Starguy Speaks

on visible light from a Black Hole


Astronomers from Kyoto University in Japan have made an announcement that’s being reported as “Visible Light from a Black Hole Spotted by Telescope, a First” and “How to spot a black hole from your back garden: Researchers say astronomical phenomena CAN be seen using visible light”. But black holes are supposed to be black. They are supposed to be places where nothing, not even light, can escape. So what’s going on?

Well, if we’re nitpicking, the headlines are technically incorrect (the best kind of incorrect).

If you consider the black hole as just the, well, black hole… no, you can’t see it. What you learned about black holes – their gravity is so intense that it bends spacetime enough that even light cannot escape – is true. What astronomers are actually seeing when we say we can see a black hole, is light being emitted from material around the black hole. That is what Mariko Kimura and her colleagues are actually claiming to see here.

An accurate simulation of a black hole, from Interstellar, courtesy of Double Negative. The light from the accretion disk is HEAVILY distorted by the black hole.

Of course, if you consider the black hole as including the surrounding material, the headlines are still wrong, because this is decidedly not the first time we’ve seen a black hole in visible light. It’s the smallest black hole we’ve seen in visible light (which is the interesting and noteworthy bit here) but it’s not the first. I’ll explain what the real first visible-light black holes were later, but for now I want to talk about what the surrounding material is.

What’s the material? There are two general categories.

First, there’s a disk of material orbiting the black hole. It’s a disk because angular momentum likes to force things to collapse into flat disks aligned with the rotation axis (the entire solar system, for instance, is more or less aligned with the equator of the Sun). As gas and dust sloshes around in the disk around the black hole, it hits other particles, and those collisions (aka friction, aka drag) will transform some of its kinetic energy into heat (or, more properly, thermal radiation). That’s the light we see. Eventually the friction will cause the particles to lose enough momentum that they spiral inwards beyond what’s known as the “innermost stable circular orbit”. There, non-Newtonian gravity really takes over, and the black hole will suck the gas or dust up. The technical term for this is “accretion”, and the disk is called an “accretion disk”.

Incidentally, the thermal radiation produced in the accretion disk is described by the same equations as the light from stars*. The hotter the material is, the more light is produced, and the bluer that light becomes (from less energetic forms of light like infrared, to red, to blue, to more energetic ultraviolet light). If the disk was uniformly 5770 degrees Kelvin, it would produce a spectrum that looked a bit like the Sun.

Second, there’s charged particles swept up by the black hole’s magnetic field. The magnetic fields suck up charged particles from the disk and can funnel them into gigantic jets that shoot out of the poles. THESE emit primarily in the most energetic kinds of light: gamma rays, X-rays and ultraviolet light, rather than the less-energetic infrared and optical. In fact, when black holes are “seen”, they’re usually seen in X-ray radiation, because in there, they’re not competing with the light from stars. That’s particularly important because the kind of stars that are massive enough to form black holes always have companions… and the companion generally gives off quite a lot of thermal radiation, enough to overwhelm a (comparatively) small accretion disk.

V404 Cygni is probably like this, except that the stellar companion is too far away to be dumping material onto the black hole. Image Credit: ESO/L. Calçada/M.Kornmesser

V404 Cygni is a binary system composed of a 9-solar-mass black hole and a slightly-less-than-solar-mass star. It did follow the typical pattern for a long time: if you looked at it in optical or infrared light, you saw the solar-mass companion; if you looked at it in X-rays, you saw the black hole. But what’s happened here is that V404 Cygni has undergone some kind of extraordinary event such that that the visible light from the accretion disk is now making its presence known (and it briefly became one of the brightest X-ray sources in the sky). THAT is unprecedented for a black hole that formed from a star. The press release says it’s NOT because V404 Cygni’s black hole just started eating something huge, which was my first guess. Instead, they seem to think it’s just that the disk is dumping matter onto the black hole in an extremely erratic way. This is going to be an interesting story to watch, as we learn WHY V404 Cygni’s black hole is suddenly so bright.

What was the actual first black hole to be seen in visible light? I’m not sure which one, exactly, but quasars certainly pre-date this discovery. Quasars are actively accreting supermassive black holes at the centers of galaxies (by supermassive, I mean enormous. The Milky Way’s supermassive black hole, for example, is 4.2 million solar masses, not 9; we should all be glad it’s not currently eating much). Quasars have tremendously large accretion disks, and are some of the most luminous objects in the universe. They are most definitely detectable in visible light. One of the first was mistakenly assumed to be a star** and was given the name BL Lacertae. It is only two magnitudes dimmer than V404 Cygni in visible light (despite being 100,000 times farther away), and might still be visible in a moderate-sized backyard telescope.

UPDATE: As suggested by Angelle Tanner, if you actually want to try to see this black hole yourself, there’s this page from StarDate which claims it can be found “near the line that connects the swan’s beak with the center of its body”. Or, if you have a telescope that can slew to sky coordinates, it’s at 20 24 03.83 +33 52 02.2. Unfortunately, January is a bad time of year to try to see it; it’ll set only an hour or so after sunset.




*And also colder things like planets, moons, iron forges, and people, believe it or not.

**To be fair, NOBODY was quite sure what quasars (QUAsi-StellAr Radio sourceS) were until the 1970s-1980s. One amusing relic of that earlier time is the 1967 Star Trek episode The Galileo Seven, wherein the Enterprise observes a “quasar-like phenomenon”. Star Trek famously tried to present (semi-)realistic science, but their guess at a “quasar” was an extraordinarily dense star cluster, rather than a black hole. It’s not a terribly bad guess for “compact object with the brightness of a million suns”, but it looks quite dated now. At least they hedged their bets with “quasar-like”…