Massive stars, supernovae and what they leave

When a massive star's core turns to iron, it runs out of fuel within days and collapses.

Living fast

NASA divides stellar fates at about eight times the Sun's mass. A star above that line starts like any other, collapsing out of a cloud and fusing hydrogen on the main sequence, but it sits at the hot, bright upper left of the HR diagram. It burns its fuel at a ferocious rate and, as NASA puts it, the whole process takes just a few million years, against about 10 billion for the Sun. Betelgeuse, about 15 times the Sun's mass according to NASA, is roughly 10 million years old and already a red supergiant.

Layers like an onion

When the core runs out of hydrogen, a massive star fuses helium into carbon, as smaller stars do. But its core is heavy enough to keep going. After helium it fuses carbon into neon, then neon into oxygen, oxygen into silicon, and finally silicon into iron. Each new fuel burns in the core while the earlier ones keep burning in shells around it, so the star's interior ends up layered like an onion, with iron at the centre. NASA notes that each new fuel buys less and less time. By the time silicon is fusing into iron, the star runs out of fuel in a matter of days.

Iron is the end of the line. Fusing iron into anything heavier takes energy instead of releasing it, so the core loses the push that held it up. It collapses in a fraction of a second until the nuclei are packed so tightly that they resist, and the core rebounds. The rebound drives a shock wave out through the star, and the result is a supernova, an explosion that can briefly outshine a whole galaxy. NASA points out that the material it throws out enriches future clouds, so the next generation of stars and planets contains elements made inside earlier stars.

Neutron stars and black holes

What survives depends on mass. NASA says a star of about 8 to 20 solar masses leaves a neutron star: more than the Sun's mass packed into a sphere about as wide as Manhattan Island is long, roughly 20 km. Its density is beyond anything on Earth; spreading 1.4 Suns of mass through a ball 10 km in radius gives about 7 × 10¹⁷ kg per cubic metre. Many neutron stars spin rapidly and sweep beams of radiation across space like a lighthouse; when a beam crosses Earth we see a pulsar. Some, called magnetars, have magnetic fields up to a thousand times stronger than a typical neutron star's.

For stars heavier than about 20 Suns, NASA says the collapse produces a stellar-mass black hole, where gravity is so strong that not even light escapes from inside the event horizon. For a black hole of one solar mass that horizon would be only about 3 km in radius. Black holes are found by their effect on nearby matter: gas pulled from a companion star heats up in a swirling disk and glows in X-rays. NASA expects Betelgeuse to end as either a neutron star or a black hole, depending on how much mass remains after its supernova, probably about 100,000 years from now.