A red giant is a kind of star which has a large diameter (tens to hundreds of times bigger than our Sun’s) and whose photosphere is relatively cool (2500 to 5000 K or so) … so such a star is certainly a giant, but not really red (if one were as bright as the Sun, it would appear just as ‘white'; it’s only ‘red’ in a relative sense … it looker redder than other stars).
Red giants have masses that are not that different from the mass of our own Sun, about half to eight sols (1 sol = mass of the Sun), and most are the last ‘fuel burning’ stage in the evolution of stars of this mass (not counting white dwarfs in close binaries) … they have used up all the hydrogen in their cores, are burning hydrogen in the shell around the (inert) helium core, and are not massive enough to be able to create high enough temperatures to initiate helium burning (the most massive red giants do go on to burn helium … unless they lose too much mass!). In about five billion years’ time our own Sun will become a red giant, so big that it will swallow both Mercury and Venus (and perhaps Earth too).
Losing lots of mass is what most red giants seem to be doing, by blowing it away in the form of a strong solar wind (being so big, the gravitational force at their surfaces is low, so mass loss is easier than for the Sun), dumping it onto companion stars (if the red giant is a member of a close binary, it can be big enough to fill its Roche lobe), and by pulses (all red giants are pulsating variables, and in some the pulses are great enough that the outer layers are shed at maximum).
Have you ever seen a red giant? Sure you have! For example, Betelgeuse – in Orion, visible in both the northern and southern hemisphere – is a red giant …
Want more? Try NASA’s Ask An Astrophysicist (Definition of a Red Giant Star), Red Giant Evolution (University of Oregon), and Red Giant Stars and the Death of the Sun (Wolfram Demonstrations Project).
Here at Universe Today, we just love red giants … we have so many articles on them! For example: Red Giant Brightness Variations Still Mysterious, Will Earth Survive When the Sun Becomes a Red Giant?, and Unprecedented Images Show Betelgeuse Has Sunspots.
Red giants feature in several Astronomy Cast episodes, all of them worth your time! There’s The Life of Other Stars, Different Sized Stars, Stellar Roche Limits, and Famous Stars (yep, some famous stars are red giants!).