A red giant is a late-stage star that has exhausted the hydrogen in its core. Hydrogen fusion continues in a shell around the core, inflating the outer layers to tens or hundreds of times the original radius and cooling the surface to a red glow.

Will the Sun become a red giant?

Yes. In about 5 billion years the Sun will leave the main sequence and expand well past the orbit of Mercury. Earth will be rendered uninhabitable long before that, as the Sun's brightness rises steadily over the next billion years.

What comes after the red giant phase?

For stars up to about 8 solar masses, the star sheds its envelope as a planetary nebula and the exposed core becomes a white dwarf. More massive stars go on to collapse and explode as supernovae.

How big can a red giant get?

The largest known red supergiants, such as UY Scuti or Stephenson 2-18, exceed a thousand times the Sun's radius. Placed where the Sun is, their surfaces would reach beyond the orbit of Jupiter.

Red Giantとは何か？

A red giant is a star that has run out of hydrogen in its core and responded by swelling to enormous size. The surface cools to a deep orange-red even as the star's total output rises by a factor of a thousand — a dying star, briefly at its brightest.

When core hydrogen runs out, fusion does not stop; it moves. A thin shell of hydrogen around the inert helium core keeps burning, and it burns faster than the core ever did, because the contracting core underneath it is hotter. The extra energy has nowhere to go but outward, and the envelope inflates. Surface gravity drops, the outer layers cool to around 3,000 to 4,500 K, and the star turns red while growing to tens or hundreds of times its former radius.

The interior is now grotesquely lopsided: a helium core roughly the size of Earth but containing a large fraction of the star's mass, wrapped in a tenuous envelope larger than the orbit of Mercury. Eventually the core ignites helium — in stars around the Sun's mass this happens explosively, in a runaway called the helium flash. Later, on the asymptotic giant branch, the star pulses, drives off its envelope as a planetary nebula, and leaves the bare core behind as a white dwarf.

This is the Sun's future. In roughly five billion years it will leave the main sequence, expand past the orbit of Mercury and almost certainly Venus, and scorch the Earth long before it reaches us. Aldebaran and Arcturus are red giants you can find in the night sky tonight; Betelgeuse is the far more massive supergiant version, on a much faster track toward a supernova.