The Life Cycle of a Star With One Solar Mass

Protostars

• Every star arises from a nebula, a cloud of mostly hydrogen gas containing also some helium and dust. At some point, a gravitational collapse occurs, causing matter within the nebula to spin together, its particles moving faster, heating up and glowing. The result is a hot, shining ball of gas called a protostar.

Main Sequence

• As the core of the protostar gets hotter and denser it eventually reaches a temperature (about 10 million degrees Kelvin) sufficient to kick off the process of hydrogen fusion. Hydrogen atoms fuse into helium, releasing high-energy photons in the process. This radiation exerts an outward pressure which tips the scales against gravity, halting the collapse of the protostar. Equilibrium between inward and outward pressures is reached, and a star, as they say, is born.

  This first stage of the star's life is called the main sequence. It will last for about 90 percent of the star's existence. Our Sun is in its main sequence right now.

Red Giant

• The main sequence ends when the star's core runs out of hydrogen nuclei. Without the radiation pressure generated by hydrogen fusion, equilibrium is lost. The star's core, made up almost entirely of helium now, begins to collapse. As in the protostar stage, temperatures rise with increasing density.

  Some hydrogen remains in the outer shell of the star. Being farther out than the hydrogen at the core, it never reached a temperature high enough for nuclear fusion. It will reach that temperature now. As the core heats up, it heats the hydrogen shell the way a stovetop heats a kettle.

  As shell hydrogen fuses into helium, it generates radiation pressure. Because gravity is weaker in the shell than at the core, this outward-moving pressure overcomes gravity so that the outer layers of gas expand. They cool down and turn red as they drift away from the core. The star is now a red giant.

  The red giant's core will continue to rise in temperature until, at about 100 million Kelvin, helium begins to fuse into carbon and oxygen. The red giant phase will continue until there is no more helium in the core.

White Dwarf

• The end of the red giant phase is similar to the end of the main sequence. The core runs out of helium. Nuclear fusion ceases. The core begins to collapse and heat up, causing the helium in the outer shell to heat up also. Nuclear fusion occurs in the shell, causing it to expand.

  Meanwhile, the core, consisting mostly of carbon and oxygen now, simply keeps collapsing. Unlike the core of a much larger star, it will never reach the temperature required to fuse these heavier elements. Instead, it becomes a small, dense, relatively cool object known as a white dwarf. The remains of its shell surround it, a cloud of matter known as a planetary nebula.

Timeline

• Single-solar-mass stars live a very long time. Our Sun, for example, has been in its main sequence for 4.5 billion years and will continue in that phase for another four or five billion years. Once the Sun runs out of core hydrogen, its conversion into a red giant will take about 250 million years.