A red giant is a star that has exhausted the primary supply of hydrogen fuel at its core. An average-sized star like our Sun will spend the final 10 percent of its life as a red giant. In this phase, a star's surface temperature drops to between 3,140 and 6,741°F (1,727 and 3,727°C) and its diameter expands to 10 to 1,000 times that of the Sun. The star takes on a reddish color, which is what gives it its name.
With no hydrogen left at the core of a star to fuel the nuclear reaction that keeps it burning, the core begins to contract. The core's contracting releases gravitational energy into the surrounding regions of the star, causing it to expand. Consequently, the outer layers cool down and the color of the star (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.
Black hole: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.
Fusion: Combining of nuclei of two or more lighter elements into one nucleus of a heavier element; the process stars use to produce energy to support themselves against their own gravity.
Nebula: A cloud of interstellar dust and gas.
Neutron star: Extremely dense, compact, neutron-filled remains of a star following a supernova.
Variable star: Star that varies in brightness over periods of time ranging from hours to years.
White dwarf: Cooling, shrunken core remaining after a medium-sized star ceases to burn.
This change marks the start of a dynamic process in which the star develops into a variable star. It becomes alternately brighter and dimmer, generally spending about one year in each phase. The star continues in a variable state until it completely runs out of fuel.
While the star is in a puffed-up state, helium continues to accumulate at its core. Since the helium initially is not hot enough to undergo fusion (the process by which two atoms combine, releasing a vast amount of energy), it becomes denser and denser. Finally, pressure alone forces the atoms to fuse, forming carbon and oxygen. At the same time, the core shrinks and the star becomes bluer and smaller.
Using helium as a fuel, the star's core continues to burn normally for a while, although the star shines less brightly than it did in its expanded state. At the same time, any remaining hydrogen in regions farther out from the core fuse into helium. The core becomes so hot that it may pulsate (vary in brightness). This stage does not last long, however, since the helium burns quickly.
As the helium runs out, the star again puffs up—this time to about 500 times the size of the Sun, with about 5,000 times the brightness of the Sun. Buried deep inside the star's unstable atmosphere is a hot core about the size of Earth, but with 60 percent of the Sun's mass. As a final act, the atmosphere dislodges from the core and floats off as a planetary nebula (cloud). The glowing core, called a white dwarf, is left to cool for eternity.
More massive stars exit the red giant stage with a bang, transformed by a supernova (explosion) into a neutron star (dense, neutron-filled remains of a star) or a black hole (a single point of infinite mass and gravity).