Nebula

[DB]

What exactly are nebulas and do they form stars and even solar systems? Are they mostly iron???

Ty

[mathman]

Nebula (as opposed to galaxies, which at one time were called nebula) are clouds of gas, mostly hydrogen. Whether or not stars can be born there depends on the size, density, etc.

[meteor]

Nebulae are made of dust and gas; in some parts of the nebula clumps of dust are formed and they collapse gravitationally to form stars. There are to classes of nebulae: diffusse nebulae and dark nebulae. Diffusse nebulae can be divided into emission nebulae and reflection nebulae
I will point also that stars don't form only in nebulae, they also form in giant molecular clouds

Are they mostly iron???

Not quite. Most nebulae are formed of about 90% hydrogen and 10% helium

[Chronos]

... Most nebulae are formed of about 90% hydrogen and 10% heliumWell, I would say closer to 75-25, but it's just a fine point.

[meteor]

Well, I would like to see a reference for that, I obtained my info from here
http://www.seasky.org/cosmic/sky7a05.html o:)

[mathman]

I believe the discrepancy has to do with what ratio we are talking about for H to He. The mass ratio is about 3-1, while the number ratio is 12.5-1. I hope this clears it up.

[Chronos]

Once we convert to the same units, we will find agreement. about 90-10 is ratio by number of hydrogen v helium atoms, 75-25 is the ratio by mass. I had to check cause I pulled 75-25 off the top of my head [a sometimes risky practice]. Anyways, those are the text book numbers [The New Solar System, Beatty & Chaikin]. No online version of that book but I have a link that includes the relevant table from the book.
http://www.genesismission.org/educate/scimodule/PlanetaryDiversity/plandiv_pdf/SupermarketST.pdf

[Nereid]

Interestingly, this implies that your average 'nebula gas' is close to primordial (in terms of composition). For the most part, that's quite reasonable (much of the non-primordial stuff in the dark nebulae, reflection nebulae, etc is in the form of dust, not gas), but not always ... for example, nebulae that are 'purely' supernova remnants will likely be deficient in H (and He).

[meteor]

Thanks Nereid
One of the links says that there are 5 types of nebulae: planetary nebulae,supernovae remnants, emission nebulae, reflection nebulae and dark nebulae. But, as there are supernovae remnants, there are also nova remnants. I wonder why these should be marginalized and not included as nebulae

[Chronos]

Good point meteor. I was thinking in terms of protosolar nebulae: the kind that birth stars. That was the spirit of the original question as I perceived it. Those of course can be expected to be very similar to the primordial gas clouds from which most stars are formed. Nebulae formed by events of a different nature are, as you note, horses of a different color.