<blockquote>
  <p>If I have understood correctly, stars form in big clouds of gas and dust that are pulled together by their gravity. And the stars are often ignited when something disturbs the cloud, such as a passing massive body, or a wave like the once which create spirals in galaxies.</p>
</blockquote>

<p>You have the sequence backwards. Protostars ignite by themselves once their cores reach a critical temperature and pressure. The gravitational collapse that eventually results in protostars (and then a stars) oftentimes needs a little help from an external disturbance such as a passing star or a supernova shock.</p>

<blockquote>
  <p>Are there any phenomena on earth that behave like these clouds, which could be used to illustrate and explain star formation?</p>
</blockquote>

<p>Not really. The interstellar gas clouds that eventually collapse to form stars are very large, very massive, very cold, and very tenuous. The best way to understand them is to understand them directly rather than looking for a poor analogy. There's nothing on Earth like them.</p>

CC BY-SA 3.0
2016-11-14T15:45:51.373Z
Are you saying we stars can't form without the help from external forces? If there is a nebula undisturbed by external forces, will it ever give birth to a star?
fahadash
CC BY-SA 4.0
2022-08-23T11:01:03.947Z
@fahadash No? He's literally saying "collapse [...] _oftentimes_ needs a little _help_ from an external disturbance". As long as there is way for the nebula to lose gravitational energy, it will collapse - eventually. Needless to say, the earliest stars _had_ to form this way - there were no stars or supernovae to provide the "disturbance". The disruption accelerates this process through creating density variation in the nebula, but ultimately it's all about shedding that gravitational energy _somehow_ .
Luaan
CC BY-SA 3.0
2015-06-24T12:55:49.553Z
Interesting info. Let's wait a little before accepting to see if someone else has any input.
Lii
CC BY-SA 3.0
2015-06-23T20:19:20.083Z
2015-06-23T20:26:56.837Z
2015-06-23T20:26:56.837Z
David Hammen
<p>To begin with, star-formation occurs when relatively stable and huge clouds of gases and dust are disturbed by external disturbances such as shock waves from a supernova explosion. This brings some material closer when gravity kicks in, bringing together more of the gases, which accumulate and grow dense to pull in more material.  </p>

<p>This process continues till an enormous amount of material has been accumulated, making it dense at the core i.e. the "center" of the gigantic gas cloud, as well as raising the temperatures and pressures there. This is when the star "ignites", meaning that a fusion reaction kicks in, releases the outward-energy required to balance the inner-crushing gravity of the star.</p>

<p>Since the magnitude of the pressures, temperatures, size of the gas clouds etc. involved here are huge, it's impossible to find such phenomena on the earth.</p>

<p>You can view the episode "Extreme Stars" from the series How the Universe Works (Season 1 Episode 4) that were aired on The Discovery Channel for a detailed visual understanding of the Star formation process.  </p>

CC BY-SA 3.0
2015-06-29T10:23:54.293Z
2015-06-29T10:23:54.293Z
Riptide
<p>If I have understood correctly, stars form in big clouds of gas and dust that are pulled together by their gravity. And the stars are often ignited when something disturbs the cloud, such as a passing massive body, or a wave like the once which create spirals in galaxies.</p>

<p>Are there any phenomena on earth that behave like these clouds, which could be used to illustrate and explain star formation? Something people might be familiar with and which you could show a video clip of?</p>

<p>I'm thinking about some gas that is disturbed then takes solid form? Or something completely different which looks similar?</p>

CC BY-SA 3.0
2015-06-26T15:13:23.337Z
The spread of star formation to nearby regions via shocks has often been said to resemble forest fires.
eshaya
CC BY-SA 3.0
2015-06-26T17:05:30.030Z
@EdShaya: That's very interesting! Do have a link to an example?
Lii
CC BY-SA 3.0
2015-06-26T17:55:28.983Z
I would give credit to this idea to Phil Seiden in his landmark paper: ["Stochastic star formation and the evolution of galaxies"](http://adsabs.harvard.edu/full/1979ApJ...232..702S)
eshaya
CC BY-SA 3.0
2015-06-30T16:05:54.210Z
I like this question because it illustrates just how unintuitive some astronomical processes are. You're asking for an analogy to nuclear fusion (the word "ignite", in this context, might confuse people, btw) which just doesn't occur, except in the most horrible of circumstances, here on Earth.
blamblambunny
<star-formation>
Star formation analogy
Lii
# Star formation analogy

