What is Red giant?
[[File:Sun red giant.svg|thumb| The small white star on the [[left]] is the sun the way it is now. The large red star on the right is what the sun will look like when it turns into a red giant.]] A '''red giant''' is a [[giant star]] that has the mass of about one-half to ten times the mass of our [[Sun]]. Red giants get their name because they appear to be colored [[red]] and they are very large. Many red giants could fit thousands and thousands of suns like ours inside of them. Right now, our Sun is a [[main-sequence star]], not a red giant. However, five billion years from now, [[scientist]]s believe our sun will become a red giant. It will be about 200 times bigger in [[diameter]] than it is now. It will become so big it will swallow up [[Mercury (planet)|Mercury]], [[Venus]] and possibly the [[Earth]].<ref>Though their orbits will be further out than at present.</ref><ref>Jones M.I. ''et al'' 2014. The properties of planets around giant stars. ''Astronomy & Astrophysics'' '''566''': A113. [http://www.aanda.org/articles/aa/abs/2014/06/aa23345-13/aa23345-13.html]</ref> == How a star becomes a red giant == {{main|Stellar evolution}} All new stars change [[hydrogen]] to [[helium]] through [[nuclear fusion]]. This makes a lot of energy (e.g. light and heat). In a normal star, like our Sun and all other main-sequence stars, this change happens at the very center of the star. Sooner or later, almost all of the hydrogen at the center has changed to helium. This causes the nuclear reaction to stop. The center will start to get smaller due to the star's [[gravity]]. This makes the layer just outside the center get hotter. This layer still has hydrogen. This hydrogen will fuse to make helium. [[File:Evolutionary track 1m.svg|thumb|273x273px|Changes of a star like the Sun in brightness and temperature as the Sun grows old. You can see the three red giant phases.]] With this new source of power, the outer layers of the star will get much, much bigger. The star will get brighter, sometimes as much as ten thousand ti