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SYFY WIRE Bad Astronomy

What Kind of Object Can Survive a Close Encounter With a Monster Black Hole?

By Phil Plait

In or around May 2014, a truly remarkable thing happened: An object, dubbed G2, survived a very close encounter with a very large black hole.

The black hole in this case is the supermassive one in the center of our own galaxy, the Milky Way. Our best estimate of its mass is about 4.3 million times the mass of the Sun, so it’s a bruiser.

Using the largest optical and infrared telescopes in the world, astronomers can actually see the movements of stars orbiting the black hole. Its fierce gravity accelerates them so strongly that they have ridiculously high velocities. Over months and years, even from our vast distance of 26,000 light-years—260 quadrillion kilometers—from the black hole we can physically see their movement.

G2 was discovered a few years ago, and it wasn’t—and still isn’t—clear just what it is. It looks like a big cloud of dust, and it was on a trajectory that was predicted to take it only about 30 billion km from the black hole. That’s close enough that tides from the black hole were expected to tear G2 apart.

But it survived! This means it can’t be just a dust cloud; it must have a star embedded in it, and the star’s gravity held the dust cloud together. (In fact it’s more complicated than this; the star may be a binary. I did a BA Video on this very topic.)

New observations have now put some numbers, and a face, to G2. The image above is a composite of observations taken using the Very Large Telescope in Chile. Each blob is actually G2 seen at a different time; the yellow one is form 2006, then 2010, 2012, and the red one, right before peribothron (closest approach to the black hole*) in February 2014.

The last image, in blue, was taken in September 2014. It appears smeared out, but that apparently is not real but an instrumental effect from the telescope; the object itself doesn’t appear significantly extended.

The coloring is significant: It represents how fast G2 was moving away from Earth as it rounded the black hole (the colors were added later to represent red and blue shift). The velocity in February 2014 was away from us at about 2,700 km/sec, just under 1 percent the speed of light! In September it had rounded the black hole and was headed toward us at more than 3,300 km/sec, an incredible speed. I’ll note it’s so far away that even at that speed it would take millennia to reach Earth, and in fact it’s in a closed orbit around the black hole, so it’s not going anywhere anyway.

Here’s a video of the observations showing the movement of G2 around the black hole:

The astronomers were able to determine the orbital characteristics, too. It takes G2 about 260 years to go around the black hole once. The orbit is extremely elongated (for tech-types: the eccentricity is 0.976, nearly a parabola!) and takes it about 0.1 light-years—a trillion kilometers—out from the black hole before it begins its centurylong dive back in.

Interestingly, when I made the video and wrote about this object a few weeks ago, there was a thought that it used to be a binary star (two stars orbiting each other) that had merged into a single star after encountering the black hole. This would expel a lot of gas and dust, explaining the peculiar properties of the cloud. The astronomers in this newer study model it as a single star that is very young, and still drawing in (accreting) material from the cloud. What they find fits the observations decently well. We’ve gone from being baffled by this object to now having more than one good explanation for what it is! Hopefully, more observations over time will be able to figure out which of these two models is correct … or if neither is. That’s science.

And if you’ll indulge me a moment …

This image is amazing; it made the hairs on the back of my neck stand up. No joke: We are so good at what we do that we have built telescopes and detectors that can directly detect the motions of objects trillions of kilometers away! We can watch as another bizarre object is caught in the clutches of a black hole and forced into a tight orbit that takes it so close that it moves through space at a fraction of the speed of light itself.

This is what we do, we evolved apes. So many people pooh-pooh science for so many reasons, sometimes claiming other or higher sovereignty. But when I think on things like this, what we see and what we do, I can only shake my head and smile ruefully.

If you want to stare into the face of awesome, science is an excellent way to do it.

*Normally, a close encounter with a star or object like it would be called periastron. But this is a black hole, so this term peribothron is used; literally, close to the hole. Brian Koberlein at One Universe at a Time explains.

Correction, April 14, 2015, at 17:00 UTC: I originally misstated that the black hole was 260 trillion kilometers away. It’s actually 1,000 times farther than that: 260 quadrillion kilometers. Sorry if anyone panicked.

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