Have we spotted the birth of a black hole?

Astronomers using a small telescope on top of California’s Mt Palomar, outside of San Diego, have detected not one but two examples of a new type of supernova explosion, witnessing, possibly for the first time, the birth of black holes.

Both discoveries were made by a sky survey called the Zwicky Transient Facility, which uses a relatively small telescope (1.22 metre) to scan the entire northern sky every three days looking for anything out of the ordinary.

The first supernova was observed on 8 June 2019, in a galaxy a billion light-years away. The second was seen on 11 February 2021 in another galaxy, coincidentally about the same distance away.

What made them interesting was that follow-up observations showed that the debris blasted outward by the explosions was rich in carbon, oxygen and neon.

This had never been seen before in supernova explosions, says Avishay Gal-Yam of the Weizmann Institute of Science, in Rehovot, Israel, who reported his findings at a press conference sponsored by the American Astronomical Association (AAS) on 12 January. His study is published in Nature.

But, Gal-Yam says, it is consistent with the explosion of a massive type of star called a Wolf-Rayet star, in which fusion has progressed so far that instead of being primarily composed of the two lightest elements, hydrogen and helium, the star’s outer layers are rich in heavier elements such as carbon, oxygen and neon.

Such stars are a common type of supermassive stars, and it has long been believed that they should explode at the end of their lives, Gal-Yam says, but none had ever before been observed to do so: which raises the question of why such explosions haven’t been seen before.

The answer, says Daniel Perley, an astrophysicist at Liverpool John Moores University who studied the 2021 explosion, is that these stars don’t truly detonate the way one might expect. Rather, they blow off only a small fraction of their mass, with the rest rapidly collapsing into a black hole. Perley’s study has been accepted in Astrophysical Journal.

The clue to puzzling this out, Perley said at the AAS press conference, came from studying changes in the second supernova’s brilliance over the course of two to three months.

Normally, he says, it takes months for supernova brilliance to fade. That’s because material from the initial blast continues to be heated by the decay of highly radioactive material thrown out from deeper in the star.

But in this case, the supernova’s brilliance dropped incredibly rapidly, so fast that it could no longer be seen in about 80 days.

What that appears to mean, he says, is that only a small portion of the star’s mass was blown away by the explosion, and the explosion was only seen at all because this material slammed into the star’s pre-blast solar wind. That created a hot, bright shock wave that quickly faded and vanished.

As for the rest of the star’s mass? It appears simply to have vanished – i.e., it collapsed into a black hole.

It’s an exciting find, Perley adds, because it shows that Wolf-Rayet stars can indeed form black holes… and might even do so invisibly, if the pre-explosion solar wind isn’t strong enough to produce the super-bright shock wave seen in these two cases.

“This could be happening more often than we realise,” he says.