Ask Ethan: When Do Black Holes Become Unstable?

The simulated decay of a black hole not only results in the emission of radiation, but the decay of the central orbiting mass that keeps most objects stable. Black holes are not static objects, but rather change over time.EU’S COMMUNICATE SCIENCE

There are quite a few ways to make the black holes we know about in the Universe, from core-collapse supernovae to merging neutron stars to the direct collapse of tremendous amounts of matter. On the smallest end, we know of black holes that may be merely 2.5-to-3 times the mass of our Sun, while on the largest end, supermassive ones in excess of 10 billion solar masses reside at the centers of galaxies. But is that it? And how stable are black holes of different masses? That’s what Nyccolas Emanuel wants to know, as he asks:

Is there a critical size for black hole stability? [A] 1012 kg [black hole] is already stable for a couple of billion years. However, a [black hole] in the range of 105 kg, could explode in a second, thus, definitely not stable… I guess there is a critical mass for a [black hole] where the flow of gained matter will equal to the Hawking evaporation?

There’s a lot going on here, so let’s unpack it all.

Black holes will devour whatever matter they encounter. Although this is a great way for black holes to grow, Hawking radiation also ensures that black holes will lose mass. Deriving when one defeats the other is not a trivial task.X-RAY: NASA/CXC/UNH/D.LIN ET AL, OPTICAL: CFHT, ILLUSTRATION: NASA/CXC/M.WEISS

The first thing to start with is the stability of a black hole itself. For any other object in the Universe, astrophysical or otherwise, there are forces that hold it together against whatever the Universe might do to try and tear it apart. A hydrogen atom is a tenuously held-together structure; a single ultraviolet photon can destroy it by ionizing its electron. An atomic nucleus needs a much higher-energy particle to blast it apart, like a cosmic ray, an accelerated proton, or a gamma-ray photon.

But for larger structures, like planets, stars or even galaxies, the gravitational forces holding them together are enormous. Normally, it takes either a runaway fusion reaction or an incredibly strong, external gravitational pull — such as from a passing star, black hole, or galaxy — to tear such a megastructure apart.

NGC 3561A and NGC 3561B have collided and produced huge stellar tails, plumes and even possibly “ejecta” that are condensing to make tiny “new” galaxies. Hot young stars glow blue where rejuvenated star formation is taking place. Forces, such as those between galaxies, can rip stars, planets, or even entire galaxies apart. Black holes, however, will remain.ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA

For black holes, however, something is fundamentally different. Rather than their mass being distributed over a volume, it’s compressed down into a singularity. For a non-rotating black hole, that’s just a single, zero-dimensional point. (For rotating ones, it’s not much better: an infinitely-thin, one-dimensional ring.)

Furthermore, all of the mass-and-energy-containing contents of a black hole are contained within an event horizon. Black holes are the only objects in the Universe that contain an event horizon: a boundary where, if you slip within it, it’s impossible to escape. No acceleration, and hence no force, no matter how strong, will ever be able to pull matter, mass, or energy from inside the event horizon outside to the Universe beyond.

Artist’s impression of the active galactic nucleus. The supermassive black hole at the center of the accretion disk sends a narrow high-energy jet of matter into space, perpendicular to the disc. A blazar about 4 billion light years away is the origin of many of the highest-energy cosmic rays and neutrinos. Only matter from outside the black hole can leave the black hole; matter from inside the event horizon can ever escape.DESY, SCIENCE COMMUNICATION LAB