A red giant is an average-size star, similar to our sun that will spend about the final ten percent of its life as a red giant.  In this phase the stars surface temperature will drop to between 3,100 and 6,700 degrees Fahrenheit (1,700 and 3,700 degrees Celsius).  The diameter of the star expands between ten and one thousand times that of our sun.  The star appears to take on a reddish color, which is why it is given this name.

            To become a red giant, a star must use up all of its hydrogen at its core.  (There is still hydrogen around the core).  With nothing left to fuel the nuclear reaction at the core begins to contract.  These contractions release gravitational energy into the surrounding regions of the star, causing it to expand.  The out layers, being further away from the core, start to cool down, and the color of these layers (which is a function of temperature) becomes red.  The star may slowly shrink and expand more than once as it evolves into a red giant!

         A red giant is a variable star.  A variable star changes in brightness, and this is what happens to a red giant!  The red giant is a variable until it runs completely out of fuel, in which case it becomes a white dwarf star.  But sometimes the red giant star wont become a white dwarf star.  This occurs when a star, usually at least eight times larger than our sun, becomes a red giant.  The star then explodes in a supernova, and becomes either a neutron star or a black hole!