It and It's

Its and It’s

Its is an irregular possessive form (see The Possessive Apostrophe) and it’s is a contraction of it is. If your sentence does not make sense when writing it is, then you do not need an apostrophe.

When the sun reaches the end of its life (the life belonging to the sun), when it burns through all of its fuel (the fuel that the sun is burning), it’s not (it is not) going to become a supernova!