<p>Don't downvote this question. It's a good question. The answer is:</p>
<p>Large gravitational deflection and acceleration only occurs when an object is <em>very close</em> to another massive object compared to it's size. At the scale in which stars exist, the actual stars are like grains of sand in diameter. They are very unlikely to swing so close to each other that they experience large gravitational acceleration.</p>
<p>They DO gain enough energy to swing them in a random direction. That and the pull of the cluster as a whole is what makes stars drift away, then swing back  through the cluster's center of mass and shoot out the other side.</p>
<p>But remember, every time 1 star speeds up, the other star slows down. And it all adds up to zero system momentum change. But that motion keeps the cluster from collapsing into a black hole.</p>
<p>But still, it does happen, and that's why there are orphaned objects floating around the galaxy.</p>

CC BY-SA 4.0
2024-06-25T11:04:13.723Z
Don't _download_ this question??
Nilay Ghosh
CC BY-SA 4.0
2024-06-25T11:46:19.243Z
Don't down VOTE the OP question. I hate when that happens. You can proofread your stuff 100 times, but it happens anyway.
Miss Understands
CC BY-SA 4.0
2024-06-26T01:33:52.647Z
I had thought that in a 3 body interaction it would just be a matter of time for them to get into a configuration where one of them has energy enough to scape. And that any cluster has many 3-body systems. And therefore we would see many more orphan objects in the sky. I thought of that after learning about 30 Dor #16.
Roberto
CC BY-SA 4.0
2024-06-26T16:13:53.093Z
Evaporation of stars from clusters is not primarily due to close encounters. Indeed for a uniform spatial distribution, collisions at each decade (factor of 10) in distance contribute equally (leading to logarithmic divergence with the size of the system, i.e. the [Coulomb logarithm](https://en.wikipedia.org/wiki/Coulomb_collision#Coulomb_logarithm)).
Sten
CC BY-SA 4.0
2024-06-27T17:58:49.983Z
why is it related to black holes?
user721108
CC BY-SA 4.0
2024-06-28T12:22:37.350Z
@Roberto I think you maybe asked the wrong question by accident. A runaway star isn't just a star that escapes its cluster, it's one that's moving bizarrely fast compared to its environment, usually in an odd direction. Even if we only look at interactions that give one of the bodies enough energy to escape, in most cases that'll be just barely over escape velocity -- that means by the time the star is visibly not part of the cluster anymore, it's moving fairly slowly. You need an extreme interaction to give a star enough kick to wind up moving so fast that it's notable.
Darth Pseudonym
CC BY-SA 4.0
2024-06-30T01:45:10.317Z
@DarthPseudonym you are completely right about my wrong assumption. So is it common to have escaped stars with low speed? How are they called?
Roberto
CC BY-SA 4.0
2024-07-01T03:48:55.880Z
I don't think we have a special name for stars that just aren't in a cluster, but maybe somebody else knows better than me.
Darth Pseudonym
CC BY-SA 4.0
2024-06-24T23:10:48.973Z
2024-06-25T11:44:26.787Z
2024-06-25T11:44:26.787Z
Miss Understands
<p>The natural evolution of a star cluster is first to relax into a elliptical configuration, and then, on a much longer time scale, evaporate by ejecting stars. The poster children for this are globular clusters.</p>
<p>The evaporation time depends on the density of stars in the cluster. Even for the high density of stars in a globular cluster, the time is billions of years.</p>

