Artwork of subgiant Sun seen from the surface of a barren Earth. A subgiant is a star that has exhausted the hydrogen in its core and which as a result has left the main sequence and began to swell. A subgiant star typically has about three times the diameter of the star it was on the main sequence. The Sun will become a subgiant in 6.3 billion years, before it transforms into a red giant. The Moon is also visible, more distant from Earth than it is today.