Our Milky Way galaxy is a sprawling collection of gas, dust, and hundreds of billions of stars, arrayed in a more-or-less flat disk. In the very center of the galaxy – just as in countless other large galaxies like ours – lies a hidden monster: a black hole. And not just any black hole, but one with four million times the Sun’s mass.
It’s called a supermassive black hole for a reason.
Usually, it’s not doing a whole lot except sitting there being black and holey. But sometimes it gets a little snack, and when it does it can let out a cosmic-sized belch. A very, very, very hot belch. Like it did in July 2012:
[Click to schwarzschildenate.]
These images were taken with NASA’s newest X-ray satellite, NuSTAR (more on that in a sec). NuSTAR can detect high-energy X-rays coming from space, and happened to be pointed toward the black hole when it erupted. On the left is an overview of the region near the center of our galaxy. The whitish area is the stuff immediately surrounding the black hole (the pink glow is most likely from a supernova, a star that exploded in centuries past). On the right is a series of three images showing that region getting very bright in X-rays, then fading away: a flare.
OK, so I know what you’re thinking. How can a black hole – famous for gobbling down everything nearby, even light – get bright and emit so much energy?
Basically, it doesn’t. The stuff around it does.
A black hole by itself is dark. But if a gas cloud gets near, very interesting things happen. The gravity from the black hole stretches out the cloud, because the part of the cloud nearer the hole gets pulled by the gravity harder than the part of the cloud farther away. Also, the cloud probably doesn’t just fall straight it; like an orbiting planet around the Sun it has some sideways motion. This means the hole whips it around, pulling out a long tendril which then spirals ever closer to the Point Of No Return.
This video may help. It shows a star getting torn apart by a black hole, but the principle is the same.
So some of the stuff may get flung away, but a lot of it falls toward the black hole. As it nears the hole, it forms a flat disk, called an accretion disk. The material in this disk is tortured by unbelievable forces: the inner part of the disk is whirling madly around the black hole, while the outer part is moving more slowly. The gas is literally heated up by friction as the different parts of the disk rub against each other (other forces like magnetism play a role too). The heating can be HUGE: the gas can reach temperatures of hundreds of millions of degrees!
Gas that hot emits X-rays, which is how this flare was seen by NuSTAR. Probably, a smallish cloud found itself too close to the black hole, got torn apart, and flew down into it. As it did it got extremely hot and blasted out X-rays. But when the whole thing was gobbled down, the X-rays stopped… because there was nothing left to emit them.
So maybe saying this was a belch is a bit misleading, since you do that after you eat something. This is more like your food screaming loudly and incoherently and flailing around while you’re actually eating it. Is that better?
This is a pretty cool observation. For one thing, our local big black hole is usually pretty quiet, so even getting a chance to see something like this is pretty nifty. Second, it can tell us what the environment is like near the black hole. And also, it helps us understand what happens right before some unfortunate object takes The Final Plunge. As I mentioned, every big galaxy has a supermassive black hole – ours is actually rather paltry compared to others; the one in the center of the Andromeda Galaxy is probably ten times more massive than ours – so anytime we can observe something going on with ours, we learn more about how they behave in other galaxies, too.
Also, I’m proud of NuSTAR. I worked on the project for a while, as part of the Education and Public Outreach team. I wrote quite a bit about the mission at the time, and was very pleased when it launched in June. It almost never got off the ground; the mission was actually canceled at one point, but was eventually reinstated.
I’m very glad it was! Now we can watch black holes in our galaxy (and others) as they eat and act rudely. Maybe it’s impolite to stare, but c’mon. When one puts on a fun show like this, it would be wrong not to.
- Astronomers see ANOTHER star ripped apart by a black hole! (including this original post and this followup)
- NuSTAR opens its X-ray eye
- The long reach of the Centaur’s dark heart
- Desktop Project Part 22: A black hole belches out a hurricane
Sorry I didn’t post this when it happened, but some good news: In late June, NASA’s NuSTAR X-ray observatory saw first light! This is the traditional moment when a telescope first opens up its eye and sees light from the external Universe. It’s like a baby-naming ceremony for astronomers.
