Red giant is a large, bright star that glows with a reddish light. Red giants range in diameter from about 10 times to 100 times the diameter of the sun. They shine tens to hundreds of times as bright as the sun does. Their reddish appearance results from relatively low surface temperatures, around 2000 to 4500 K. One kelvin (K) equals one Celsius degree above absolute zero (–273.15 °C). Well-known red giants include Arcturus , the fourth brightest star in the night sky, and Aldebaran, the brightest star in the constellation Taurus .
Formation.
Each red giant was once a star like our sun, called a main-sequence star . A main-sequence star 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 giant forms when a main-sequence star with about 1/2 to 8 times the sun’s mass (amount of matter) uses up all the hydrogen in its core. The exact range of masses depends on the star’s metallicity—that is, the proportion of elements heavier than helium that the star contains.
When the core runs out of hydrogen, fusion ceases there. 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. The extra energy causes the star’s outer layers to expand enormously. These layers cool as they expand, growing redder. As the star’s surface area grows, its brightness increases, and it becomes a red giant.
Evolution.
Depending on its mass and composition, a star may remain a red giant for a few hundred million to a few billion years. As the star changes into a red giant, 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. Soon after the star becomes a red giant, the core becomes hot enough to fuse helium nuclei into carbon nuclei.
In red giants with less than about 2.3 times the sun’s mass, helium fusion begins rapidly in an explosive event called the helium flash. During the helium flash, a star’s brightness increases briefly. The star’s size and brightness then decrease until the energy from the helium fusion halts the core’s contraction. The star then settles into a period of helium fusion. Stars with high metallicities remain red giants as they fuse helium. Stars with low metallicities contract further and grow hotter, becoming a type of star called a horizontal-branch star.
For red giants with masses greater than about 2.3 times that of the sun, helium fusion begins more smoothly. Such a star shrinks and dims slightly as helium fusion begins. When the energy from helium fusion stops the core’s contraction, the star becomes hotter and more orange in appearance. The star then grows cooler and redder again as it consumes helium.
Once a red giant has used up all the helium in its core, fusion there ceases and the star begins to contract again. The contraction begins a series of changes that ultimately transform the star into a type of burned-out star called a white dwarf . A horizontal-branch star will expand to become a red giant once more before eventually ending its evolution as a white dwarf.
The sun.
Astronomers predict that, around 5 billion years from now, the sun will become a red giant. Its outer layers will probably expand nearly to the current orbit of Mercury. The sun will remain a red giant for about a billion years. Over the roughly 100 million years that follow, it will develop into a horizontal-branch star and eventually cast off its outer layers to become a white dwarf.
See also Main-sequence star ; Star (Red giant phase) ; Sun (Evolution of the sun) .
