Stars born in clusters, found in single

[dotancohen]

So far as I understand, most stars found in the galaxy are single or double stars. However, they are born in nebulae by the hundreds. What happens to spread them out such? I would think that gravity would keep them together in the way that they were formed, by the hundreds. Or am I misunderstanding how stars are born?

Thanks in advance.

 

[marcus]

That's an interesting question. Some groups of stars, which presumably formed together in the same molecular cloud, are gravitationally bound---like the Pleiades---and stay together.
So-called "open clusters".

A group of stars which is not gravitationally bound will naturally disperse in the course of several revolutions around the galactic center. One reason is that stars further out from galactic center orbit more slowly. And individual random motions take their toll.

The question is, are the original molecular clouds themselves gravitationally bound? (I suspect not entirely, but pose the question. Eventually clouds disperse with time due to radiation pressure.)
But if the original cloud were bound, then why should not the stars that form in it be bound as well?

http://en.wikipedia.org/wiki/Open_cluster

I have to go do some other things, but will try to return to this later today. Maybe someone else will respond in the meantime.

 

[dotancohen]

Thanks, Marcus. The Open Cluster link was a good read. I would still like to know why the majority (it seems) of stars are not found in such clusters.

By the way, my nine-year-old niece came up with the question. I love how children get us thinking!

 

[turbo]

A group of stars which is not gravitationally bound will naturally disperse in the course of several revolutions around the galactic center. One reason is that stars further out from galactic center orbit more slowly. And individual random motions take their toll.

The rotational speed curve of spiral galaxies is generally pretty flat farther out from the galactic center. Are you perhaps thinking of clusters toward the galactic center where the curves are much steeper?

 

[marcus]

The rotational speed curve of spiral galaxies is generally pretty flat farther out from the galactic center. Are you perhaps thinking of clusters toward the galactic center where the curves are much steeper?

No I wasn't thinking of a steep section of the rotation curve, actually. A flat or nearly flat speed curve would be adequate to eventually disrupt a group of stars, would it not?
If their random individual motion didn't do it first. You may think the effect is negligible, and it would be in some cases certainly. But stars at different distances from galactic center will have different orbital periods even if they have identical speeds.

Turbo, I suspect you have a good way of summarizing the different effects that gradually disperse a crowd of stars which is not gravitationally bound. Why don't you take a stab at responding?

 

[marcus]

By the way, my nine-year-old niece came up with the question. I love how children get us thinking!

That's a nice niece. You should have more nieces like that and they should keep asking questions!

Thanks, Marcus. The Open Cluster link was a good read. I would still like to know why the majority (it seems) of stars are not found in such clusters.

An open cluster is a gravitationally bound group of stars, as the article explains. (The term "open" helps distinguish from another type of gravitationally bound group, like the "globular" cluster. Globular clusters are a different topic---they're gravitationally bound but have a definite overall shape.)

OK so you realize that Turbo and I haven't answered the question yet and you persist on wanting to know---this is great. We may get Turbo or someone else to give a clear concise answer. In the meantime I'll tell you what I think.

You are pointing to a paradox. If the cloud out of which some 100 stars formed is gravitationally bound, and not just a temporary overdensity in the interstellar medium that is destined to go away----if the original cloud is bound then why aren't the 100 stars also bound? After all, it is the same mass just rearranged. It should have the same gravitational behavior!

Here's what I think: the original cloud held together temporarily because it had enough material for 1000 stars. But after 100 stars formed, their light blew the remaining 90% of the gas cloud away!
So then there wasn't enough mass left within the original confines of the cloud to keep the 100 stars (and any residual gas) bound together.

How does that sound to you Dotancohen? Do you see anything that doesn't make sense? Radiation pressure is a really important factor in star-forming regions. See what your nineyearold niece thinks about light-pressure. The same thing that makes a solar sail work. Light actually exerts very gentle pressure. Too gentle to feel but eventually enough so a star can "blow away" most of the gas around it, once it starts to shine properly.

 

[turbo]

Here's what I think: the original cloud held together temporarily because it had enough material for 1000 stars. But after 100 stars formed, their light blew the remaining 90% of the gas cloud away!
So then there wasn't enough mass left within the original confines of the cloud to keep the 100 stars (and any residual gas) bound together.

That's a good explanation for the appearance of some star-nurseries. If a cloud condenses very rapidly, it can form very massive and energetic stars and the radiative pressure from those stars can easily evacuate the gas and dust envelope that surrounded their nursery. Over-densities in that material as it is "herded" by radiative pressure can result in the condensation of smaller stars that will be much longer-lived than the their massive progenitors.

 

[Chronos]

Open clusters are somewhat unusual. They appear to form in relatively dense knots in relatively empty space, and tend to disperse rapidly - after a couple rotations around the galaxy [roughly 500 million years]. The abundance of massive young stars that typically populate open clusters is a clue.

