<p>Stars do not stay where they are born. They orbit in our galaxy and their orbits are subject to relative perturbing forces caused by galactic tides and encounters with spiral density waves and other giant molecular clouds. A typical velocity dispersion for stars in the solar neighbourhood is <span class="math-container">$\sim 20$</span> km/s (or about 20 pc per million years). Thus once stars are billions of years old, they can have moved relative to the stars they were born with by a distance of order the size of the Milky Way.</p>
<p>Thus the positions of old stars are essentially randomised<span class="math-container">$^1$</span> and their density (near the Sun) is of order 0.1 per cubic parsec (including stars of all masses greater than about 0.1 solar masses - the smaller ones are much more numerous).</p>
<p>A typical giant molecular cloud in the earliest stages of star formation might be of order 10-100 pc in diameter, with a volume of 500 to 500000 cubic parsecs. You would thus expect about 50-50000 normal stars to exist within the ​boundaries of a giant molecular cloud of this size, simply at random. Note that a molecular cloud is far too sparse to offer any mechanical resistance to the motion of a star into it or through it.</p>
<p>Simply scale to the size you have in mind for your molecular cloud to get a different answer.</p>
<p>If you restrict the stellar types to say 0.8-1.2 solar masses, then you can reduce this number by about a factor of 10. If you restrict it to stars older than 4.5 billion years, then you could reduce the numbers by a further factor of 2-3, since the rate of star formation has been approximately constant for the last 10 billion years and this is comparable to the main sequence lifetime of a 1 solar-mass star.</p>
<p><span class="math-container">$^1$</span> Note that there is a tendency for stars to oscillate around a mean relative position in an orbit (an epicycle) rather than wander randomly through the galaxy, but it still has the effect of smoothly randomising their relative positions.</p>

CC BY-SA 4.0
2022-02-10T16:30:42.397Z
Thanks but this question was answered long ago on other sites, and the discovery a while back confirmed it https://www.livescience.com/earth-trapped-in-local-bubble
Alan Davies
CC BY-SA 4.0
2022-02-10T17:26:33.260Z
It is unclear to me how you think that article answers your question. @AlanDavies
ProfRob
CC BY-SA 4.0
2022-02-15T01:03:37.047Z
It answered it well enough as it shows that our solar system has moved though this bubble area and will move into a nebula concentration area soon. As nobody on this site or others gave me an example of an old star in a nebula, although answers said it is possible, this was the most confirming evidence. But thanks for your estimates, I don't know how true they are as they don't take into account the volume of old stars kicked out by starburst formation forces but thanks all the same.
Alan Davies
CC BY-SA 4.0
2022-02-15T05:17:33.770Z
@AlanDavies which molecular cloud do you think the Sun will pass through soon?  No old stars are" kicked out by starburst formation forces" (not sure what forces you are imagining).
ProfRob
CC BY-SA 4.0
2022-02-07T23:23:56.143Z
2022-02-07T23:23:56.143Z
ProfRob
<p>A molecular cloud is a cloud of hydrogen and other molecules in space.</p>
<p>You may have noticed that in the atmosphere of Earth there are many clouds of dust and water vapor, and that solid objects like birds and airplanes pass thorugh those clouds with ease.</p>
<p>Molecular clouds are dense compared to most regions of interstellar space, but they are very thin vacuums compared to the clouds in Earth's atmospthere.  They have no ability to bounce away a solid object which reaches their boundries.  Instead the solid object will pass right through the molecular cloud.</p>
<p>A solid object passing thorugh a molecular cloud will sweep up some of the molecules near its path, creating a tunnel of even thinner vacuum through the cloud.  Its gravitational and/or magnetic interactions with molecules it passes might change their paths through space and cause thinner and denser regions to form in the cloud.</p>
<p>The stellar density is the number of stars per unit of volume of space.</p>
<blockquote>
<p>In the solar neighborhood, this value can be determined from surveys of nearby stars, combined with estimates of the number of faint stars that may have been missed. The true stellar density near the Sun is estimated as 0.004 stars per cubic light year, or 0.14 stars pc−3. When combined with estimates of the stellar masses, this yields a mass density estimate of 4×10−24 g/cm3 or 0.059 solar masses per cubic parsec. The density estimate varies across space, with the density decreasing rapidly in the direction out of the galactic plane.<a href="https://en.wikipedia.org/wiki/Stellar_density" rel="nofollow noreferrer">1</a></p>
</blockquote>
<p><a href="https://en.wikipedia.org/wiki/Stellar_density" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Stellar_density</a></p>
<p>If you calculate the volume of a molecular cloud in cubic light years or cubic parsecs, you can calculate the average number of stars which can be expected to be within that volume of space.  And if you estimate or choose the volume of space which you count as being near a molecular cloud, you can calculate the average number of stars which would be near the molecular cloud at any one time.</p>
<p>At any one time a molecular cloud might contain one or more stars which are forming or have already formed, but have not yet dispersed.  And a large enough molecular cloud will have a large enough volume that it will be statistically probable that one or more older stars that did not form in that molecular cloud will be passing though the cloud at any one time.</p>

