Red Giants: Stars on the Way Out

Our star will swell to become a red giant on its way to becoming a planetary nebula. B. Jacobs/Wikimedia Commons

You may have heard of the term "red giant" before and wondered what it means. In astronomy, it refers to stars that are evolving toward their deaths. In fact, our Sun will become a red giant in a few billion years. 

How a Star Becomes a Red Giant

Stars spend much of their lives converting hydrogen into helium in their cores. Astronomers refer to this period as the "main sequence". Once the hydrogen that fuels this fusion process is gone, the star's core begins to shrink in on itself. That makes the temperature hotter. All the extra energy moves out from the core and pushes the outer envelope of the star outward, like air expanding a balloon. At that point the star has become a red giant.

Properties of a Red Giant

Even if the star is a different color, like our yellow-white Sun, the resulting giant star will be red. This is because as a star increases in size its average surface temperature decreases and the wavelength of light it puts out (its color) will be mostly red.

The red giant phase comes to an end once the core temperature gets so high helium begins fusing into carbon and oxygen. The star shinks, and becomes a yellow giant. 

Not Everyone Gets to be a Giant: It's an Exclusive Club

Not all stars will become red giants. Only stars will with masses between about half and six times the mass of our Sun will eventually evolve into red giants. Why is this? 

Smaller stars transfer energy from their cores to their surfaces by the process of convection, which spreads the helium created by fusion throughout the star. The process of fusion ends at helium and the star "stagnates". But, it doesn't get hot enough to become a red giant. 

Usually, we ascertain the fate of stars by studying them at different evolutionary states and mapping out their probable life cycles, which are compared to theoretical models of the physical interactions and mechanisms of the star. However, the smaller a star is the longer that it spends doing hydrogen fusion in its core. Theoretically, stars smaller than about a third of our Sun's mass would have lifetimes greater than the current age of the Universe. So, we haven't seen any go farther than hydrogen fusion. 

Planetary Nebulae

Low- and medium-mass stars, like our Sun, become red giants and evolve to become planetary nebulae.

When the core begins to fuse helium into carbon and oxygen the star becomes highly volatile. Even very small changes in core temperature will have a dramatic effect on the rate of nuclear fusion.

Should the core temperature get too high, either by random dynamics in the core, or because of the amount of helium that has been fused, the runaway fusion rate that results will once again push the outer envelope of the star out into the interstellar medium. This puts the star into a second red giant phase. Because of the ever-increasing core temperature and because the star has become so large, its outer layers lift away and expand out to space. That cloud of material creates a planetary nebula around the core of the star.

Eventually all that's left of the star is a core made of carbon and oxygen. Fusion stops. And, the core becomes a white dwarf. It continues to smolder for billions of years. Eventually, the glow from the white dwarf will also fade, and there will only be a cool, dim ball of carbon and oxygen left behind.

High-mass Stars

Larger stars do not enter a normal red giant phase. Instead, as heavier and heavier elements are fused in their cores (up to iron) the star oscillates between various supergiant star phases, including the related red supergiant.

Eventually, these stars will exhaust all of the nuclear fuel in their cores. When it gets to iron, things go catastrophic. The fusion of iron takes more energy than it produces, which stops the fusion and causes the core to collapse.

Once this occurs the star will start down the path leading to a Type II supernova, leaving either a neutron star or black hole behind.

Think of red giants as way stations in the life of an aging star. Once they go red, there's no going back.