21.4 Life Cycles of Stars

What changes do stars undergo in their lifetimes?

Stars have a life cycle, just like people: they are born, grow, change over time, and eventually grow old and die. Most stars change in size, color, and class at least once in their lifetime. What astronomers know about the life cycles of stars is because of data gathered from visual, radio, and X-ray telescopes.

Star Formation

As discussed in the chapter The Solar System, stars are born in clouds of gas and dust called nebulas, like the one shown in Figure below.

The Main Sequence

For most of a star’s life, nuclear fusion in the core produces helium from hydrogen. A star in this stage is a main sequence star. This term comes from the Hertzsprung-Russell diagram shown in the Figure above. For stars on the main sequence, temperature is directly related to brightness. A star is on the main sequence as long as it is able to balance the inward force of gravity with the outward force of nuclear fusion in its core. The more massive a star, the more it must burn hydrogen fuel to prevent gravitational collapse. Because they burn more fuel, more massive stars have higher temperatures. Massive stars also run out of hydrogen sooner than smaller stars do.

Our Sun has been a main sequence star for about 5 billion years and will continue on the main sequence for about 5 billion more years (Figure below). Very large stars may be on the main sequence for only 10 million years. Very small stars may last tens to hundreds of billions of years.

Red Giants and White Dwarfs

As a star begins to use up its hydrogen, it fuses helium atoms together into heavier atoms such as carbon. A blue giant star has exhausted its hydrogen fuel and is in a transitional phase. When the light elements are mostly used up, the star can no longer resist gravity and starts to collapse inward. The outer layers of the star grow outward and cool. The larger, cooler star turns red in color and so is called a red giant.

Eventually, a red giant burns up all of the helium in its core. What happens next depends on how massive the star is. A typical star, such as the Sun, stops fusion completely. Gravitational collapse shrinks the star's core to a white, glowing object about the size of Earth, called a white dwarf (Figure below). A white dwarf will ultimately fade out.

Supergiants and Supernovas

A star that runs out of helium will end its life much more dramatically. When very massive stars leave the main sequence, they become red supergiants (Figure below).

Unlike a red giant, when all the helium in a red supergiant is gone, fusion continues. Lighter atoms fuse into heavier atoms up to iron atoms. Creating elements heavier than iron through fusion uses more energy than it produces, so stars do not ordinarily form any heavier elements. When there are no more elements for the star to fuse, the core succumbs to gravity and collapses, creating a violent explosion called a supernova (Figure below). A supernova explosion contains so much energy that atoms can fuse together to produce heavier elements such as gold, silver, and uranium. A supernova can shine as brightly as an entire galaxy for a short time. All elements with an atomic number greater than that of lithium were formed by nuclear fusion in stars.

Neutron Stars

After a supernova explosion, the leftover material in the core is extremely dense. If the core is less than about four times the mass of the Sun, the star becomes a neutron star (Figure below). A neutron star is more massive than the Sun, but only a few kilometers in diameter. A neutron star is made almost entirely of neutrons, relatively large particles that have no electrical charge.

Black Hole

If the core remaining after a supernova is more than about five times the mass of the Sun, the core collapses into a black hole. Black holes are so dense that not even light can escape their gravity. With no light, a black hole cannot be observed directly. But a black hole can be identified by the effect that it has on objects around it, and by radiation that leaks out around its edges.

Summary

• Stars spend most of their lives on the main sequence, fusing hydrogen into helium for energy.
• As stars burn up their hydrogen and fuse helium into larger atoms they begin to collapse and become red giants. When the helium is gone they become white dwarfs.
• When a massive star has no more elements left to fuse it explodes as a supernova, from which the chemical elements heavier than lithium form.
• An extremely massive core will collapse after a supernova explosion to become a black hole, which is black because no light can escape it.

Review

1. Why do some stars become red giants and others become supernovae?
2. Why are supernovae crucial to the evolution of the universe?
3. How does a star become a black hole? What are the characteristics of a black hole?

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