Last night, I gave a talk at Science Pub RVA, part of NOVA ScienceNOW‘s initiative to bring scientists and the public together in an informal setting. Many thanks to Cynthia Gibbs, the Richmond organizer, for inviting me to speak and for getting this great event going!

The topic for my presentation was black holes—specifically, the real black holes astronomers have studied over the decades. While I’ve written about the subject before (and black holes were the subject of a popular planetarium show at my old job), I’ve never given a public talk specifically about them. However, I love black holes. Did I mention I’ve written about them before?

Consider this post a mild recap of my talk, without the technical difficulties (such as the power going out in the building several times before the presentation began), but with all the slides and bonus links. This is a very graphics-intensive post, in other words, so please be patient while everything loads!

Black Holes Don’t Suck

(Yes, I’ve used that joke before. So sue me.)

Discussions of black holes fall into two distinct categories. The first is the sexy string theory/quantum gravity/Stephen Hawking category, all about time warps, wormholes, extra dimensions, Bekenstein entropy, and baby universes; the second discusses the real black holes discovered in our galaxy and beyond. While the sexy stuff is a lot of fun to talk about, that’s not what I discussed: it’s speculative, and at the present time impossible to test. (Some of it by its very nature is impossible to test, since we can’t get access to the region inside a black hole. More on that shortly.) However, I think real astronomical black holes are just as interesting, and over the last several decades astronomers have realized how important they are in shaping the galaxies they inhabit.

The Monster in the Galaxy’s Basement

The center of the Milky Way lies in the constellation of Sagittarius, but you can’t tell just by looking. Even on a dark night in a place with few lights, the center of the galaxy is hard to spot simply because it’s full of stars and obscured by cosmic dust. Our best telescopes using visible light can’t do much better: the dust and gas are opaque in those wavelengths. However, we have other tools: telescopes using X-rays and infrared light. Just as medical X-ray machines can see through the squishy parts of your anatomy (that being the technical term), X-rays from cosmic sources can pierce the dust. The image at the right from the Chandra X-ray observatory shows the galactic center, and you can clearly see a bright source of light.

Known as Sagittarius A* (pronounced “a star”), this is one of the brightest objects in the X-ray sky. [Correction: Sagittarius A* is relatively faint, though still brighter than the majority of objects. I’m mixing up its X-ray signature with its brightness in radio waves. Thanks to Peter Edmonds for catching that!] Since X-rays are some of the most energetic forms of light, it takes a lot to make them in these quantities. (Stars, including our Sun, are X-ray sources. However, even with the billions and billions of them in the Milky Way, a few other objects—mostly remnants of supernovas—outshine all of them put together.) The picture becomes even more interesting when Sagittarius A* was examined in infrared light, by the Keck telescope in Hawaii.

The animation at the left is constructed from real data taken from observations lasting over a decade. Each circle is a star (or “stellar object”, labelled “SO”), and with so many years of observation, we can see their orbits. What they are orbiting is invisible in infrared light, but using Kepler’s laws of motion we can learn a few things: the hidden object is about 4 million times the mass of the Sun, and it occupies a space much smaller than our Solar System. No single star can grow that large, and the space the hidden object occupies is too small to be a cluster of stars. The only thing we know of that fits the bill is a supermassive black hole.

A supermassive black hole has a lot of mass. (Subtle concept!) As it turns out, nearly every galaxy we’ve observed contains a supermassive black hole at its heart, ranging from millions to billions of times more massive than the Sun. And just like Sagittarius A* in the Milky Way, these black holes are often very bright in X-rays, radio light, or both. To understand why, let’s look at a black hole up close (well, kind of).

Anatomy of a Black Hole

A black hole is defined by a boundary known as its event horizon. Anything that crosses that boundary cannot ever return to the outside Universe—and that “anything” includes light. Even if you could survive the passage across the event horizon, you’d never be able to tell your story, because your radio signal back to Earth would be trapped inside the event horizon with you.

Despite the intensity of the gravitational field surrounding a black hole, nothing is actually “sucked in”. Far from a black hole, it acts like anything else. In fact, if you removed the Milky Way’s black hole from the galaxy, the Sun’s orbit wouldn’t be noticeably affected—we’re just too far away (about 26,000 light-years) for the black hole’s gravity to make a profound difference.

Close by is a different story. Near a black hole, the gravitation is strong enough to accelerate particles nearly to light speed. This is the case for the first black hole ever discovered: Cygnus X-1. Stars are made of plasma: a hot gas consisting of electrons and bare nuclei. When the Cygnus X-1 strips those charged particles off a companion star, they experienced strong acceleration, which in turn causes them to emit photons, both in radio light and X-rays. (This phenomenon is known variously as cyclotron radiation, Bremsstrahlung, and other fun science-fictiony names.) In case that isn’t enough, when charged particles move, they generate magnetic fields, which in turn can cause more acceleration.

