When you look at the night sky, do you wonder how stars are born? Here is an explanation that is easy to understand.
The universe, though it may seem to you, is not a complete vacuum. Actually, there are many materials for star formation. This material, mainly dust and gas, is called ISM, Interstellar Medium. The ISM is the building blocks for stars, including our sun. There are many ways for stars to form. They form in different nebulas (the stars' birthplace):
These ISM may gather in some places and form what we call GMC, Giant Molecular Cloud (aka dark nebula). Inside the GMC, the dust blocks out most starlight, thus making the temperature inside very low. Also, because of the lack of starlight, it is nearly impossible to observe a dark nebula with an ordinary light telescope. Only other telescopes, such as infrared telescopes can do so.
The GMC is mainly composed by hydrogen molecular gas. That is why hydrogen is the most abundant chemical element in the universe, making up 75% of the universe's mass. In our galaxy, the GMC tend to concentrate in the spiral's arms.
Emission nebulae are called so because they emit light on their own. Contrary to dark nebulae, they are so hot that the gas inside is ionized, producing light because of a reaction between ions and electrons. The Ring Nebula is an emission nebula.
Reflection nebulae are blue simply because the dust grains scatter blue light. They are much less dense than emission nebulae.
Now that all kinds of nebulae are introduced, let's move on to the next stage of star formation.
Stars form when parts of a GMC collapses and clump together. However, a GMC does not collapse without some sort of trigger. There are three possible triggers:
An expanding shock from a near supernova explosion (a supernova is what happens when some kinds of stars die)
Some moving stars or clouds
Radiation from young big stars
A spiral density wave (something in spiral galaxies)
The gravitational collapse of an entire GMC would not produce a single star. Rather, it will break into many fragments and a whole cluster will be formed. As the collapsing cloud gets denser and hotter due to inside collisions, it becomes a protostar. Once the core temperature reaches 100 million K, nuclear fusion starts. Hydrogen is fused into helium. Eventually, the gravitational energy matches the radiating energy from fusion, so the star reaches a hydrostatic equilibrium. The star will not suddenly have changes in size for a long period of time. The protostar has now become a star.