Black Holes that may not have explosive origins
Most black holes start their life in a supernova, but there are some that have much quieter origins
Some stars, it seems, end their lives not with a bang, but with a whimper. When large stars – specifically those greater than about eight solar masses in size – run out of hydrogen at their core, they are unable to support themselves through nuclear fusion and they collapse. The result is a dramatic explosion which we all know as a supernova. That’s the process at its most basic. But the details are tricky.
Depending on the precise mass of the exploding star – and on the physics of the collapse – the result might be a neutron star, a black hole, or the total destruction of the star leaving no compact remnant behind at all. Stars that are only just big enough to go bang are believed to suffer the latter fate, producing supernovae but not black holes.
We do need a way to explain the creation of low mass black holes, though, and astronomers are on the case. They’ve been looking for stars that just seem to disappear without fuss, ending their lives without a supernova, instead collapsing directly into a black hole. The first – and so far only serious – candidate for such an event is a star called N6946BH1 in a galaxy called NGC 6946, which is about 18 million lightyears away.
N6946-BH1 was once a massive red supergiant like Betelgeuse and Antares, but in 2009 it briefly brightened and then faded away. Since then it’s been mostly visible in the infrared range, but even on this wavelength it is now gradually fading away. This infrared emission is believed to come from dust in the ejected outer envelopes of the star, with the central parts having collapsed to form a black hole.
Understanding this system could therefore give us clues as to how black holes form, but studying it is difficult without knowing more about the progenitor star. Instead, a team of US-based astronomers has been looking hard at its neighbourhood. The idea is that other, nearby stars offer easy ways to work out the age of a population. You can study what’s called the main sequence turnoff, for example: the luminosity above which all the stars have stopped ‘burning’ hydrogen in their cores. Or you can look at
“This infrared emission is believed to come from dust in the ejected outer envelope of the star, with the central parts having collapsed to form a black hole”
easy-to-understand, helium burning stars and use them to calculate how recently star formation has been taking place.
When they bring all the tools of modern stellar astrophysics to bear on the problem, they find that the last great burst of star formation in the region happened about 10.5 million years ago. So, making what seems a reasonable assumption that N6946BH1 hasn’t just wandered in from some other region of space completely, that means 10.5 million years is how old it is likely to be as well. If it that old, and it just reached the end of its life, it must have had a mass of about 18 times that of the Sun.
Assuming that’s right, we now know of one kind of star capable of producing, quietly, a black hole at the end of its life. The errors on the measurement are large though, and there’s not much more to be done, other than keep a close eye on nearer galaxies to see whether they too harbour vanishing stars.
CHRIS LINTOTT was reading… The Progenitor Age and Mass of the Black-Hole-Formation Candidate N6946BH1 by Jeremiah W Murphy et al. Read it online at https://arxiv.org/abs/1803.00024
2007 2015 N6946-BH1 before and after: in 2009 it flared to over a million times brighter than the Sun then imploded into a black hole
CHRIS LINTOTT is an astrophysicist and co-presenter of The Sky at Night on BBC TV. He is also the director of the Zooniverse project