CC BY-SA 4.0
2024-06-25T15:55:37.890Z
There are hardly any open clusters (as a fraction) with ages > a few hunderd Myr.  This isn't how they evaporate/disperse.
ProfRob
CC BY-SA 4.0
2024-06-25T16:00:34.587Z
@ProfRob Indeed: they blow their gas away and disperse before they get a chance to relax.
John Doty
CC BY-SA 4.0
2024-06-25T13:37:54.793Z
2024-06-25T13:37:54.793Z
John Doty
<p>From a different perspective:</p>
<p>tldr; Chaos runs its course</p>
<p>In a chaotic system, many objects bump into each other. They change their orbits in a short time. But soon, there are way fewer objects with more mass each. They're harder to push around. Surprisingly soon, <em>the only objects left are in stable orbits.</em></p>
<p>You can see it with your own eyes.</p>
<p>Look up the moon. Notice that there's light stuff and dark stuff. The light stuff is impact craters and the dark circles are lava after big impacts. The lava covered all the craters that happened earlier.</p>
<p>So you'd expect the dark stuff to be young, right?</p>
<p><a href="https://i.sstatic.net/65igCUSB.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/65igCUSB.jpg" alt="enter image description here" /></a></p>
<p>No. Lava covered all the existing craters <em><strong>3.9 billion years ago.</strong></em></p>
<p>And its dark surface looks smooth even today.</p>
<p>All those white craters on the moon happened in the first 500 million years of its life.</p>
<p>Chaos runs its course, then long-term  stability takes over.</p>

CC BY-SA 4.0
2024-06-26T16:07:58.423Z
This is reasonably accurate for some systems, such as the solar system. However it is incorrect for galaxies and globular star clusters. Those systems are not stable; their evaporation time scales are simply longer than the age of the universe.
Sten
CC BY-SA 4.0
2024-06-26T05:31:23.587Z
2024-06-26T10:18:16.440Z
2024-06-26T10:18:16.440Z
Miss Understands
<p>If stars are clustered together, isn’t a third star being ejected (becoming a runaway star) just a matter of time?</p>
<p>Then shouldn’t we expect most of stars to be runaway stars?</p>

CC BY-SA 4.0
2024-06-24T11:51:46.673Z
Please could you clarify what do you mean by "runaway star"? If you mean a star with enough velocity to leave the Galaxy than the answer is simple - the velocity of most stars ejected from clusters is not that high.
Heopps
CC BY-SA 4.0
2024-06-24T15:11:08.017Z
Runaways aren't necessarily at escape velocity, at least as I understand it.  They're just going abnormally fast.
Darth Pseudonym
CC BY-SA 4.0
2024-06-24T19:13:08.363Z
Clusters do dissipate with time. though the close interaction rate between stars is usually only high in the initial star formation stage. Many three body interactions will not impart a velocity high enough to escape the cluster.
ProfRob
CC BY-SA 4.0
2024-06-25T04:27:36.360Z
Are you specifically asking about triple star systems? Or star clusters in general?
PM 2Ring
CC BY-SA 4.0
2024-06-26T16:16:04.777Z
There are a lot of answers now, but I still can't tell if the question is about star systems, star clusters, or galaxies.
Sten
CC BY-SA 4.0
2024-06-27T09:50:26.570Z
I’m asking about star clusters.
Roberto
<runaway-star>
Why don’t runaway stars happen more often?
Roberto
# Why don’t runaway stars happen more often?

(Asked by: username_0 on 2024-06-24)

If stars are clustered together, isn’t a third star being ejected (becoming a runaway star) just a matter of time?

Then shouldn’t we expect most of stars to be runaway stars?

**Tags:** runaway-star
> **Comments:**
> **username_1**: Please could you clarify what do you mean by "runaway star"? If you mean a star with enough velocity to leave the Galaxy than the answer is simple - the velocity of most stars ejected from clusters is not that high.  
> **username_2**: Runaways aren't necessarily at escape velocity, at least as I understand it.  They're just going abnormally fast.  
> **username_3**: Clusters do dissipate with time. though the close interaction rate between stars is usually only high in the initial star formation stage. Many three body interactions will not impart a velocity high enough to escape the cluster.  
> **username_4**: Are you specifically asking about triple star systems? Or star clusters in general?  
> **username_5**: There are a lot of answers now, but I still can't tell if the question is about star systems, star clusters, or galaxies.  
> **username_0**: I’m asking about star clusters.  

