What is a nebula?

1 Answer
Dec 18, 2015

Nebulae are formed from stars.


Nebulae are giant gas clouds in space mostly consisting of Hydrogen and Helium. As these are the two most common (ie: most stable) compounds, this is not surprising.

One primary way in which nebulae are formed is when a star very much like our sun undergoes a significant change. As a star is a delicate balance (hydrostatic equilibrium) of the crushing force of gravity and the explosive force of the fusion reactions in it's core, tampering with this balance is often catastrophic.

As our sun grows older, it will start to burn hotter. As a result of this excess of fusion, a change in the equilibrium occurs, and the volume of the sun increases. This causes two problems: increased fuel consumption, and stellar nucleosynthesis. The star basically gets so hot that nuclear fusions beyond helium occur, and heavy atoms like calcium and nitrogen and what not are made. All the elements right up to iron (more or less...) can be generated in large stars.

So all this fuel consumption and excess density (ie: more gravity) causes the core of the sun to collapse. Why the core? Because the sun is liquid/gas/superfluid something. It's not solid is my point, and the heavy things sink to the middle where they fuse to form heavier things and so on.

So the core collapses into a white dwarf, and sends out a massive shock wave when it does it. This blows the outer layers and atmosphere of the star outwards in wildly varying patterns and formations, and bada bing bada boom you have a nebula. The pretty colours come from the radiation from the star exciting electrons in the atoms of the gas cloud. Neat.