(Asked by: username_0 on 2015-06-23)

If I have understood correctly, stars form in big clouds of gas and dust that are pulled together by their gravity. And the stars are often ignited when something disturbs the cloud, such as a passing massive body, or a wave like the once which create spirals in galaxies.

Are there any phenomena on earth that behave like these clouds, which could be used to illustrate and explain star formation? Something people might be familiar with and which you could show a video clip of?

I'm thinking about some gas that is disturbed then takes solid form? Or something completely different which looks similar?

**Tags:** star-formation

> **Comments:**
> **username_1**: The spread of star formation to nearby regions via shocks has often been said to resemble forest fires.  
> **username_0**: @EdShaya: That's very interesting! Do have a link to an example?  
> **username_1**: I would give credit to this idea to Phil Seiden in his landmark paper: ["Stochastic star formation and the evolution of galaxies"](http://adsabs.harvard.edu/full/1979ApJ...232..702S)  
> **username_2**: I like this question because it illustrates just how unintuitive some astronomical processes are. You're asking for an analogy to nuclear fusion (the word "ignite", in this context, might confuse people, btw) which just doesn't occur, except in the most horrible of circumstances, here on Earth.  

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## 2 answers

### Answer by username_3 on 2015-06-23. Score: 4

> If I have understood correctly, stars form in big clouds of gas and dust that are pulled together by their gravity. And the stars are often ignited when something disturbs the cloud, such as a passing massive body, or a wave like the once which create spirals in galaxies.

You have the sequence backwards. Protostars ignite by themselves once their cores reach a critical temperature and pressure. The gravitational collapse that eventually results in protostars (and then a stars) oftentimes needs a little help from an external disturbance such as a passing star or a supernova shock.

> Are there any phenomena on earth that behave like these clouds, which could be used to illustrate and explain star formation?

Not really. The interstellar gas clouds that eventually collapse to form stars are very large, very massive, very cold, and very tenuous. The best way to understand them is to understand them directly rather than looking for a poor analogy. There's nothing on Earth like them.

> **Comments:**
> **username_0**: Interesting info. Let's wait a little before accepting to see if someone else has any input.  
> **username_4**: Are you saying we stars can't form without the help from external forces? If there is a nebula undisturbed by external forces, will it ever give birth to a star?  
> **username_5**: @username_4 No? He's literally saying "collapse [...] _oftentimes_ needs a little _help_ from an external disturbance". As long as there is way for the nebula to lose gravitational energy, it will collapse - eventually. Needless to say, the earliest stars _had_ to form this way - there were no stars or supernovae to provide the "disturbance". The disruption accelerates this process through creating density variation in the nebula, but ultimately it's all about shedding that gravitational energy _somehow_ .  

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### Answer by username_6 on 2015-06-29. Score: 3

To begin with, star-formation occurs when relatively stable and huge clouds of gases and dust are disturbed by external disturbances such as shock waves from a supernova explosion. This brings some material closer when gravity kicks in, bringing together more of the gases, which accumulate and grow dense to pull in more material.

This process continues till an enormous amount of material has been accumulated, making it dense at the core i.e. the "center" of the gigantic gas cloud, as well as raising the temperatures and pressures there. This is when the star "ignites", meaning that a fusion reaction kicks in, releases the outward-energy required to balance the inner-crushing gravity of the star.

Since the magnitude of the pressures, temperatures, size of the gas clouds etc. involved here are huge, it's impossible to find such phenomena on the earth.

You can view the episode "Extreme Stars" from the series How the Universe Works (Season 1 Episode 4) that were aired on The Discovery Channel for a detailed visual understanding of the Star formation process.


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