Earliest ever supernova sheds light on the first stars

Astronomers have captured a massive star exploding just moments after the universe emerged from the cosmic dark ages, shedding light on the birth and death of the first stars.

When stars run out of fuel and explode, they produce a burst of powerful light called a supernova. Supernovae may appear extremely bright in our local universe, but light from an exploding star in the early universe can take billions of years to reach Earth, after which it has dimmed and become too faint to see.

For this reason, astronomers can generally only see very distant supernovae in special cases, such as for Type Ic supernovae, which are stellar cores that have lost their external gas and produce an exceptionally bright gamma-ray burst. But the most typical Type II supernovae, which are the most common stellar explosions we see in our galaxy and occur when a massive star runs out of fuel, are normally too faint to see.

Now, David Coulter of Johns Hopkins University in Baltimore, Maryland, and colleagues have spotted a Type II supernova called SN Eos from when the universe was just a billion years old, using the James Webb Space Telescope.

The stellar explosion was fortunately placed behind a massive cluster of galaxies, whose powerful gravity amplified its light and made it dozens of times brighter than it would normally appear, and therefore easier to study in detail.

Researchers analyzed the spectrum of light coming from SN Eos, making it the first spectroscopically confirmed supernova. The results clearly show that it is a Type II supernova, which means it must have come from a massive star.

It also shows that the star that produced it contained very small amounts of elements other than hydrogen or helium – less than 10% of the amounts found in our Sun. This is what astronomers think the early universe looked like, because several generations of stars didn’t have much time to form, die, and produce heavier elements.

“This immediately tells us what type of stellar population [the star] “High-mass stars explode very, very quickly after birth,” says Or Graur of the University of Portsmouth, UK. In cosmological terms, about a million years is nothing. So they tell you about the star formation happening in this galaxy.

When we see light at these distances, it usually comes from small galaxies, where you can infer the average properties of the stars that might be in those galaxies. But studying individual stars at such distances is not usually possible, says Matt Nicholl of Queen’s University in Belfast, UK.

“We can see this individual star, with beautiful data, at a [distance] where we have never seen an isolated supernova and the data is good enough to see that the stars are different from most stars in the local universe,” he says.

This would have happened just a few hundred million years after a period in the history of the universe known as the reionization epoch, Graur explains. That’s when light from the first stars began stripping electrons from neutral hydrogen, which blocks most forms of radiation, and turning it into ionized hydrogen, which is transparent. Before then, the universe was opaque, so SN Eos is actually as distant a supernova as we could hope to see.

“It’s very, very close to this reionization period where the universe came out of its short, dark period and photons were able to flow freely again and we were able to see things,” Graur explains.