Video Transcript
List the following stages of stellar evolution in order, for a certain star that has a mass of 80 percent of the Sun: a) A white dwarf, b) A nebula, c) A red giant, d) A main sequence star, and e) A protostar.
So we’ve got stages of stellar evolution. That’s the evolution of a star. And we know that this star has a mass of 80 percent of the Sun. The reason that this piece of information is useful is because a star that has a mass roughly similar to that of the Sun will have the same stellar evolution as that of the Sun. Basically, the stages in the stellar evolution of the Sun and of this star should be the same. And that’s because their masses are roughly similar. And we can see that this is actually true. The five stages that’ve been listed for us happened to also be stages in the Sun’s stellar evolution. They’re all stages that either the Sun has already been through or is currently going through or will go through in the future. So all that remains is for us to rank them in the correct order. In order to do this, let’s all gather a seat around the campfire because it’s story time.
Once upon a time in outer space, there was a clump of dust and gas. This clump of dust and gas was rather special because it was not a sparse distribution of dust and gas, but rather it was a clump. That meant that all of this dust and gas had gathered together in a small region of space. And it was this clumping together of that dust and gas that was very special, because this gas cloud was known as a nebula. In Latin, the word nebula meant cloud or fog. And this made sense. We essentially had a cloud of dust and gas. And, like we said earlier, all of this gas was localized to a fairly small region, rather than being distributed over large areas of space.
And this meant that the effects of gravity could start to become evident because whenever lots of particles happen to find themselves in a region of space, clumped together, then the effect of gravity becomes strong. And this clump starts to contract into something that now looks like the beginning of a star. This is known as a protostar. The word proto means young or primitive or early. So a protostar is a young star. The really cool thing about this protostar was that all the gravitational collapse was causing the temperatures, especially in its core, to increase rapidly. Because whenever gravity tries to shove a lot of gas into a small amount of space, the temperature in that gas increases.
Eventually, the temperatures in the protostar were high enough that the gas in the core was no longer a gas but rather a plasma. There were lots of electrons floating about, ripped apart from the nuclei of the atoms they came from. And these nuclei were mainly protons because the gas that we had initially in the nebula was mainly hydrogen gas.
Now, as gravity continue to take hold, the temperatures in the core of the protostar continue to rise even further. Eventually, all the hydrogen nuclei in the core could start fusing. Nuclear fusion had started to occur. At this point, our little baby star was no longer a little baby star. But it was, instead, a main sequence star. The hydrogen nuclei in its core started to be fused together to form helium nuclei. And this was to be the main part of the star’s life, fusing hydrogen in its core into helium.
And this continued for millions of years until suddenly, the hydrogen began to run out. All of it was being fused together to form helium. There wasn’t much hydrogen left. At least, there wasn’t in the core of the star. There was some hydrogen left, however, in the shells surrounding the core. At this point, the temperatures in the star began to rise again. The hydrogen in the shell surrounding the core started to fuse into helium. And all of the helium inside the core could now stop fusing into much heavier elements, elements such as carbon or oxygen. This phase of the star’s life was known as the red giant phase.
It continued to fuse helium at its core forming carbon and oxygen until suddenly, even the helium run out. At this point, the star had reached its limit. It was not large enough to increase its temperature so high that it could start to fuse even carbon and oxygen. So instead, it shed all of its outer layers leaving only the core behind. The core that was left was formed of carbon and oxygen due to all the fusion processes. And all of the shells that surrounded the core were expelled. These shells contained some hydrogen, some helium, and some heavier elements. And they would later go on to form a new nebula, the start of another star’s life.
However, what we were left with was a core that was not large enough to fuse carbon and oxygen. So instead, all it did was gave out some heat that was left over from all the energetic processes it had been through in its lifetime. This was known as a white dwarf. And this was the final stage in the evolution of the star. The end.
I hope you enjoyed that story. And I hope it helps you remember the stages of stellar evolution for a star that’s roughly the same mass as our Sun. And by the way, our Sun has already been through the nebula phase and the protostar phase. And it’s currently at the main sequence star phase. It will probably, eventually, go on to become a red giant and then a white dwarf.
But anyway, the solution to our problem, the answer to our question is that the stages of stellar evolution, in the correct order, are: a nebula followed by a protostar followed by a main sequence star followed by a red giant and then finally, a white dwarf.