Here’s the bouncing baby black hole they looked at:
Cygnus X-1 was the first black hole ever found, and is still the nearest one known. It orbits a hot, massive star, and is sucking down matter from that star. As the material falls in, it forms a big flat disk that gets incredibly hot just above the Point of No Return. Really hot things like this emit X-rays, and Cyg X-1 is one of the brightest in the sky. So historically as well as practically it was a good choice for NuSTAR’s first light.
In the diagram above, the left part shows where the black hole is in the constellation of Cygnus. On the upper right is an X-ray image of Cyg X-1 from the European INTEGRAL spacecraft, and below it the shot from NuSTAR. As you can see, the resolution of NuSTAR is much higher, which is kinda the reason it was built.
NuSTAR, by the way, is short for NUclear Spectroscopic Telescope Array, and it launched into space in June 2012. I’ll note that in an earlier post I included some of the history of this star-crossed spacecraft that a lot of folks might not know about. I was involved in this mission literally from the start (developing the education and public outreach effort for it) so to me this isn’t just some story, it’s personal.
Remember, you see these pictures from space taken by fancy observatories, but there’s a deep and usually very rich history behind them. When you see something you like, dig deeper. You may find the story adds to the experience of learning about the astronomy itself.
Image credits: NASA/JPL-Caltech; A. Hobart, CXC
At 16:00 UTC June 13, 2012, the NuSTAR X-ray observatory began its successful journey into orbit! The satellite was launched using a Pegasus rocket, a smaller vehicle that is literally dropped from an airplane and blasts away into space. This method saves a huge amount of fuel by starting the rocket a few kilometers above the ground. [The image shown here is from a different Pegasus drop and is for explanatory purposes; I'm hoping to get some nice images of the actual NuSTAR launch soon.]
NuSTAR (NUclear Spectroscopic Telescope Array) is designed to detect high-energy X-rays emitted by some of the most violent objects in the Universe: exploding stars, matter falling into black holes, and magnetars (super-magnetic neutron stars that are capable of fierce blasts of energy).
X-ray astronomy is a lot harder to do than regular-old visible light astronomy. For one thing, our air absorb X-rays, so we have to launch telescopes into orbit to see these objects at all.
For another, at these high energies, it’s not possible to focus X-rays in a normal way. Photons of visible light bounce off of mirrors, so we can focus them to a point and see distant objects clearly. X-rays are different, though. Think of them as little bullets zipping along; if they hit a mirror they’ll penetrate right through it! So instead of using a reflective mirror, X-rays can be focused by letting them graze against a gently angled sheet of metal, like skipping a rock on the surface of a lake.
So X-ray observatories like Chandra, XMM-Newton, and NuSTAR use very long cylindrical mirrors. But not precisely cylinders; they gently taper at one end a bit like a thimble. X-rays graze these cylinders and bounce at a shallow angle, coming to a focus. The problem with this is that the angle is so shallow it takes a long path (called the focal length) to get the X-rays to a focus. So the telescopes have to be many meters long.
However, to fit on the diminutive Pegasus rocket, NuSTAR has to be only 2 meters long. Engineers solved this length problem in a very cool way: an extendable boom, like an accordion, that expands after launch and lengthens the spacecraft to over 10 meters! At one end of the extended mast are the mirrors, and at the other end are the detectors, as in the drawing above.
I’ll note I worked on NuSTAR a few years back. When I was at Sonoma State University we developed educational activities based on NASA missions. We were asked to work on the original proposal for NuSTAR to NASA, so my boss Lynn Cominsky and I wrote the original Education and Public Outreach part of the proposal, and once it was accepted I wound up writing a lot of the verbiage for the NuSTAR educational website as well.
NuSTAR has a bit of a checkered history. I wrote about this a while back; in 2006 NuSTAR was abruptly canceled just before the final proposal was submitted to NASA (and I was, um, fairly angry about that), then reinstated a year or so later. But now NuSTAR is in orbit after a wonderfully perfect launch, and I’m very, very happy.
My congrats to the NuSTAR team, especially to Dr. Fiona Harrison — to the best of my knowledge the first female Principal Investigator of a NASA astrophysics mission — on finally getting this important mission into space and peering at some of the most interesting objects in the Universe!
Image credits: NASA/JPL-Caltech/Orbital; NASA/JPL-Caltech