CC BY-SA 4.0
2022-01-08T17:37:37.713Z
2022-01-08T18:09:20.113Z
2022-01-08T18:09:20.113Z
M. A. Golding
<p>From what I have read as molecular clouds are the birth place of stars, their population as you would assume are young stars or stellar remnants but so far I have not found out if there are main-sequence stars of the age of ours or older that have either wandered into a molecular cloud or are positioned close by.</p>
<p>I was wondering if we have observed any solar systems similar to the age of ours or older within or close to molecular clouds?</p>

CC BY-SA 4.0
2022-01-07T15:40:10.167Z
Stars get dispersed from their birth cloud. See https://astronomy.stackexchange.com/q/43425/16685
PM 2Ring
CC BY-SA 4.0
2022-01-07T19:38:50.590Z
@PM2Ring I don't mean stars from that nebula as the nebula would have formed long after, I mean old stars that are near the area or somehow wonder into the area.
Alan Davies
CC BY-SA 4.0
2022-01-07T21:07:20.940Z
From [wiki](https://en.wikipedia.org/wiki/Molecular_cloud#Occurrence) they are about 1% of the volume and located in the spiral arms near the plane of the MW. I can't think of anything that would make stars less likely to be in these regions.
Keith McClary
<nebula><main-sequence>
Are older main-sequence stars found in or near molecular clouds?
Alan Davies
# Are older main-sequence stars found in or near molecular clouds?

(Asked by: username_0 on 2022-01-07)

From what I have read as molecular clouds are the birth place of stars, their population as you would assume are young stars or stellar remnants but so far I have not found out if there are main-sequence stars of the age of ours or older that have either wandered into a molecular cloud or are positioned close by.

I was wondering if we have observed any solar systems similar to the age of ours or older within or close to molecular clouds?

**Tags:** nebula, main-sequence

> **Comments:**
> **username_1**: Stars get dispersed from their birth cloud. See https://astronomy.stackexchange.com/q/43425/16685  
> **username_0**: @username_1 I don't mean stars from that nebula as the nebula would have formed long after, I mean old stars that are near the area or somehow wonder into the area.  
> **username_2**: From [wiki](https://en.wikipedia.org/wiki/Molecular_cloud#Occurrence) they are about 1% of the volume and located in the spiral arms near the plane of the MW. I can't think of anything that would make stars less likely to be in these regions.  

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## 2 answers

### Answer by username_3 on 2022-02-07. Score: 3

Stars do not stay where they are born. They orbit in our galaxy and their orbits are subject to relative perturbing forces caused by galactic tides and encounters with spiral density waves and other giant molecular clouds. A typical velocity dispersion for stars in the solar neighbourhood is $\sim 20$ km/s (or about 20 pc per million years). Thus once stars are billions of years old, they can have moved relative to the stars they were born with by a distance of order the size of the Milky Way.

