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Protostar is an interesting word, is it not? In fact, it is a word that might give an amateur astronomer pause, as we are accustomed to very straightforward names for celestital objects (e.g. white dwarf, red giant and, of course, black hole). Therefore, to understand what "protostar" means, let's consider the word's etymology.
I think portion of the word "star" should be a simple enough for us to understand, but what does "proto" mean?
Proto is of greek origin, meaning "first" or "earliest form of". With this in mind, we can begin to understand the word.
I encourage you to take a guess at protostar's meaning!
I think we may have all come to the same (and correct) conclusion. A protostar refers to an infant star, one in its earliest stages of life.
Stars form magnificently, they are conceived in nebulae, where they accrete gasses until their internal temperatures are hot enough to ignite nuclear fusion and carry it on independently.
But how do these stars come together? The answer, as usual, is gravity.
It is nearly impossible for objects in our universe to be evenly spread throughout space. That would mean everything would have to be of uniform mass or, somehow, divinely placed according to their gravitational attraction to all other things so that our universal system reached an equilibrium.
Well, as you can imagine, that doesn't really happen.
Therefore, in a nebula, stars form when hydrogen and helium begin to clump together, creating a cloud that will become increasingly hot as more mass is pulled in towards its center. When it reaches it's critical mass the core will begin to shrink, pulled in by its own gravity, it will begin to spin quickly (let us note that this ball was already in motion, due to the natural movement of the elements in the nebula) and throw off a disk that will orbit around the somewhat spherical protostar.
What is even more remarkable is that as this protostar spins and grows, the disk now orbiting it will become a planetary nebula. This will be the first step towards creating a solar system.
Equilibrium will have been reached when the force of gravity pushing in is equal to the force of the gasses atoms resisting one another.
From here one of two things can happen:
- The temperature of the star isn't hot enough, and it withers and becomes a brown dwarf
2)The temperature of the star is hot enough to ignite fusion and the protostar becomes a full fledged star.
Whether it is on the main sequence or not is determined by how much mass it accreted in the earliest phases of its life. The star, regardless of size, will then push off the disk of dust that has been orbiting it. If enough time as elapsed between the formation of that disk and the disk's exile from the star, then it is possible that protoplanets have formed!
From here, the magnificent life of a full-fledged star begins- and this is where it gets interesting!