“Closest black hole” is the astro-news this past week. And let’s briefly explore it, just so you know the real story behind this scary-sounding object. Because black holes are headline-grabbers and objects of public fascination, but the truth about them is more fascinating than the hype.
First let’s be clear: They’ve had very bad “PR” – which makes people think a black hole goes around sucking everything into them. Actually you’d have to venture right up to a black hole’s edge, its so-called “event horizon,” in order to be forced inward. At any sort of normal distance, they’re not hazardous at all.
Say our Sun collapses to be a black hole later today. To do that, it simply needs to shrink smaller and smaller until the “escape velocity” at its gassy surface, the velocity you need in order to leave it—which is currently 360 miles per second—climbs all the way up to the speed of light, which is 186,282 miles per second. To achieve that kind of escape speed you’d need to boost its surface gravity by making it collapse into a ball just two miles wide.
For Earth to achieve a black hole’s escape velocity (the speed of light) our planet would have to shrink down to be only the size of a marble. Obviously, that’s not going to happen.
But say it did happen. Say the sun collapses and becomes a black hole. Then do we get pulled in? Not at all. Its overall gravity stays the same because its mass hasn’t changed. Sure, at its new surface, nothing could escape, not even light, which is why it’s now a black hole. But here at Earth’s distance of 93 million miles away, we feel the same solar gravity we always have, and nothing changes at all. We keep orbiting it just as we did before. We’re not pulled toward it in the slightest.
Only very heavy and old stars, typically weighing more than a half dozen Suns, have enough mass to self-collapse from their own gravity. Then they can become a black hole.
Read more about black holes in my post, Where in Space Are We Headed?
The bright blue light in the center is HR 6819, a triple system with two stars and the closest black hole to Earth. Credit: ESO.
The Closest Black Hole
Until last week, the nearest was Cygnus X-1 in the “Swan” constellation, located about 6,000 light-years away. We could never see that black hole, of course, since it neither emits not reflects any light, but we do see its gravitational effects as it pulls on its surrounding objects; in this case, its companion is visible star, an ordinary heavy sun that orbits it once a week. The way it’s tugged shows us the mass of the unseen object. The fact that it also emits pulses of X-rays is another key bit of evidence, since that’s what should happen if tiny atom-sized bits of matter are spiraling inward. And its X-ray pulsations fluctuate rapidly, which only a very small object could display. So, no, we can’t see it, but we know it’s massive, tiny, emits X-rays, and changes its intensity very quickly and therefore it must be super-tiny. In a court of law it would be “case closed”—it’s a black hole.
And now we’ve found another black hole—roughly 2,500 light-years closer—in the dim, far-southern constellation of Telescopium in the star system HR 6819. We can see this star system with the naked eye, though it’s too far south to ever rise for U.S. observers. However, from the Southern Hemipshere, during wintertime, it appears as a brilliant blue star gleaming overhead. It’s actually a two-star system with two stars that are so near that they appear as one.And now we know it’s a triple system that also includes the closest black hole to Earth ever found.
As the image at the top of this page shows, HR 6819 is made up of an inner star, this newly discovered black hole, and a third star in a wider orbit. Scientists discovered the black hole by accident; they wanted to learn why the pair of stars was orbiting each other in an odd way, and found that it was locked in orbit with a massive invisible partner over four times the mass of our sun. And this black hole is just 1,000 light-years from us.
Even if this black hole were as close as the sun, it wouldn’t affect us. So it’s newsworthy for our intellects. But it’s not information that would arouse the attention of your insurance company.
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