Question

At which point in the life cycle of a star does nuclear fusion begin?
1) Black Hole
2) Main sequence
3) Nebula
4) Protostar
(I ruled out 1 already, but I am not sure about the rest)

Answer 1

When nuclear fusion begins is when the star "turns on" or "lights up". You are correct in eliminating answer 1. Think about the other answers in this way: Is this the stage when the star "turns on"?

Answer 2

I believe so @ospreytriple

Answer 3

In other words,
a) Does the star "turn on" when it is forming in the nebula?
b) Does the star "turn on" when it is a protostar?
c) Has the star "turned on" when it moves to the main sequence?

Answer 4

Thanks so much! I appreciate it :)

Answer 5

You're welcome.