Red supergiant is a huge, extremely bright star that glows with a reddish light. Red supergiants have diameters that range from 100 times to more than 1,000 times that of the sun. They shine thousands to hundreds of thousands of times as bright as the sun does. Their reddish appearance results from relatively low surface temperatures of less than about 4000 K. One kelvin (K) equals one Celsius degree above absolute zero (–273.15 °C). Well-known red supergiants include Betelgeuse, the 10th brightest star in the night sky, and Antares, the brightest star in the constellation Scorpius.
Formation.
Each red supergiant was once a type of star called a main-sequence star. A main-sequence star, such as our sun, produces most of its energy by combining hydrogen nuclei to form helium nuclei in its core. This reaction, a type of nuclear fusion, produces a tremendous amount of energy. The energy creates an outward pressure that balances the star’s tendency to contract (shrink) under its own gravitational pull. A red supergiant forms when a main-sequence star with more than about 8 times the sun’s mass (amount of matter) uses up all the hydrogen in its core. The minimum mass a star must have to become a red supergiant depends partly on the star’s metallicity—that is, the proportion of elements heavier than helium that the star contains.
Without the energy from fusion pushing outward, the entire star—including the core—begins to contract. The contraction quickly heats the star’s interior. When temperatures there become high enough, hydrogen fusion begins again, this time in a thin shell surrounding the core. This fusion produces even more energy than was given off by fusion in the core, causing the star’s outer layers to expand. These layers cool as they expand, growing redder. Hydrogen fusion in the shell continues to convert hydrogen to helium. The helium that is produced adds mass to the core, causing it to contract and heat. Eventually, the core becomes hot enough to fuse helium nuclei into carbon nuclei. By this time, the outer layers of the star have expanded and cooled enormously, and the star has become a red supergiant.
Evolution.
When a red supergiant’s core runs out of helium, a helium-burning shell develops around the now contracting carbon core. This causes the star’s outer layers to expand and cool even further. Eventually, the carbon core heats up enough to begin fusing carbon. Carbon fusion creates a variety of elements, including magnesium, sodium, neon, and oxygen. After the carbon in the core is entirely consumed, the star goes on to fuse heavier and heavier elements in stages. Each time the fusion of a particular element ceases in the core, it continues in a thin shell surrounding the core. Eventually, the star’s central region takes on a layered appearance resembling an onion, with an iron core surrounded by successive shells fusing primarily silicon, oxygen, neon, carbon, helium, and hydrogen.
Depending on its mass, a star may remain a red supergiant for up to a few million years. The fusion of each successive element in the core takes less time. A red supergiant with 25 times as much mass as the sun and a similar chemical composition might fuse helium for about 700,000 years, carbon for 1,000 years, neon for 9 months, oxygen for 4 months, and silicon for about 1 day. By the end of this series of fusion reactions, all the silicon in the star’s core has been fused to iron. The fusion of iron consumes energy, so the red supergiant can no longer produce energy through fusion. Without this energy pushing outward, the core collapses, resulting in a supernova—that is, a violent explosion of the entire star.
Some smaller red supergiants, those with less than roughly 11 times the sun’s mass, will stop fusing nuclei once they have exhausted the carbon in their cores. Rather than exploding in a supernova, this kind of red supergiant slowly sheds its outer layers, which form a planetary nebula. Eventually, the remaining core of the star becomes a type of burned-out star called a white dwarf.
See also Antares ; Betelgeuse ; Star (High-mass stars) .