 

[dotancohen]

<quote>That's a nice niece. You should have more nieces like that and they should keep asking questions!</quote>

She has two little sisters and they both take an interest as well. So does my oldest daughter (only 3, but she can name Cappella and Jupiter in the sky), the youngest is only 1 year old so it is a bit early to tell. I have promised them that I'd buy a telescope in the indefinite future, but it is out of our budget right now. The nine-year old makes due with Stellarium and loves it!<quote>Here's what I think: the original cloud held together temporarily because it had enough material for 1000 stars. But after 100 stars formed, their light blew the remaining 90% of the gas cloud away!
So then there wasn't enough mass left within the original confines of the cloud to keep the 100 stars (and any residual gas) bound together.

How does that sound to you Dotancohen? Do you see anything that doesn't make sense? Radiation pressure is a really important factor in star-forming regions. See what your nineyearold niece thinks about light-pressure. The same thing that makes a solar sail work. Light actually exerts very gentle pressure. Too gentle to feel but eventually enough so a star can "blow away" most of the gas around it, once it starts to shine properly.</quote>

It sounds logical to me, but I know very little about the processes. Online sources seem to confirm that the phenomenon of blowing out 90% of the gas does in fact happen, and I agree that it sounds plausible that it would change the gravity map of the region enough to break the gravitational bond between the stars.

I will ask Noam (the nine-year old niece) what she thinks about that.

Thanks!

 

[marcus]

...
<quote>Here's what I think: the original cloud held together temporarily because it had enough material for 1000 stars. But after 100 stars formed, their light blew the remaining 90% of the gas cloud away!
So then there wasn't enough mass left within the original confines of the cloud to keep the 100 stars (and any residual gas) bound together.
...</quote>

It sounds logical to me, but I know very little about the processes. Online sources seem to confirm that the phenomenon of blowing out 90% of the gas does in fact happen, and I agree that it sounds plausible that it would change the gravity map of the region enough to break the gravitational bond between the stars.

I will ask Noam (the nine-year old niece) what she thinks about that.
...

Here's some optional extra discussion intended partly for you and partly for Noam.

Let's forget about the gas cloud and just think of a cluster of 1000 stars. Every star has a little bit of random motion. Some are falling in towards center, some have already fallen thru and are sailing outwards (but not fast enough that they will escape, eventually they'll slow down and fall back into the heart of the cluster). Other stars are going every which way. But no one is going fast enough to break away from the community because the combined mass of stars is too large. It will always slow a departing star down and eventually turn it around and pull it back.

We can use technical terms and say that the cluster of stars is gravitationally bound because no single star, in its random motion, has escape velocity----enough speed to escape from the combined pull of all the other 999 stars.

The point of the example is that it could very well be that a small subset of the cluster, if you could magically make the rest disappear, would NOT be gravitationally bound.
If you took this milling mob of 1000 stars, and selected 100 to keep, and magically made the other 900 vanish-----but without changing the individual motions of the remaining 100---then the remaining 100 stars might not have enough mass collectively to keep themselves together as a bunch.

The collective mass would be less, so the necessary escape velocity would be less, so many or all of the stars would suddenly find that they had enough speed to coast out of the cluster and go on their own independent way. And the cluster would dissipate.

So you can have something that is gravitationally bound, and makes a stable blob or mob or system, and that if you take away 90% of the mass will no longer be gravitationally bound.

The story with the starforming region---the molecular cloud---is similar except that 90% of the mass is hydrogen gas and when the 100 stars begin to shine they heat it up and blow it away---so you don't need magic to make the 90% go away.

And I think that the gas that is blown away might eventually cool down and begin to gather again and merge with other clouds of gas somewhere else and might then collect into another starforming region and give birth to another 100 stars. Things keep happening. The story can repeat.

 

[Widdekind]

http://www.scientificamerican.com/article.cfm?id=the-long-lost-siblings-of-the-sun

The Long-Lost Siblings of the Sun

• The sun is a solitary star, and astronomers have traditionally assumed it formed as such. Yet most stars are born in clusters, and scraps of evidence from meteorites and from the arrangement of comets suggest that our sun was no exception.
• Its birth cluster could have contained 1,500 to 3,500 stars within a diameter of 10 light-years—a big, unhappy family whose larger members bullied the small fry and which broke up not long after our solar system came into being.
• Although the sun’s siblings have long since dispersed across the galaxy, observatories such as the European GAIA satellite will be able to look for them. Their properties might fill in the gaps of the solar system’s deep history.

As Chronos observed, OC's disperse over several revolutions around the Galaxy. The article cites numerical simulations, to show that over 4.5 Gyr, the Sun's "stellar family" will have spread out around an arc stretching nearly 180 degrees around the Galactic core, all at about a constant radius.



See also: http://www.scientificamerican.com/article.cfm?id=solar-twins-search-galaxy

But Portegies Zwart changed his mind after calculating the sun's velocity backward over the past 4.6 billion years. From that putative starting point in the Centaurus constellation, he then projected how stellar members of our birth cluster would actually disperse forward in time.

"To my surprise," he says, "these stars all sort of stayed in the neighborhood with their proper motions and positions pretty much intact. Whether we can really recognize these stars is still not completely clear. Out of about a million stars within some 300 light-years, the expectation is that about 50 of them are siblings."