One thing I've always wondered..

[Manok101]

My thing is understanding nebulae and the one thing I've never been able to comprehend is how a massive cloud of gas condenses into a star or anything for that matter... it doesnt seem like there should be enough mass in any gas. Or is it much much denser than gasses here on earth?

[ollypenrice]

The nebulae are much less dense than gas clouds on Earth. In fact they are so tenuous that they are less dense than all but relatively recent laboratory vacuums. In most of the books I have read the density is given as less than a lab vacuum but a couple of Manchester University physicsts I know said that this is no longer true. However, M42 is not very dense!

So why do they condense? My understanding is that most of the time they don't. They are, though, very cold so their atoms are not moviing around fast enough to repel each other if they are given a nudge bringing them together. Such nudges come from events like supernova explosions sending out shockwaves or passing massive bodies attracting gas atoms towards them. Once a slightly overdense patch has formed the process is irreversible and denser patches become denser, collapsing towards their own newly formed centre of gravity. Any motion they contain (angular momentum) is conserved and, as they shrink, this 'winds up' ever faster. Soon (in astronomical terms!) you have a spinning ball of gas heating up as it becomes smaller and denser until nuclear reactions are triggered in its core. Then the outflowing radiation begins to blast a cavity in the nebula in which the star or cluster of stars were born. On the deep sky imaging board at the moment are some excellent images of the Rosette Nebula, a fine example of the above in progress.

Olly

[Manok101]

excellent explanation thanks very much.

[Talitha]

Thanks Olly, that's the most understandable explanation i've ever read.

[brantuk]

Great description Olly - I've allways pondered this myself and feel I understand it a lot better now. Good question Manok

[Space Beagle]

Wow! How's that for an answer! Good stuff Olly!

Blimey, there are some really rather clever peeps on this forum!

Doc

[NGC 1502]

Thanks Olly, when it's well explained like that, we can get our

heads around it.

But can you do the same for Quantum mechanics please

Cheers, Ed.

[ollypenrice]

Thanks Olly, when it's well explained like that, we can get our

heads around it.

But can you do the same for Quantum mechanics please

Cheers, Ed.

On his deathbed Heisenberg (or it might have been Schroedinger??) was asked if he had any questions for God. He said, Yes, quantum mechanics and turbulent flow. I think I might get an answer on quantum mechanics...'

A lot of my friends here are paraglider pilots and I often think of this. They are relying on flows described by equations that a great physicist considered beyond the reach of God! This cannot really be a good idea...

Olly

[sbooder]

Top description Olly, write a book?

[r.t.swing]

Explanations like this are why I'm liking this forum more and more.

[Manok101]

This begs another question could there not be miniature stars? So small we don't have the capability to detect?

[Mark-mck]

i could and most probably am wrong (no degree in astrophysics) but i belive that a star has to reach a certain size (critical mass?) before the nuclear reaction starts to take place. so i think they would have to get upto a certain minimum size before it turns into a star and not just a stupidly big gas giant.

but as i say i'm no physist so all of what i just said is probably wrong, and at the best only partly right.

[sbooder]

i could and most probably am wrong (no degree in astrophysics) but i belive that a star has to reach a certain size (critical mass?) before the nuclear reaction starts to take place. so i think they would have to get upto a certain minimum size before it turns into a star and not just a stupidly big gas giant.

but as i say i'm no physist so all of what i just said is probably wrong, and at the best only partly right.

No, you are spot on.

[Mark-mck]

well all that time on wikipedia paid off then:D lol

[Ags]

Another cause of compression of gas clouds is 'bow shock' - when one large object like a galaxy collides with another. The Tarantula Nebula is a good example of a gas cloud collapsing spectacularly because the Large Magellanic Cloud is crashing through the tenuous gas around the Milky Way.

Of course, at the beginning of the universe, gas clouds did collapse all by themselves without the help of supernovae. The collapses must have been spectacular, forming the globular clusters we admire today (and of course all the galaxies and supermassive black holes!).

[Manok101]

Thanks a lot, great explanations all round