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

Sky at Night Magazine - - BULLETIN -

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

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

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