# Sun's formation as “part of a star-forming-cluster…”?

I was wondering, do we in fact know if our Sun in particular was created as part of a star-forming-cluster, or, was it more of a solo creation?

(Or, are all stars created as part of star-forming-clusters, is that the norm and how all stars come to be?)

If yes, indeed are we aware of which stars around us, are our sibling star? (So, there's a list somewhere "stars known to likely be sibling to our Sun from a certain star-forming-cluster".) Or have they dispersed over the 4 billion years and it's impossible to know, or is it just the case that "all the stars anywhere nearby" are/were indeed siblings from "our" star-forming-cluster?

Further, I'm having trouble grasping the order of magnitude of star-forming-clusters. Consider say "all the stars in our arm of the galaxy": Consider the question "in how many star-forming-clusters did all those stars originate?" Is the answer like "3", "a few hundred" or "100 million" ??

Perhaps most simply: if sol formed as part of a star-forming-cluster, what's the order of magnitude of how many stars formed with it?

Fortunately, there is an extensive and highly respected Annual Reviews of Astronomy and Astrophysics article on this very question - Adams (2010), https://arxiv.org/abs/1001.5444

At the moment it is debated whether most stars form in clusters. They may do, but there is a lot of evidence for hierarchical structure formation that results in lots of stars forming in relative isolation.

Regardless, there are a number of strands of evidence that can be used to suggest that the Sun formed in a cluster of somewhere between a thousand and ten thousand siblings - a bit like the Orion Nebula cluster.

Adams argues for this on the basis of: (1) We have a relatively well-behaved solar system inside the orbit of Neptune. This argues the Sun did not (or rather was not likely to) form in a massive and dense cluster where it suffered lots of very close encounters. On the other hand, the wacky orbits of some more distant objects like Sedna do suggest some early close encounter with another star was required. (2) If the Sun was born in a very massive cluster, it would likely have been born close to some very massive stars. The UV emission from such stars could photoevaporate a circumsolar disk before it formed planets. (3) In tension with this is the probability that the protosolar disk was enriched with radioactive supernova products. This requires the Sun to be born close to moderately massive stars (that would explode on a short timescale), which are only found in moderately massive clusters.

Adams puts these constraints together to estimate a probability that the Sun was born in a cluster of a particular size. Figure 7 of the review shows this to be reasonably sharply peaked between a thousand and ten thousand.

An order of magnitude uncertainty is at least warranted and some of the assumptions and calculations are open to criticism. But it is a starting point for any further research into the matter.

• i would like to hear more about this idea of stars forming in isolation. That must be a new idea, because most of the evidence I know about supports the statement in the abstract of that very review article you cited: "Since most stars form within groups and clusters, the question becomes one of determining the nature of the birth aggregate of the Sun." – Ken G Sep 30 '16 at 0:54
• @KenG The rival picture is more hierarchical with a continuum of birth environments. This paper has been very influential in the field: arxiv.org/abs/1009.1150 – Rob Jeffries Sep 30 '16 at 6:17
• That is an interesting article, thank you. I find this particularly interesting: "only a low fraction (< 26%) of stars are formed in dense environments where their formation/evolution (along with their circumstellar disks and/or planets) may be affected by the close proximity of their low-mass neighbours." So they are not saying most stars don't form in clusters, they are saying they are not affected by proximity effects-- which is not at all the same thing. – Ken G Sep 30 '16 at 13:09
• Thanks for yet another superlative answer on this question! That PDF will keep me reading for awhile. I am wondering, let's say the "Adams picture" is correct the Sun was created in a cluster of 5000 stars. Regarding the other 4999 stars, for me to understand the general picture is it the case that: (i) we just have utterly no clue where they would be, (ii) they would be randomly distributed in the whole galaxy, (iii) we have no clue which they are, but they must all be in our same arm, (iv) we have no clue which they are, but they must all be kicking around quite close to us. – Fattie Sep 30 '16 at 13:10
• @JoeBlow How far stars are dispersed and radially migrate is very much an active research topic. Velocity dispersion certainly increases with age. Most likely, the siblings of the Sun are spread in an annulus right around the Milky Way, of uncertain width, but it could be as much as a few kpc. The siblings would have a very close chemical match to the Sun, which allows us to find candidate members of the "solar cluster". Most of the siblings would have lower masses than the Sun. Siblings of more than a few solar masses have long since evolved into white dwarfs (or exploded). – Rob Jeffries Sep 30 '16 at 13:47

Stars typically do form in clusters, because to get the gravitational instability to initiate, you need a very large mass-- much larger than the mass of a single star. A typical mass for the gravitational instability is about a million solar masses, though it can vary by an order of magnitude depending on the situation. But you don't get a single star of that mass because after the instability starts (on the scale of a "giant molecular cloud"), it fragments into smaller bits, which are the star clusters, which fragment into further bits, which are the stars. There are two basic types of clusters, "open" and "globular", where the open clusters are not gravitationally bound so the stars wander away from each other after forming (which is presumably what happened to the Sun). The globular clusters stay bound for very long times and can even wander away from the plane of the spiral galaxy and still be together as clusters, but obviously the Sun is not in one of those and is not old enough to have completely left the plane of the galaxy (though it does oscillate up and down through the plane). A typical open cluster might be a thousand stars, so it seems likely the Sun might have about a thousand "sister" stars spread around the plane by now, but we'll never know which ones those are since the age cannot be determined precisely enough.

• It's not entirely true that stars in open clusters aren't bound. They can survive for timescales on the order of $\sim10^8$ years, which isn't insignificant compared to the lifespan of many massive stars. But their lifespans are certainly much smaller than those of globular clusters. – HDE 226868 Sep 29 '16 at 17:28
• There is no such thing as a typical cluster, but by chance you may have the answer about right. – Rob Jeffries Sep 29 '16 at 19:16
• Also not true that "we'll never know". Efforts are underway - search for "chemical tagging" and "solar twins". – Rob Jeffries Sep 29 '16 at 19:17
• "There's no such thing as a typical cluster." I'm afraid I find that statement rather meaningless. And I am quite correct, lucky or otherwise, that we'll never know what stars formed with our Sun, because what astronomers mean by a "solar twin" is never going to be proof that it formed with our Sun. Do some more research, perhaps starting with arxiv.org/abs/1608.03788. There you will find the term "solar twin" is used in the open cluster M67. By the way, no one is suggesting the Sun formed in M67! – Ken G Sep 29 '16 at 23:49
• On the issue of gravitational binding, it's true that when we see an open cluster, the stars are still weakly bound, so that's why it still exists. But those represent only some of the stars that form in that cluster, so the Sun could have been one that became unbound as the gas in the cluster dispersed. So it's true that open clusters should not be described as gravitationally unbound, better would be to say they are so weakly bound that stars in them can often become unbound. – Ken G Sep 30 '16 at 1:31