How are nebulae formed?

[farunj]

Ok this might seem like a silly question but how are nebulae formed? Are they formed out of the gas/dust emitted by supernovae?

[Tiki]

There are lots of different types of nebula. Eg. Some stars at some point in their life will shed layers of matter, this is what forms the doughnut shaped part of M57.

[Knight of Clear Skies]

The word nebula just means 'cloud', so it describes several different phenomena. This kind of thing is fairly common in astronomy, where objects were grouped together based on their observational characteristics before their nature was understood. In older literature, galaxies are often referred to as nebula.

An emission nebula is a cloud of gas and dust which glows due to ionization by a source of UV radiation, either hot stars forming inside it (e.g. Orion Nebula) or a stellar remnant such as a white dwarf (the Ring Nebula, an example of a misleadingly named planetary nebula) or neutron star (e.g. The Crab Nebula). A reflection nebula is a cloud which simply reflects light from stars (e.g. the nebulosity around the Pleiades).

Star formation begins when a cold molecular cloud reaches a critical density and begins to collapse under its own gravity. This can happen due to collisions between orbiting clouds, or compression by a nearby supernova. The hydrogen and helium in the molecular clouds originates from the big bang, but they are enriched with heavier elements which have been cooked up within stars, and released at the end of their lives either when a star becomes a planetary nebula and sheds its outer layers, or explodes as a supernova.

[Macavity]

Ok this might seem like a silly question but how are nebulae formed? Are they formed out of the gas/dust emitted by supernovae?

Interesting thought! Seems there are indeed TWO different origins of nebulae. The matter left over from supernovae (as you say) - And also stuff (atoms, dust etc.) left over from the big bang...

Well maybe not so much "dust", in the latter case? In theory, the big bang mostly gave rise hydrogen & helium (isotopes)? But specifically, no elements heavier than lithium, beryllium, (boron maybe) ... and these only at the merest trace levels.

http://en.wikipedia....nucleosynthesis etc. etc.

[farunj]

The word nebula just means 'cloud', so it describes several different phenomena. This kind of thing is fairly common in astronomy, where objects were grouped together based on their observational characteristics before their nature was understood. In older literature, galaxies are often referred to as nebula.

An emission nebula is a cloud of gas and dust which glows due to ionization by a source of UV radiation, either hot stars forming inside it (e.g. Orion Nebula) or a stellar remnant such as a white dwarf (the Ring Nebula, an example of a misleadingly named planetary nebula) or neutron star (e.g. The Crab Nebula). A reflection nebula is a cloud which simply reflects light from stars (e.g. the nebulosity around the Pleiades).

Star formation begins when a cold molecular cloud reaches a critical density and begins to collapse under its own gravity. This can happen due to collisions between orbiting clouds, or compression by a nearby supernova. The hydrogen and helium in the molecular clouds originates from the big bang, but they are enriched with heavier elements which have been cooked up within stars, and released at the end of their lives either when a star becomes a planetary nebula and sheds its outer layers, or explodes as a supernova.

Nice one! But come to think of it, the Orion neb has been along for a long time now! So is it still forming stars or it has stopped and all that is left is clouds? And btw what are these clouds made up of? People just say clouds but are they hydrogen/helium clouds?

[Knight of Clear Skies]

Glad I could be of help.

Nice one! But come to think of it, the Orion neb has been along for a long time now! So is it still forming stars or it has stopped and all that is left is clouds?

Something like 700 stars are currently forming in the Orion nebula. However, this process is slowing because the stellar wind from the young stars is blowing away the remaining gas and dust. This is why the bright stars of the Trapezium cluster at it's core are visible, the obscuring clouds there have already been ejected. In about 100,000 years most of remaining gas will be gone, leaving a young open cluster behind, rather like the Plieades.

And btw what are these clouds made up of? People just say clouds but are they hydrogen/helium clouds?

They are mostly hydrogen and helium, but with a substantial mix of heavier elements, such as oxygen, carbon, neon, iron, nitrogen and silicon. It's possible to detect the different elements by the specific wavelengths of light that they emit (spectroscopy). For example, the green tinges seen in the Orion nebula is evidence of ionized oxygen.

[Qualia]

Brilliant explanation, Knight. Thanks for that

[farunj]

Glad I could be of help.

Something like 700 stars are currently forming in the Orion nebula. However, this process is slowing because the stellar wind from the young stars is blowing away the remaining gas and dust. This is why the bright stars of the Trapezium cluster at it's core are visible, the obscuring clouds there have already been ejected. In about 100,000 years most of remaining gas will be gone, leaving a young open cluster behind, rather like the Plieades.

They are mostly hydrogen and helium, but with a substantial mix of heavier elements, such as oxygen, carbon, neon, iron, nitrogen and silicon. It's possible to detect the different elements by the specific wavelengths of light that they emit (spectroscopy). For example, the green tinges seen in the Orion nebula is evidence of ionized oxygen.

Brilliant! Thanks for that!

[badgerchap]

Don't forget that with planetary nebulae, they have little to do with planets! They were originally named this as when they were first spotted, they were seen as small, round and planet-shaped. However the only real connection they have with planets is that at some point the material from which they are comprised may end up being incorporated into planets or parts of planets.

You can be fairly certain that some of you came from planetary nebulae, and very certain that the rest of you came from some form of nebula at some point. Our Sun and the solar system were formed from, aside from primordial hydrogen, helium and lithium, the remnants of supernovae which took place long ago. Elements heavier than iron are formed in these events, and as far as we know nowhere else. As for the elements lighter and including iron, these were formed in the hearts of stars as they consumed their nuclear fuel. These elements may be from stars that died leaving planetary nebulae, or as part of supernova explosions. So to use what is rapidly becoming an uncomfortable cliche, you are made of stardust.

[nameunknown]

I think its fair to say that almost all the nebulae we see nowadays are probably stuff made in supernovae. Either the collapse of clouds under supernova shock-waves into new star forming regions, or novae puffing out clouds from old stars that went "bang" after their cores collapsed. The basic answer to "how are nebulae formed?" is gravity, its the only force (apart from dark energy) that has any effect on a cosmic scale. So, gravity provides all the interest in astronomy and we haven't really a clue how it works...

P

[Ganymede12]

So what eventually happens to nebula? Do they exist until all the material has become a Star/Planet/Asteroid/Comet? Or can the cloud "thin out" to the point where it's no longer a cloud?

[Knight of Clear Skies]

So what eventually happens to nebula? Do they exist until all the material has become a Star/Planet/Asteroid/Comet? Or can the cloud "thin out" to the point where it's no longer a cloud?

The radiation pressure from the stars forming within the nebula disperses the cloud, until it is no longer dense enough to form any more stars. Only about 10% of the total mass in a nebula ends up as stars before this happens and the rest escapes. As it drifts further away, the plasma gradually cools and recombines, causing it to stop glowing. Eventually, some of the material will combine with other molecular clouds, and possibly take part in further rounds of star formation.

[Ags]

So what eventually happens to nebula? Do they exist until all the material has become a Star/Planet/Asteroid/Comet? Or can the cloud "thin out" to the point where it's no longer a cloud?

A large nebula is intrinsically unstable - it must either collapse under its own weight, or disperse because the gas is too hot for the cloud's gravity to contain itself. There is no middle ground. If the cloud is cool, it collapses under its own weight, and the knots of dense gas form stars, which heat up the remaining nebular gas, which ultimately cause it to evaporate.

Planetary nebulae and supernova remnants are always flying apart, so they are always very 'young' structures we glimpse for the short time they exist.