The life of a Sun-like star

In about 5 billion years the Sun will swell into a red giant large enough to engulf Mercury and Venus.

Born in a cold cloud

Stars form in molecular clouds, vast regions of cold gas and dust. NASA describes them as holding from 1,000 to 10 million times the Sun's mass and spanning up to hundreds of light-years. Because the gas is cold, it clumps. Some clumps collide or gather more material, their gravity grows, and eventually they collapse. As a clump shrinks, it heats up and starts to spin faster, flattening the leftover gas into a disk. The hot, collapsing centre is a protostar. At this point its energy comes from the squeeze itself, not from fusion. The Sun formed this way about 4.6 billion years ago, and the disk around it became the planets.

When the core reaches millions of degrees, hydrogen fusion switches on and the outward push of energy halts the collapse. The star has joined the main sequence, the longest stage of its life. Mass sets the pace. NASA explains that more massive stars must burn their fuel faster to hold themselves up, so they shine brighter and die sooner, while low-mass stars burn longer, dimmer and cooler. The Sun's main-sequence life is about 10 billion years in total, and it is a little less than halfway through.

Running out of hydrogen

When the core runs out of hydrogen, fusion there stops and gravity wins for a while. The core shrinks and heats up, and hydrogen starts fusing in a shell around it. That extra energy makes the outer layers swell enormously and cool, and the star becomes a red giant, which NASA notes looks more orange than red. For the Sun this begins in about 5 billion years. NASA says the red giant Sun will become so large that it will engulf Mercury and Venus, and possibly Earth as well. Arcturus, about 25 Suns wide, shows what this stage looks like today.

Red giants become unstable and pulsate, puffing out and throwing off parts of their atmosphere. Eventually all the outer layers drift away as an expanding, glowing shell called a planetary nebula. The name dates from small telescopes in which these round shells looked a little like planets; they have nothing to do with planets. NASA's example is the Helix Nebula, 650 light-years away in Aquarius. What remains in the middle is the star's exposed core.

A white dwarf

That core is a white dwarf, usually about the size of Earth but hundreds of thousands of times more massive, as NASA puts it. It makes no new heat, so it slowly cools over billions of years, glowing from blue-white through to red. Sirius B is a nearby example: in the model, the Earth sitting next to the white dwarf at the same scale is slightly larger. Stars below about eight times the Sun's mass end this way, which covers the great majority of stars.