---

## 3 answers

### Answer by username_6 on 2024-06-24. Score: 3 | Accepted answer

Don't downvote this question. It's a good question. The answer is:

Large gravitational deflection and acceleration only occurs when an object is *very close* to another massive object compared to it's size. At the scale in which stars exist, the actual stars are like grains of sand in diameter. They are very unlikely to swing so close to each other that they experience large gravitational acceleration.

They DO gain enough energy to swing them in a random direction. That and the pull of the cluster as a whole is what makes stars drift away, then swing back through the cluster's center of mass and shoot out the other side.

But remember, every time 1 star speeds up, the other star slows down. And it all adds up to zero system momentum change. But that motion keeps the cluster from collapsing into a black hole.

But still, it does happen, and that's why there are orphaned objects floating around the galaxy.

> **Comments:**
> **username_7**: Don't _download_ this question??  
> **username_6**: Don't down VOTE the OP question. I hate when that happens. You can proofread your stuff 100 times, but it happens anyway.  
> **username_0**: I had thought that in a 3 body interaction it would just be a matter of time for them to get into a configuration where one of them has energy enough to scape. And that any cluster has many 3-body systems. And therefore we would see many more orphan objects in the sky. I thought of that after learning about 30 Dor #16.  
> **username_5**: Evaporation of stars from clusters is not primarily due to close encounters. Indeed for a uniform spatial distribution, collisions at each decade (factor of 10) in distance contribute equally (leading to logarithmic divergence with the size of the system, i.e. the [Coulomb logarithm](https://en.wikipedia.org/wiki/Coulomb_collision#Coulomb_logarithm)).  
> **username_8**: why is it related to black holes?  
> **username_2**: @Roberto I think you maybe asked the wrong question by accident. A runaway star isn't just a star that escapes its cluster, it's one that's moving bizarrely fast compared to its environment, usually in an odd direction. Even if we only look at interactions that give one of the bodies enough energy to escape, in most cases that'll be just barely over escape velocity -- that means by the time the star is visibly not part of the cluster anymore, it's moving fairly slowly. You need an extreme interaction to give a star enough kick to wind up moving so fast that it's notable.  
> **username_0**: @DarthPseudonym you are completely right about my wrong assumption. So is it common to have escaped stars with low speed? How are they called?  
> **username_2**: I don't think we have a special name for stars that just aren't in a cluster, but maybe somebody else knows better than me.  

---

### Answer by username_9 on 2024-06-25. Score: 2

The natural evolution of a star cluster is first to relax into a elliptical configuration, and then, on a much longer time scale, evaporate by ejecting stars. The poster children for this are globular clusters.

The evaporation time depends on the density of stars in the cluster. Even for the high density of stars in a globular cluster, the time is billions of years.

> **Comments:**
> **username_3**: There are hardly any open clusters (as a fraction) with ages > a few hunderd Myr.  This isn't how they evaporate/disperse.  
> **username_9**: @ProfRob Indeed: they blow their gas away and disperse before they get a chance to relax.  

---

### Answer by username_6 on 2024-06-26. Score: 1

From a different perspective:

tldr; Chaos runs its course

In a chaotic system, many objects bump into each other. They change their orbits in a short time. But soon, there are way fewer objects with more mass each. They're harder to push around. Surprisingly soon, *the only objects left are in stable orbits.*

You can see it with your own eyes.

Look up the moon. Notice that there's light stuff and dark stuff. The light stuff is impact craters and the dark circles are lava after big impacts. The lava covered all the craters that happened earlier.

So you'd expect the dark stuff to be young, right?

[![enter image description here](https://i.sstatic.net/65igCUSB.jpg)](https://i.sstatic.net/65igCUSB.jpg)

No. Lava covered all the existing craters ***3.9 billion years ago.***

And its dark surface looks smooth even today.

All those white craters on the moon happened in the first 500 million years of its life.

Chaos runs its course, then long-term stability takes over.

> **Comments:**
> **username_5**: This is reasonably accurate for some systems, such as the solar system. However it is incorrect for galaxies and globular star clusters. Those systems are not stable; their evaporation time scales are simply longer than the age of the universe.  

---

