Video: Understanding How Stars Form

Parth Gharfalkar

Which of the following correctly describes a protostar? [A] A protostar is a type of star which emits no light but a very high number of protons. [B] A protostar is the type of star that only appeared in the early universe. [C] A protostar is a type of star that only appears close to the galactic center. [D] A protostar is a newly formed star. [E] A protostar is a hot dense cloud of gas that has not yet begun nuclear fusion in its core.

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Video Transcript

Which of the following correctly describes a protostar?

Now we’ve been given five different statements. So let’s go through them one by one and see which one is a good description of a protostar. Number one, a protostar is a type of star which emits no light, but a very high number of protons. Now if we didn’t know anything about a protostar, then we’ll be tempted to say that this was the correct definition, because we’ve got the word “proto” and we’re talking about protons. However, that’s not what the word “protostar” means. As we’ll see in a moment, the word “proto” means something else entirely. Also, protostars do emit light. Therefore, this statement is not the correct description of a protostar.

Number two then, a protostar is the type of star that only appeared in the early universe. Is this true? Well, no it’s not. We can still see protostars to this day. But if they only appeared in the early universe, then we wouldn’t see any around now. So this description is also incorrect.

Number three then, a protostar is a type of star that only appears close to the galactic center. What this statement is saying is that protostars only appear close to the centers of galaxies. That’s what galactic center means. More specifically, it might be saying that they only appear close to the center of the galaxy the Milky Way, because this statement says the galactic center rather than a galactic center. And so it might be referring to the galaxy that we’re in, the Milky Way.

However, this statement is also untrue. Protostars can be found all over galaxies, not just near their center. In fact, our Sun used to be a protostar. And our solar system is no way near the center of the Milky Way. In fact, if this is a diagram of the Milky Way, then our solar system is found somewhere on one of these tails. This means that we’re really far away from the center of the galaxy. Hence, statement number three is also incorrect.

Number four then, a protostar is a newly formed star. Oh! Now this is getting so close to the correct answer. But it’s not quite right. Basically, a protostar is what a star is before it becomes a star officially. And so the state in a star’s evolution after the protostar is when we can officially start calling it a star.

In other words, a protostar is the last stage of evolution before it becomes a star. So no it’s not a newly formed star. It’s almost a newly formed star, very nearly a newly formed star. So this means that number five must be our correct description. So let’s read it and find out.

Number five, a protostar is a hot dense cloud of gas that has not yet begun nuclear fusion in its core. Now this is the correct description of a protostar. Basically, when stars form, there’s a cloud of gas that starts to contract under its own weight due to gravity. And eventually, this gas collapses into what’s known as a protostar. But in this protostar, nuclear fusion has not begun in its core yet. As soon as nuclear fusion begins in the core, that’s when it’s known as a star and no longer known as a protostar.

So the protostar stage, as we said earlier, is the stage before it becomes a star. And it becomes a star when nuclear fusion begins. So it looks like number five is our correct definition.

Another thing to note is that the word “proto” actually means young or primitive. So a protostar is a young star or a primitive star. And as we said earlier, our Sun used to be a protostar before nuclear fusion began in its core. And at this point, we can say that statement five is a correct description of a protostar. A protostar is a hot dense cloud of gas that has not yet begun nuclear fusion in its core.

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