by Sarah Scoles
When really massive stars die, they collapse into black holes. But no one has actually observed this process in action. So what happens, exactly, when a star stops being a star and starts being the universe's weirdest object? How does it look, and what signals would allow scientists to point at some tiny spot in the sky and say, "Hey, right there, a black hole was just born"?
A new paper suggests one such signal, and, almost as importantly, this paper has a great title: "Taking the 'Un' out of 'Unnovae.'" Good job, sole author theoretical astrophysics postdoctoral researcher Anthony L. Piro.
Piro suggests that the death of a star and the subsequent birth of a black hole--the Simba-style circle of cosmic life--is marked by a specific type of flash.
Wait, did you just use the word 'unnova' and proceed to write another sentence before defining it?
Sorry. While some stars-turning-black-holes produce spectacular explosions that are supernovae or supernova-like and/or include ultra-energetic gamma-ray bursts, others may simply implode and disappear without all the fanfare. This "now it's there, now it's not" version is the theoretical-and-as-yet-unobserved unnova. As in, not a nova.
But it is something, even if it is not a nova. When a massive star is turning into a black hole, it collapses into a neutron star first, its protons and electrons so squished together that they combine into neutrons. As the star is essentially cramming itself into a medium-sized city's limits, it is also releasing neutrinos--nearly massless particles that travel almost as fast as light and don't interact much with the rest of the universe. As a result, the star loses about a tenth of its mass in the course of a few seconds.
Consider losing a tenth of your mass. Like you would, the star feels its loss of gravitational strength. While you would probably just run faster or jump higher or be really hungry, the star can no longer hold on to its outer layers. As they expand, they create a shock wave in the star's envelope, which is consequently ejected. When the wave reaches the star's surface, the surface heats up and glows. It's a bit like a slow-mo, low-energy supernova...which is not a supernova at all and is, in fact, an unnova. The resulting light, though it would last for about a year, is too faint for our telescopes to reliably identify at the moment.
Piro's paper asks the question, "Is there a brighter signature within the unnova-ing process that scientists could latch onto?"
The short answer: Yes! A flash.
The longer answer: When the shock wave hits the star's surface, he predicts there will be a blast of light. Not nearly as bright as a supernova, but 10-100 (I know, I know, check out that range) times brighter than the "glow."
In other words, if scientists can find instances of this flash, which Piro estimates should occur detectably once per year, they will be able to say, "Look at that unnova! Probably," and will suspect that they have seen a black hole being born. And who doesn't want to be in the delivery room to witness that?
Stats on the flash:
- it would last 3-10 days (so "flash" is perhaps not the best word to describe it, except to astronomers, who think on million-year timescales at the smallest)
- it would equate to a temperature of 10,000 kelvin, or 17,540F
- it would travel at a max speed of 200 km/second or 124 miles/second
- it wouldn't show any evidence of nuclear synthesis, but hydrogen would be prevalent
- it would have a luminosity of 10^40-41 ergs/second, or 2.6 million-26 million times as bright as the Sun
- it would be brightest in UV light but a similarly strong blue component
So keep your transient-identifying, world-class UV-sensitive telescope's eyes open, and maybe you'll be the first one to see a black hole progenitor's final moments, before it Houdinis itself out of the visible universe.
Piro, A. (2013). TAKING THE “UN” OUT OF “UNNOVAE” The Astrophysical Journal, 768 (1) DOI: 10.1088/2041-8205/768/1/L14