Where is Betelgeuse's cluster?

[EDG]

My understanding of stellar formation was that stars tended to form in clusters. The Pleiades open cluster is less than 100 million years old and the Hyades open cluster is about 625 million years old, and both still contain a lot of stars within a volume of a few cubic lightyears.

Now I look at Betelgeuse, which is only about 10 million years old, and I can't find any mention of any cluster around it. Same goes for Antares, another supergiant that is probably about the same age as Betelgeuse.

So, are we just not able to see the cluster of stars that these supergiants are part of (which seem unlikely to me) or do really massive stars simply not form in clusters but rather form on their own? Or is there some other process going on that's very rapidly cleaning out the clusters in which they formed?

[tony873004]

That's a great question. The stars you mention are certainly young enough to still live with their parents (the cluster), even though their mass makes them old in terms of MS lifetime. But they don't. Even if they were ejected from their cluster early on, not enough time has passed for them to have significantly distanced themselves from their siblings, unless they were the demise of their birth cluster. But you would think a few siblings would remain in stable orbits. Are these stars part of a multi-star system too small to be considered a cluster? Or at least the neighborhood surrounding these stars would be denser, as the former members haven't had time yet to spread throughout the galaxy. I don't know? Perhaps there's something to your conclusion that there may be "...some other process going on" in the birth of massive stars.

[EDG]

Doing a bit more digging...

Jim Kaler says that all of Orion is one big OB Association (the OB1 Association - http://stars.astro.illinois.edu/sow/orihunter.html ), but if that's true then this 'association' is huge - the stars in it range from about 1100 ly to 1600 ly distance from us, and Betelgeuse is apparently a "runaway" member of OB1. Wiki says that Associations can span up to 1500 lightyears in diameter ( http://en.wikipedia.org/wiki/Stellar...r_associations ).

Antares is apparently part of the Sco-Cen OB Association (OB2) - http://en.wikipedia.org/wiki/Scorpiu...us_Association , and OB2 contains hundreds of stars. I guess that means that clusters like the Hyades and Pleiades are much smaller-scale examples of star formation?

[01101001]

So, are we just not able to see the cluster of stars that these supergiants are part of (which seem unlikely to me) [...]

What would you expect the collection of Betelgeuse's siblings, if any, to look like?

[StupendousMan]

Doing a bit more digging...

Jim Kaler says that all of Orion is one big OB Association (the OB1 Association - http://stars.astro.illinois.edu/sow/orihunter.html ), but if that's true then this 'association' is huge - the stars in it range from about 1100 ly to 1600 ly distance from us, and Betelgeuse is apparently a "runaway" member of OB1. Wiki says that Associations can span up to 1500 lightyears in diameter ( http://en.wikipedia.org/wiki/Stellar...r_associations ).

Antares is apparently part of the Sco-Cen OB Association (OB2) - http://en.wikipedia.org/wiki/Scorpiu...us_Association , and OB2 contains hundreds of stars. I guess that means that clusters like the Hyades and Pleiades are much smaller-scale examples of star formation?

Yes. The Hyades and Pleiades are compact groups of stars which were formed within a compact, dense blob of gas within a much larger region of gas. If you look at regions of star formation, you'll see clumps of different sizes and overdensities at many different scales.

Look at a picture of the Andromeda Galaxy. Your eye will pick out "bright clumps" within the spiral arms which are several kiloparsecs across. If you zoom in on one of these giant regions, you'll see it breaks up into smaller clumps ... and those clumps into smaller clumps.

It's very difficult to define the size of a region of star formation, as the definition depends on the resolution and sensitivity and wavelength of your detector.

[grant hutchison]

Solstation has Betelgeuse as a runaway from Orion OB1a (the closest and oldest members of the Orion OB1 association). When the heaviest member(s) of a gravitationally bound multiple system evolve off the main sequence and lose mass, the system can unbind, with the escape of some members.

Grant Hutchison

[EDG]

I guess what surprises me is how quickly the association 'evaporates'. Or are these stars actually forming very far apart in a very big molecular cloud?

and 011etc: I guess I thought that we'd be able to see lots of other stars of a similar age in close proximity to Betelgeuse and Antares, like the Pleiades. Given the young age of the supergiants I thought there wouldn't be much time for the stars to disperse after formation - but it seems that associations are not the same as the smaller clusters like the Pleiades.

[grant hutchison]

My understanding is that open clusters like the Pleiades are gravitationally bound, whereas the larger associations are not.
Another possible mode of ejection for Betelgeuse would occur as the chaotic non-hierarchical orbits of an open cluster settled into a standard hierarchical multiple with a "nested binary" structure. As I understand it, the formation of these binaries involves the gravitational interaction of three stars: two settle into stable binary orbits, and the third carries off the excess energy.

Grant Hutchison

[EDG]

My understanding is that open clusters like the Pleiades are gravitationally bound, whereas the larger associations are not.

So the open clusters are all forming in one closely-packed clump where they can interact with eachother, whereas the stars in associations form on their own?

I get the impression that the stellar nurseries of the Orion Nebula or the big "pillars of creation" (in the Eagle nebula?) are examples of places where these massive stars are forming that ultimately end up becoming associations.

