Read this excerpt from "Why Are They Called Black Holes?" Gravity is caused by matter, and as matter increases, the pull of gravity increases too. Stars are born when enough matter collects that gravity makes it collapse into a hot, dense protostar. When the star gets big enough and hot enough, the central core has enough energy to fuse hydrogen atoms into helium atoms.

According to the context clues, which phrase best defines the underlined word?

a star that has recently begun to exist
the sun that is seen in the Earth's sky
the planet that orbits closest to the sun
a star whose existence is about to end