Blue giant is a large, bright star that glows with a bluish-white light. Blue giants shine from a few tens to many thousands of times as bright as does the sun. Their bluish color results from relatively high surface temperatures, generally greater than 15,000 K. One kelvin (K) equals one Celsius degree above absolute zero (–273.15 °C). Blue giants make up only a small fraction of the stars in our galaxy. Because they shine so bright, however, they can be seen even at a great distance. For this reason, we can see many blue giants in the night sky. Spica, the brightest star in the constellation Virgo, is perhaps the best-known blue giant.
Like most other stars, blue giants combine hydrogen nuclei to form helium nuclei in their cores. This reaction, a type of nuclear fusion, produces a tremendous amount of energy. Stars that produce nearly all of their energy by fusing hydrogen in their cores are called main-sequence stars. Blue giants rank as the hottest main-sequence stars and those with the greatest mass (amount of matter). Such extremely hot and massive stars are known as upper main-sequence stars.
The heat and pressure in a star’s interior drive the fusion reaction. In general, a star’s interior heat and pressure increase with its mass. Because blue giants have much greater masses than most other stars, their cores have relatively high temperatures and pressures. As a result, blue giants fuse hydrogen more quickly and give off more energy than do less massive stars.
Blue giants use up their hydrogen relatively quickly. Depending on its mass, a blue giant may fuse all the hydrogen in its core in tens of millions or hundreds of millions of years. The star’s outer layers then begin to expand. The most massive blue giants then spend a relatively short period of time as huge blue supergiants or as Wolf-Rayet stars. Wolf-Rayet stars are hot, bright stars that give off an unusually massive stellar wind (stream of particles). Eventually, however, the expanding star grows cooler, transforming into a huge, relatively low-temperature star called a red supergiant. Because blue giants burn out so quickly, astronomers know that all the blue giants we can see are relatively young. In fact, astronomers searching for regions where new stars are forming often look for areas with relatively high concentrations of blue giants.
See also Blue supergiant .
