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black holes

Invisible Fascination

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Black Holes

Artists conception of a black hole being fed by surrounding material and shedding excess energy via near-light-speed jets that stream away from the black hole. Courtesy Chandra X-Ray Satellite.
Artist's conception of a black hole being fed by surrounding material and shedding excess energy via near-light-speed jets that stream away from the black hole. Courtesy Chandra X-Ray Satellite.

Black holes fascinate people. How do I know this? It’s one of those topics that people bring up when they find out I’m “into” astronomy. I’ve been asked about black holes in supermarket checkout lines, at book signings (back when I used to do those when I had a book to hawk), at public lectures, on planes, and online. I even had a medical professional ask me about them as I was being wheeled on a gurney into a procedure last year. Fortunately they hadn’t given me any anesthetic yet. I can just imagine what my answers to their questions would have sounded like if they had already started giving me the famous “I don’t care” cocktail!

Now, the funny thing about my perception of black holes is that I’m still amazed that they exist. Back when I was a kid and I first heard about this theoretical mathematical construct called a singularity (the high-falutin’ name for a black hole), few people thought we’d actually find any of these things. That was back a few decades ago, and in the years since then, not only have we found black holes, we’ve found them with such regularity that the American Astronomical Society has regular black hole briefings at their twice-a-year meetings. Hubble Space Telescope is getting so good at spotting the effects of black holes on surrounding matter that when a press release comes out describing the latest find, some of us joke about HST finding another “damned black hole.”

What is it about the black holes that fascinate people? Just judging by what they tell me, I’d say it’s the concept of the singularity itself. What it represents. At the very least, it’s a place where space and time act very differently from what we know out here in the non-black hole universe. A black hole is a place where gravity is so strong that light can’t escape it. And, since light carries information that could tell us what is happening inside the singularity, we’ll never be able to see “pictures” of the interior of the black hole.

Black holes are the ultimate “black boxes” of the universe; we can see the effect they have on surrounding space and matter, but we can’t exactly see the black holes themselves. They’re all over the universe, it turns out—from the stellar black holes that form when supermassive stars explode as supernovae, to the centers of galaxies, where black holes that have the mass of millions or billions of stars wallow in the galactic cores, having effects on galaxy evolution that we are only just now beginning to understand.

And, that’s the current story of black holes: not just that we’re discovering them, but that we’re finding out the roles they play in the cosmos are interesting, and possibly even necessary for the ongoing evolution of the universe.

Fascinating!

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What’s It Like Inside a Black hole?

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Back when I used to lecture in the planetarium I would solicit questions from the audience at the end of each presentation. Every once in a while somebody would ask me what it’s like inside a black hole.

Trick question, right?

Well, probably for some of the more smart-aleck audience members it was. But, I always had an answer. I’d go into a little discussion about how we don’t know exactly, and based on a number of factors (including the laws of physics, some Einsteinian laws, etc.), we’ll probably never have a chance to explore the inside of one (and, if the gravity is so strong that light can’t even escape, is the inside of a black hole REALLY a place we want to be?). Following that there’d be a silence as people digested the idea of “being there”. Then we’d get into a discussion about what it be like to be right next to a black hole, which is a lot easier to describe, even if it IS a shrieking maestrom of radiation and searing temperatures.

At the time I was first in school, back in the dark ages of the early 60s, black holes were sort of a mathematical curiosity, a physics problem for which we didn’t have any good examples. That all changed with the advent of telescopes and detectors able to “see” the effects of black holes, including the jets that spray out from the vicinity of one as matter (stars, gas, dust) spiral into the hole. Moreover, black holes have gravitational effects on nearby stars and gas and dust that we CAN track with spectroscopic observations of the light emanating from the nearby region.

The Milky Ways Black Hole Courtesy European Space Agencys Integral Mission
The Milky Way's Black Hole Courtesy European Space Agency's Integral Mission

Which brings us to the center of our own galaxy, where a supermassive black hole about a million times the mass of our Sun lies hidden by gas and dust clouds and star clusters. This SMBH (for short), also known as Sagittarius A* (or SgrA*), radiates tremendous amounts of energy which we can detect in gamma rays. As luck would have it, we have a spacecraft called INTEGRAL that “sees” that radiation. In the image above, INTEGRAL shows us a gamma-ray view of the region near the center of the Milky Way.

Now, SgrA* is a pretty quiet and harmless black hole, and isn’t quite the powerhouse of radiation that others are—like, say, the black hole at the center of galaxy M87, which sports a very active jet. Yet, in the past, the Milky Way’s resident black hole has been restless, and whenever it acts up, the surrounding clouds light up with the evidence.

Right near SgrA* is a cloud of gas called SgrB2, and the two are about approximately 350 light-years apart. Sgr B2 is being exposed to a blast of gamma rays emitted by Sgr A* that went off about 350 years ago. The cloud absorbed the radiation and has been emitting it. Interestingly enough, the astronomers studying the data think that the whole outburst took at least ten years, possibly longer. And they’re using their studies to figure out how often and how strongly “our” black hole turns on, radiates, and then turns off again.

I have to admit, it’s pretty heady growing up knowing that these weird things that scientists once thought were probably rare are now found all over the place (in many galaxies and at the death scenes of supermassive stars). And, I find it very cool indeed that we can study the near-black-hole environment and learn so much about them.