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Starts With A Bang

Surprise! The Universe has a third way to form black holes

It isn’t just supernovae or merging neutron stars. In fact, it’s the quietest way of all!

“N6946-BH1 is the only likely failed supernova that we found in the first seven years of our survey. During this period, six normal supernovae have occurred within the galaxies we’ve been monitoring, suggesting that 10 to 30 percent of massive stars die as failed supernovae.” –Scott Adams

When a massive enough star runs out of fuel in its core and collapses, the resulting Type II supernova will produce a black hole.

Cassiopeia A in X-ray light from the Chandra X-ray Observatory. It is conceivable that there is a black hole remnant at the core of this object, although the evidence is not indisputable. Image credit: NASA / CXC.

Supernovae that aren’t quite massive enough will produce neutron stars instead, which themselves will make black holes if they either accrete more matter or collide with another neutron star.

Two neutron stars colliding, which is the primary source of many of the heaviest periodic table elements in the Universe. About 3–5% of the mass gets expelled in such a collision; the rest becomes a single black hole. Image credit: Dana Berry, SkyWorks Digital, Inc.

These two processes both enrich the Universe with heavy elements: supernovae with elements like iron, silicon, sulphur and phosphorous, while neutron star collisions create gold, mercury, lead and uranium.

Illustration of a black hole tearing apart and devouring a star. Supernova explosions or neutron star mergers (which create gamma ray bursts) should expel or kick a binary companion. The observations of black hole binaries hint at a third way. Image credit: Dana Berry/NASA.

But in theory, there should be a third way: through direct collapse.

Distant, massive quasars show ultramassive black holes in their cores. It’s very difficult to form them without a large seed, but a direct collapse black hole could solve that puzzle quite elegantly. Image credit: J. Wise/Georgia Institute of Technology and J. Regan/Dublin City University.

If a massive enough gas cloud collapses under its own gravity, it should form a black hole directly, without any intervening star.

An ultra-distant quasar showing plenty of evidence for a supermassive black hole at its center. How that black hole got so massive so quickly is a topic of contentious scientific debate. Image credit: X-ray: NASA/CXC/Univ of Michigan/R.C.Reis et al; Optical: NASA/STScI.

This is one of the leading theories for how supermassive black holes begin, including at such early times in the ultra-distant Universe.

Simulations of various gas-rich processes, such as galaxy mergers, indicate that the formation of direct collapse black holes should be possible. But none have ever been directly observed until now. Image credit: L. Mayer et al. (2014), via

If direct collapse is possible, we should see some massive stars with just the right properties disappearing with no explosion.

The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation. Image credit: NASA/ESA/C. Kochanek (OSU).

For the first time, astronomers observed a 25 solar mass star just disappear.

There was a brief brightening in the optical, corresponding to a ‘failed supernova’, but then the luminosity plummeted to zero, where it has remained. Image credit: NASA/ESA/P. Jeffries (STScI).

Direct collapse is the only explanation possible.

The 30-ish solar mass binary black holes first observed by LIGO are very difficult to form without direct collapse. Now that it’s been observed, these black hole pairs are thought to be quite common. Image credit: LIGO, NSF, A. Simonnet (SSU).

As many as 30% of massive stars should become black holes in this way, which is now verified for the first time.

Mostly Mute Monday tells the astronomical story of an object, process or phenomenon in images, visuals and no more than 200 words.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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