Another question: does this mean that all the stars around Betelgeuse and Antares are about the same (very young, only a few million years old) age? I'd imagine that there'd be all sorts of stellar ages in the vicinity since other stars would be passing through the association.

[forrest noble]

EDG_,

My understanding of stellar formation was that stars tended to form in clusters. The Pleiades open cluster is less than 100 million years old and the Hyades open cluster is about 625 million years old, and both still contain a lot of stars within a volume of a few cubic lightyears.

Now I look at Betelgeuse, which is only about 10 million years old, and I can't find any mention of any cluster around it. Same goes for Antares, another supergiant that is probably about the same age as Betelgeuse.

So, are we just not able to see the cluster of stars that these supergiants are part of (which seem unlikely to me) or do really massive stars simply not form in clusters but rather form on their own? Or is there some other process going on that's very rapidly cleaning out the clusters in which they formed?

Although I think this is a good question I think you already answered it yourself.

"......stars tend(ed) to form in clusters"

This is a true statement but the meaning of "tend" is that there are plenty of exceptions. The inner galaxy has stars closer together, more clustering can be observed, also determined by the commonality of age and Molecular spectrum. The outer galaxy such as in our vicinity, tends to have less clustering. As you implied clustering is more common amongst younger stars since in time such clustering can dissipate from its point of origin such as maybe our sun has. In younger clusters run-aways are less common since there has been less time to "run." This would be especially true of the largest stars like Betelgeuse. Because of its size it could not easily be pushed around and instead would tend to be a bully. It would seem that it is the tendency of bullies not to run. It may instead have formed from a small disassociated cloud whereby it gobbled up its fair share to achieve its massive size.

Clustering usually has a commonality concerning the general age of the stars within it since most would have been created from the same molecular cloud(s).

[EDG]

This would be especially true of the largest stars like Betelgeuse. Because of its size it could not easily be pushed around and instead would tend to be a bully. It would seem that it is the tendency of bullies not to run. It may instead have formed from a small disassociated cloud whereby it gobbled up its fair share to achieve its massive size.

But then we have different sources saying that Betelgeuse is a runaway member of the OB1 association, so apparently that HAS been thrown out by other stars despite its mass (at least that's what "runaway" implies to me).

[forrest noble]

EDG,

But then we have different sources saying that Betelgeuse is a runaway member of the OB1 association, so apparently that HAS been thrown out by other stars despite its mass (at least that's what "runaway" implies to me).

sounds like a hypothesis that I wouldn't want to bet against, if somehow it could be confirmed. But if I got 2:1 on my bet I'd go the other way concerning all other possibilities.

cheers

[grant hutchison]

But then we have different sources saying that Betelgeuse is a runaway member of the OB1 association, so apparently that HAS been thrown out by other stars despite its mass (at least that's what "runaway" implies to me).

The point is that Betelgeuse isn't that massive, when compared to the total mass of an OB multiple star system. Any one of the stars in the multiple can easily be chucked out if energy is transferred from the other stars as they settle into closer orbits.

Grant Hutchison

[EDG]

That make sense. Though it looks like the other members of the OB association are spread quite thinly... when would they have encountered Betelgeuse to throw it out?

[grant hutchison]

That make sense. Though it looks like the other members of the OB association are spread quite thinly... when would they have encountered Betelgeuse to throw it out?

Many of the "stars" within the OB association are actually multiples. These start out with chaotic orbits around their centre of mass, and then evolve into more hierarchical systems by ejecting members. The idea is that Betelgeuse came from such a high-mass multiple. It's the dynamics of the single system that are important, not of the whole association.

Grant Hutchison

[EDG]

OK, but I guess my issue is that if the other visible stars in orion are supposed to be part of the OB1 association, then it seems that they've had a very short time in which to interact with eachother, and they would have had to have been much closer together in the past few million years to be able to do that.

Or could Betelgeuse have interacted with a dimmer (i.e. not O or B) system that we can't see from here that would also have been part of the association?

[grant hutchison]

OK, but I guess my issue is that if the other visible stars in orion are supposed to be part of the OB1 association, then it seems that they've had a very short time in which to interact with eachother, and they would have had to have been much closer together in the past few million years to be able to do that.

Or could Betelgeuse have interacted with a dimmer (i.e. not O or B) system that we can't see from here that would also have been part of the association?

But the Orion OB1 contains at least four separate regions of successive star formation, so it would seem that there's no particular requirement for all the stars to have been gravitationally bound in the past in order for them to be judged an "association". Nor is there a particular reason for the widely scattered star systems of the OB1 to have interacted with each other in order to eject Betelgeuse: that process just requires the dynamics of single multiple system.

Grant Hutchison

[eburacum45]

It's headed this way, and it's going to blow!

[chornedsnorkack]

It's headed this way, and it's going to blow!

Does Betelgeuse have any proper motion?

[grant hutchison]

Does Betelgeuse have any proper motion?

It does. Details from Simbad. The transverse velocity is comparable to the radial velocity.

Grant Hutchison

[eburacum45]

I presume that means it will miss us by a wide margin. Phew.