Blue supergiant is a huge, extremely bright star that glows with a bluish-white light. Blue supergiants rank among the largest, visually brightest, and hottest stars known. Their diameters can reach hundreds of times that of the sun. Blue supergiants shine from ten thousand to a hundred thousand times as bright as does the sun. Their bluish color results from relatively high surface temperatures, which range from 10,000 K for blue-white supergiants to greater than 20,000 K for the bluest supergiants. One kelvin (K) equals one Celsius degree above absolute zero (–273.15 °C). Blue supergiants are extremely rare, but because they shine so bright, they can be seen over enormous distances. Two well-known blue-white supergiants are Rigel, the seventh brightest star in the night sky, and Deneb, the brightest star in the constellation Cygnus. Most blue supergiants occur in loosely bound groupings of stars called OB associations, named for class O and class B stars, two types of bright stars common in those groupings.
Like most other stars, blue supergiants combine hydrogen nuclei to form helium nuclei in their cores. This reaction, a type of nuclear fusion, produces a tremendous amount of energy. Stars produce nearly all of their energy by fusing hydrogen in their cores during a portion of their lives called the main-sequence phase. Blue supergiants are actually extremely massive blue giant stars that have used up most of their hydrogen in their cores and begun to leave the main-sequence phase.
The heat and pressure in a star’s interior produce the fusion reaction. In general, a star’s interior heat and pressure increase with its mass (amount of matter). Because blue supergiants have much greater masses than other stars, their cores have extremely high temperatures and pressures. As a result, blue supergiants fuse hydrogen much more quickly and give off much more energy than do less massive stars.
Blue supergiants have extremely short life spans, using up all the hydrogen fuel in their cores in only a few million years. After that, a blue supergiant will expand and cool to become a huge, relatively low-temperature star called a red supergiant. Because blue supergiants burn out so quickly, astronomers know that all the blue supergiants we can see are rather young.
The energy given off by a star pushes some particles away from the star, creating a flow called the stellar wind. A blue supergiant gives off a stellar wind so strong that it continually blasts away the star’s outer layers, causing it to lose large amounts of mass.
