Red Giant Stars: Facts, Definition & the Future of the Sun

Expanding red giant stars will swallow too-close planets. In the solar system, the sun will engulf Mercury and Venus, and may devour Earth, as well.

Credit: James Gitlin/STScI AVL

A red giant star is a dying star in the last stages of stellar evolution. In only a few billion years, our own sun will turn into a red giant star, expand and engulf the inner planets, possibly even Earth. What does the future hold for the light of our solar system and others like it?

Most of the stars in the universe are main sequence stars — those converting hydrogen into helium via nuclear fusion. A main sequence star may have a mass between a third to eight times that of the sun and eventually burn through the hydrogen in its core. Over its life, the outward pressure of fusion has balanced against the inward pressure of gravity. Once the fusion stops, gravity takes the lead and compresses the star smaller and tighter.

Temperatures increase with the contraction, eventually reaching levels where helium is able to fuse into carbon. Depending on the mass of the star, the helium burning might be gradual or might begin with an explosive flash.

“Although fusion is no longer taking place in the core, the rise in temperature heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion, producing more energy than when it was a main sequence star,” the Australia Telescope National Facility says on their website.

Red giant stars reach sizes of 100 million to 1 billion kilometers in diameter (62 million to 621 million miles), 100 to 1,000 times the size of the sun today. Because the energy is spread across a larger area, surface temperatures are actually cooler, reaching only 2,200 to 3,200 degrees Celsius (4,000 to 5,800 degrees Fahrenheit), a little over half as hot as the sun. This temperature change causes stars to shine in the redder part of the spectrum, leading to the name red giant, though they are often more orangish in appearance.

This picture of the dramatic nebula around the bright red supergiant star Betelgeuse was created from images taken with the VISIR infrared camera on ESO’s Very Large Telescope (VLT). This structure, resembling flames emanating from the star, forms because the behemoth is shedding its material into space.

Credit: ESO/P. Kervella

In 2017, an international team of astronomers identified the surface of the red giant π Gruis in detail using the European Southern Observatory’s Very Large Telescope. They found that the red giant’s surface has just a few convective cells, or granules, that are each about 75 million miles (120 million kilometers) across. By comparison, the sun has about two million convective cells about 930 miles (1,500 km) across.

Stars spend approximately a few thousand to 1 billion years as a red giant. Eventually, the helium in the core runs out and fusion stops. The star shrinks again until a new helium shell reaches the core. When the helium ignites, the outer layers of the star are blown off in huge clouds of gas and dust known as planetary nebulae. These shells are much larger and fainter than their parent stars.

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