Astronomy & Astrophysics 101: Red Giant

After a star exhausts its hydrogen fuel for nuclear fusion and starts to die, a red giant emerges.

The delicate interplay between a star's inherent gravitational pull and the pressure from continuing thermonuclear fusion reactions at its core allows stars to remain stable.

That balance is destroyed when a star's core runs out of hydrogen, and the core starts to collapse. The plasma shell that surrounds the core heats up enough to start fusing hydrogen as the core collapses. The star's outer layers dramatically expand as fusion in this shell starts, and the surface can grow up to several hundred times larger than the star's original size. The swollen surface of the star cools and changes color from white or yellow to red as the energy at its surface dissipates much more. A red giant develops.

It may take hundreds of millions of years for this process to complete. It only applies to stars of intermediate mass (80% to 800% of the mass of the Sun or less), which go on to generate planetary nebulae. A more massive star turns into a red supergiant before exploding as a supernova when the hydrogen in its core runs out.

After a star exhausts its hydrogen fuel for nuclear fusion and starts to die, a red giant develops. Credit: NASA and the ESA

Hubble uses red giant stars to determine the separations between various galaxies. By contrasting the brightness of the galaxies' red giant stars with nearby red giants, whose distances have been determined by other means, astronomers can establish how far away galaxies are. Red giants can be used as a "standard candle" to measure distance because they are trustworthy milepost markers and all attain the same peak brightness in their late evolution. Hubble can see red giants in the stellar halos of galaxies thanks to its exceptional resolution and sensitivity.

U Camelopardalis, which emits a roughly spherical shell of gas every few thousand years as a layer of helium around its core starts to fuse, has been seen by Hubble. Due to Hubble's sensitivity, the gas shell, which is both larger and fainter than its parent star, is visible in fine detail. The telescope has seen that the shell of gas emitted from this red giant is almost precisely spherical, despite the fact that events that occur at the conclusion of stars' lives are sometimes highly irregular and unstable.

The red giant R Sculptoris was also photographed by the Hubble Space Telescope, providing insight into what the Sun is expected to look like in a few billion years. It has also aided astronomers in their understanding of the distribution of the components that make up the universe. Red giants eventually form in the later stages of the lives of all stars with initial masses up to around eight times that of the Sun. As a constant, dense wind emanates from the star, they begin to cool down and lose a significant percentage of their mass. Red giants like R Sculptoris contribute a significant amount of the dust and gas needed for the creation of new generations of stars and planets due to their ongoing material loss.

Additionally, Hubble has shown to us the stunning and vibrant remains of past red giants. NGC 2371, NGC 2022, and NGC 5307 are a few examples.

The hourglass-shaped Southern Crab Nebula, which was created by the interaction of a red giant and a white dwarf, was the subject of Hubble's 29th anniversary photograph. Before it too spends its final years as a white dwarf, the red giant is shedding its outer layers in the final stage of its life.

By ESA/HUBBLE