Black holes are powerhouses while feeding on gas, in other words. They may be invisible when isolated, but when they have stars or clouds of gas or other things to gorge upon, they can be some of the brightest objects in the whole Universe.

The Biggest, Baddest Black Holes

Cygnus X-1 is known as a stellar mass black hole, because it’s only about 15 times more massive than the Sun. While it’s hardly a lightweight by our standards, it’s pretty puny compared to supermassive black holes: recall Sagittarius A* is 4 million times the mass of the Sun. When a supermassive black hole at the core of a galaxy is dining in earnest, it is known as an active galactic nucleus (AGN).

One of the closest AGN to us is a bizarre galaxy called Centaurus A. The galaxy itself appears highly distorted, and has a huge lane of opaque dust running across the center. As you can see from the multiple images in various forms of light, the black hole at the Centaurus A is particularly aggressive, sending out jets of gas as wide as the entire galaxy. Jets like these aren’t completely understood, but the consensus in the astronomy community is that they are driven by magnetic fields like the ones I mentioned in the previous section.

The champion heavyweight is in the galaxy M87. The galaxy itself is known as a giant elliptical galaxy: one lacking the distinctive spiral arms that dominate many of the most beautiful astronomical images. In fact, M87 is pretty dull-looking, being a fairly spherical blob of stars…but with a long streamer of gas flowing from its core. This jet is thousands of light-years long, and emanates from the galaxy’s supermassive black hole. The inset in the image on the right shows a close-up view of the galaxy’s core, and how much swirling gas there is to feed the black hole. M87’s black hole is estimated to be about 6.6 billion times the mass of the Sun—more than a thousand times the mass of the Milky Way’s black hole!

So is the Milky Way’s black hole active? Actually, it’s not. M87 is relatively calm, while Centaurus A is very active, but the Milky Way’s core is pretty much sleeping. But has it always been that way? The answer seems to be no: in 2010, the orbiting Fermi gamma-ray observatory found two huge bubbles of gas emanating from the center of the galaxy. Given the extent of the bubbles (2,000 light-years long), they obviously formed some time ago, and may reveal an earlier epoch when the black hole was more active than it is today.

A black hole that is too active is not good: the powerful jets can sweep away gas needed to make stars, starving the galaxy. Additionally, the intense radiation from accelerating particles is bad for planets like ours. However, black holes may help star formation under other circumstances if the flow isn’t as fierce. The understanding of the mutual interaction between galaxies and their black holes is an area of active research. It’s possible that our galaxy’s black hole may have shaped the environment that allowed the Sun—and therefore Earth—to form.

Some Final Thoughts

I mentioned that most galaxies contain supermassive black holes. That obviously hints that some don’t. In rare cases, it seems that galaxies can kick their black holes out, as the result of a collision between two galaxies. Big galaxies are built of smaller ones, through long processes of collisions and mergers. If the conditions are just right, the central black hole can be knocked loose, leaving it to roam the Universe. An article I wrote for Ars Technica has a lot more detailsabout this kind of nightmarish scenario.

Someone at my talk asked a really good question: if a galaxy can have no supermassive black holes, could one also havemore than one. The image at the left shows just such a case: a galaxy that underwent a merger in the not-so-distant past (in cosmic terms at least). Each galaxy contributed one black hole to the resulting object, which may over time themselves collide and make one even larger black hole. While this isn’t enough to explain how every supermassive black hole got fat, it might help us understand at least some cases. That’s a post for another day!

Finally, many people asked if I could recommend some books. Here are a few I’ve read, so the list is not exhaustive. I’ve limited myself to books for non-scientists, so if you want some physics recommendations, please let me know in the comments, and I’ll provide them.

• Gravity’s Engines by Caleb Scharf just came out this summer, and describes the latest knowledge in black hole astronomy. He’s much more of an expert than I am! Start here if you want to know more about black holes.
• For the more speculative side of things, the classic book Black Holes and Time Warps by Kip Thorne is still a great read. Thorne is best known to non-scientists for his work on wormholes, which he did while consulting on Carl Sagan’s novel Contact.
• Finally, if you want to know what string theory has to say about all this, try Warped Passages by Lisa Randall. I recommend her book over other string theory authors (Brian Greene, etc.) because she is far more focused on what we would need to do to show the more wild ideas are correct. I’m a string theory skeptic (to put it mildly), but the work being done by string theorists on black holes is fascinating stuff.