How Long Do Stars Last?
Written by Fraser Cain
Artist's impression of a red giant star. Image credit: ESO
The mass of a star defines its lifespan. The least massive stars will live the longest, while the most massive stars in the Universe will use their fuel up in a few million years and end in a spectacular supernova explosion. So, how long do stars last?
There are factors that will define how long a star will survive; how quickly they burn through the hydrogen fuel in their cores, and whether they have any way to keep the fuel in their core mixed up. Our own Sun has three distinct layers, the core, where nuclear fusion takes place, the radiative zone, where photons are emitted and then absorbed by atoms in the star. The final zone is the convective zone. In this region, hot gas from the edge of the radiative zone is carried upwards to the surface of the star in columns of hot plasma.
Let's star with the largest stars. The largest possible stars probably have 150 times the mass of the Sun; for example, the monster Eta Carinae located about 8,000 light years from here. Eta Carinae was probably formed less than 3 million years ago. It consumes fuel so fast in its core that it gives off 4 million times the energy of the Sun. Astronomers think that Eta Carinae has less than 100,000 years to live. In fact, it could detonate as a supernova any day now…
As stars get smaller, they live longer. Our own Sun has been around for 4.5 billion years, slowly turning hydrogen into helium at its core. The Sun will run out of this hydrogen fuel in another 5 billion year or so, and it will turn into a red giant. It will expand to many times its original size and then eject its outer layers and shrink down to a tiny white dwarf star, a dense object the size of the Earth. So the total lifespan of a star with the mass of the Sun is about 10 billion years.
The smallest stars are the red dwarfs, these start at 50% the mass of the Sun, and can be as small as 7.5% the mass of the Sun. A red dwarf with only 10% the mass of the Sun will emit 1/10,000th the amount of energy given off by the Sun. Furthermore, red dwarfs lack radiative zones around their cores. Instead, the convective zone of the star comes right down to the cure. This means that the core of the star is continuously mixed up, and the helium ash is carried away to prevent it from building up. Red dwarf stars use up all their hydrogen, not just the stuff in the core. It's believed that the smaller red dwarf stars will live for 10 trillion years or more.
How long do stars last? The biggest stars last only millions, the medium-sized stars last billions, and the smallest stars can last trillions of years.
We have written many articles about stars here on Universe Today. Here's an article about the biggest star in the Universe. And here's an article about how the Earth won't survive when the Sun becomes a red giant.
Want more information on stars? Here's NASA's World Book on Stars, and more information from NASA's imagine the Universe.
We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?
Filed under: Astronomy
