In the nuclear fusion of helium, the two most likely initial steps — 4He + 1 and 4He + 4He — are unable to react, because both result in unstable isotopes that fly apart before they can react further. However, at temperatures beginning around 100,000,000K, two 4He could fuse into 8Be, giving off gamma radiation in the process, at a fast enough rate that beryllium would be produced fasted than it decayed (8Be's halflife is only 10x10-17 seconds). This 8Be could then fuse with yet another 4He to produce 12C, again producing gamma radiation. The process is named from the particles going into it — three alpha particles (helium nuclei). In a star, the energy released from this reaction can combine with energy produced in a hydrogen shell to increase thermal pressure in the core enough to overcome gravity and increase the size of the star. This increased surface grows at a faster rate than the increased energy produced, causing the surface to cool off and glow red, earning stars within which this occurs the name "red giant".
http://csep10.phys.utk.edu/astr162/lect/energy/triplealph.html http://aether.lbl.gov/www/tour/elements/stellar/stellar_a.html#3alpha http://alumni.imsa.edu/~jgbarnes/aaa.htm