Shooting stars (Image: X-ray: NASA/CXC/SAO; Infrared: NASA/JPL-Caltech; Optical: MPIA, Calar Alto, O. Krause et al)

Supernova shoot-em-ups could be responsible for Earth’s iron core. An analysis suggests that certain stars fire off massive iron bullets when they die.

Stars fuse the hydrogen and helium present in the early universe into heavier elements, like iron. When stars reach the end of their lives, they explode in supernovae, littering these elements throughout space where they can eventually form planets.

A particular kind of supernova called a type Ia, the result of the explosion of a dense stellar corpse called a white dwarf star, seems to be responsible for most of the iron on Earth.


These stars also play an important role in our understanding of distance in the universe. That’s because the white dwarfs only blow up when they reach a certain, fixed mass, so we can use the light of these explosions as a “standard candle” to tell how far away they are.

But astronomers still haven’t figured out exactly what causes white dwarfs to hit this critical limit.

“Most of our iron on Earth comes from supernovae of this kind,” says Noam Soker of the Technion Israel Institute of Technology in Haifa. “It is embarrassing that we still don’t know what brings these white dwarfs to explode.”

Lumpy stars

When a star goes supernova, it leaves behind a cloud of ejected material called a supernova remnant. This remnant should be spherical – but some have extra bumps that could offer a clue to the supernova’s origin.

Now Soker and his colleague Danny Tsebrenko say that massive clumps of iron produced within a white dwarf in the process of going supernova could be punching through the remnant like bullets, creating these bumps. The iron bullets aren’t solid chunks of metal, but a more diffuse cloud of molecules.

Some supernova remnants have two bumps on opposite sides, which the researchers call “ears”.

The iron bullets form along the rotation axis of an exploding white dwarf, firing out at either end, says Soker. A white dwarf can only be spinning fast enough to allow this if it is the result of two smaller dwarfs merging, he adds.

The bullets could also shed light on our origins. Soker and Tsebrenko estimate that these clouds of iron would be several times the mass of Jupiter. They would spread and could eventually seed dust clouds with iron that would go on to form stars and planets, providing an origin for Earth’s core, says Soker.

Reference: arxiv.org/abs/1505.02034v1