One of the things that always blows my mind about astronomy is how good we've gotten at using indirect evidence to figure out what's going on up there.
In a way, of course, it's all indirect, at least in the sense that everything we're seeing is (1) wicked far away, and (2) in the past. I remember how weirded out I was when I first ran into the latter concept, back when I was maybe twelve years old. My first inkling of it happened when I was out on a walk with my dad, and down the street there was a guy using a sledgehammer to pound in a fence post. The strange thing was, I saw the hammer's head strike the post, and then, a second or two later I heard the bang of the strike. I asked my dad why that was.
"Well," he said, after a moment's thought, "the sound takes a moment to get to your ears. It's why we always see the lightning before we hear the thunder. And the farther away it is, the longer the delay. So as we get closer to the guy, the delay should get smaller."
Which, of course, it did.
After I'd had a minute to process that, I said, "But light takes time to get to your eyes, too. A very short amount of time, but still, some time. So does that mean you're not seeing things as they are, but as they were in the past?"
My dad agreed that must be so.
Upon learning some more physics, I found out that the Sun is far enough away from the Earth that it takes a bit over eight minutes for light to travel the distance in between. So if the Sun suddenly vanished -- an unlikely eventuality, fortunately -- we not only wouldn't know it for eight minutes, there is no possible way to know it. Einstein showed that information can't travel any faster than the speed of light -- it really is the ultimate speed limit.
The nearest star, Proxima Centauri, is 4.25 light years, so we're seeing it as it was 4.25 years ago, and have no way of seeing what it looks like right now. Given that it seems to be a fairly stable star, it probably looks much the same; but the fact remains that we can't know what its current appearance is. The most distant objects we've seen through our most powerful telescopes are some of the quasars, at thirteen billion light years distant (and thus, what they looked like thirteen billion years ago). So what those quasars look like right now -- where they are, if they even exist any more -- is impossible to know. We're seeing them as they looked shortly after the universe began; what they are today is anyone's guess.
Impressively far away, but at least still in our own galaxy, is Sagittarius A*, the supermassive black hole at the center of the Milky Way. It's 26,000 light years distant. But despite how far away it is -- and the fact that massive dust clouds lie between it and us, obscuring what light it does emit -- we've been able to find out an astonishing amount about it.
This, in fact, is why the topic comes up today -- some research out of the Université de Strasbourg that found evidence of a sudden flare-up of Sagittarius A*, around two hundred years ago. For such a behemoth, it's been relatively quiet since its discovery in 1990. But astrophysicist Frédéric Marin has found a cosmic glow that resulted from a brief, powerful flare of x-rays, during which Sagittarius A* was radiating a million times brighter than it is now.
The x-rays caused the clouds of dust surrounding the black hole to fluoresce; from the distance of those clouds from the event horizon of the black hole, Marin and his team determined that they must have been hit by a strong blast of x-rays about two hundred years ago. (Keep in mind that because of the time-lag effect I described earlier, these times are all as seen from Earth; the actual flare-up occurred 26,200 years ago, or thereabouts.)
What caused the burst isn't known, but is surmised to be the sudden swallowing by the black hole of a denser blob of cosmic dust and gas. As material goes into a death spiral toward the event horizon of a black hole, it speeds up, and electrons are stripped from atoms, leaving a whirling funnel cloud of charged particles. These particles radiate away some of that energy in the form of x-rays -- the "smoking gun" that allows us to see black holes, which otherwise would be entirely invisible.
If you get a little nervous about such astronomical violence, there's no cause for alarm; neither Sagittarius A* nor any of its radiation blasts pose any sort of danger to us. We'd only be in trouble if we were a great deal closer to the galactic center.
So we can just sit back and appreciate the amazing capacity the astrophysicists have for sifting through data and painting us a picture of what the universe looks like. In this case, the last blaze of glory for a dust cloud that got sucked into a supermassive black hole 26,000 light years away.
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