Red giant

Look at the bright orange star Arcturus in spring, or Aldebaran in the eye of Taurus in winter, and you are seeing a red giant: a star in the late stage of its life, swollen far beyond its original size. One day the Sun will look like that.

How a star becomes a giant

A star like the Sun spends most of its life fusing hydrogen into helium in its core. Eventually the hydrogen there runs out. The core, now mostly helium, shrinks and heats up, and hydrogen keeps burning in a shell around it. That burning shell pours out more energy than the star made before, and the outer layers swell enormously and cool. The star becomes tens or even hundreds of times wider than before, but its surface, around 5 000 K (4 730 °C) or lower, glows orange or red.

Stars from about a third of the Sun’s mass up to about eight times it go through this stage. In stars up to about two solar masses, the helium core is so dense that when it finally reaches about 100 million degrees, helium ignites almost all at once, in an event called the helium flash. After a period of steady helium burning, the star swells again on the so-called asymptotic giant branch, before gently shedding its outer layers as a planetary nebula and leaving a white dwarf behind.

The Sun’s future

The Sun is about halfway through its hydrogen-burning life. In about 5 billion years it will begin to become a red giant. Mercury and Venus will be swallowed. Earth’s fate is less certain, because the Sun will also lose a lot of mass, loosening its grip on the planets. In 2008 Klaus-Peter Schröder and Robert Connon Smith calculated that the effect would not be enough: tidal forces and drag in the Sun’s swollen outer layers would pull Earth in. By their calculation, only a planet that today orbited at about 1.15 times Earth’s distance would survive.