluster of some ten thousand stars (Adams 2010). This is an argument based on the shaping of the early solar system by dynamical encounters and the early presence of radioactive nuclei that were likely injected by the explosion of a very nearby massive star (probably a cluster sibling).