Thus the positions of old stars are essentially randomised$^1$ and their density (near the Sun) is of order 0.1 per cubic parsec (including stars of all masses greater than about 0.1 solar masses - the smaller ones are much more numerous).

A typical giant molecular cloud in the earliest stages of star formation might be of order 10-100 pc in diameter, with a volume of 500 to 500000 cubic parsecs. You would thus expect about 50-50000 normal stars to exist within the ​boundaries of a giant molecular cloud of this size, simply at random. Note that a molecular cloud is far too sparse to offer any mechanical resistance to the motion of a star into it or through it.

Simply scale to the size you have in mind for your molecular cloud to get a different answer.

If you restrict the stellar types to say 0.8-1.2 solar masses, then you can reduce this number by about a factor of 10. If you restrict it to stars older than 4.5 billion years, then you could reduce the numbers by a further factor of 2-3, since the rate of star formation has been approximately constant for the last 10 billion years and this is comparable to the main sequence lifetime of a 1 solar-mass star.

$^1$ Note that there is a tendency for stars to oscillate around a mean relative position in an orbit (an epicycle) rather than wander randomly through the galaxy, but it still has the effect of smoothly randomising their relative positions.

> **Comments:**
> **username_0**: Thanks but this question was answered long ago on other sites, and the discovery a while back confirmed it https://www.livescience.com/earth-trapped-in-local-bubble  
> **username_3**: It is unclear to me how you think that article answers your question. @username_0  
> **username_0**: It answered it well enough as it shows that our solar system has moved though this bubble area and will move into a nebula concentration area soon. As nobody on this site or others gave me an example of an old star in a nebula, although answers said it is possible, this was the most confirming evidence. But thanks for your estimates, I don't know how true they are as they don't take into account the volume of old stars kicked out by starburst formation forces but thanks all the same.  
> **username_3**: @username_0 which molecular cloud do you think the Sun will pass through soon?  No old stars are" kicked out by starburst formation forces" (not sure what forces you are imagining).  

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### Answer by username_4 on 2022-01-08. Score: 1

A molecular cloud is a cloud of hydrogen and other molecules in space.

You may have noticed that in the atmosphere of Earth there are many clouds of dust and water vapor, and that solid objects like birds and airplanes pass thorugh those clouds with ease.

Molecular clouds are dense compared to most regions of interstellar space, but they are very thin vacuums compared to the clouds in Earth's atmospthere. They have no ability to bounce away a solid object which reaches their boundries. Instead the solid object will pass right through the molecular cloud.

A solid object passing thorugh a molecular cloud will sweep up some of the molecules near its path, creating a tunnel of even thinner vacuum through the cloud. Its gravitational and/or magnetic interactions with molecules it passes might change their paths through space and cause thinner and denser regions to form in the cloud.

The stellar density is the number of stars per unit of volume of space.

> In the solar neighborhood, this value can be determined from surveys of nearby stars, combined with estimates of the number of faint stars that may have been missed. The true stellar density near the Sun is estimated as 0.004 stars per cubic light year, or 0.14 stars pc−3. When combined with estimates of the stellar masses, this yields a mass density estimate of 4×10−24 g/cm3 or 0.059 solar masses per cubic parsec. The density estimate varies across space, with the density decreasing rapidly in the direction out of the galactic plane.[1](https://en.wikipedia.org/wiki/Stellar_density)

<https://en.wikipedia.org/wiki/Stellar_density>

If you calculate the volume of a molecular cloud in cubic light years or cubic parsecs, you can calculate the average number of stars which can be expected to be within that volume of space. And if you estimate or choose the volume of space which you count as being near a molecular cloud, you can calculate the average number of stars which would be near the molecular cloud at any one time.

At any one time a molecular cloud might contain one or more stars which are forming or have already formed, but have not yet dispersed. And a large enough molecular cloud will have a large enough volume that it will be statistically probable that one or more older stars that did not form in that molecular cloud will be passing though the cloud at any one time.


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