Tag: accretion disk

NuSTAR catches a black hole’s hot belch

By Phil Plait | October 23, 2012 1:33 pm

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.


Related Posts:

- 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

The long reach of the Centaur's dark heart

By Phil Plait | May 16, 2012 7:21 am

Every now again I get surprised by a photo, showing me something I didn’t know about. And I love it even more when that surprise is from an object I thought I knew!

So check out this incredible image of the nearby galaxy Centaurus A, a nearby galaxy harboring a whole slew of surprises:

[Click to galactinate, or get the 4000 x 4000 pixel version, or, if you're feeling frisky, cram this onto your hard drive: an image that's 8500 x 8400 pixels and 29 Mb in size! And trust me: you want to.]

Isn’t that stunning? This picture was taken by the MPG/ESO 2.2 meter telescope in Chile, and once you get over its beauty you’ll realize this galaxy is, frankly, seriously messed up.

Cen A is about 12 million light years away and has roughly the same mass as our Milky Way, containing a few hundred billion stars. The underlying glow of those stars is what makes that round background fuzz in the image, and takes on the familiar elliptical shape of many such galaxies. [Note: All the individual stars you see here are in our on galaxy, since we're inside the Milky Way looking out to Cen A. Also, the little circles next to bright stars are reflections inside the camera itself, and aren't real.]

But check out that wide swath of dark stuff across the middle! That blocks the light from stars behind it, so it’s a cold certainty that’s a dust lane: a thick, flat disk of complex molecules commonly seen in galaxies. But… it’s commonly seen in spiral galaxies like ours, not elliptical ones like Cen A. So something’s weird right off the bat. And note how the ends of the disk seem bent in opposite directions; on the right it’s bent down, and on the left it’s bent up.

Most likely, this is because Cen A ate another galaxy. Literally: a galaxy collided with it in the recent past — well, like in the past few dozen million years — and that galaxy was probably more like our own, rich with dust. As it was absorbed, the dust was stripped from it and settled into that disk. The warping at the ends is a gravitational effect, most likely a distortion from the collision itself. We see it in other galaxies that have nearby companions.

When you observe Cen A using a radio telescope it gets weirder: two huge jets of material are being shot out of the core. The image here shows those jets (click to embiggen). Cen A is a very strong emitter of radio waves; in fact that’s why it’s called Cen A: the brightest radio source in the constellation of Centaurus.

The source of those jets is a gigantic black hole in the core of the galaxy. Read More

Desktop Project Part 22: A black hole belches out a hurricane

By Phil Plait | April 16, 2012 6:30 am

[We're in the home stretch of my Desktop Project: going through all the pictures on my computer's desktop and posting one a day until they're gone. Only a few left now...]

This is the only one of my Desktop Project pictures that’s not actually a picture: it’s an illustration. It’s still pretty neat:

[Click to Schwarzschildenate.]

This drawing shows the binary star IGR J17091−3624, which is actually a normal star in the clutches of a black hole. They orbit each other, and the fierce gravity of the black hole is drawing material off the other star. This matter doesn’t fall straight into the black hole, however. Because the two stars orbit each other, the material coming off the normal star has some sideways velocity (technically, angular momentum) which causes it to spiral around the black hole and form a disk called the accretion disk.

This disk is hot. Very way incredibly yikes hot: probably something like 10 million degrees Celsius (27 million F). The heat comes from lots of forces including magnetism and plain old friction as particles rub against each other pretty violently before The Final Plunge.

Stuff that hot emits X-rays, and this binary is blasting them out. What’s so very interesting is that astronomers studying this black hole found that something was absorbing X-rays from the disk. Their best guess is that this is vaporized iron blasting away from the disk in a kind of black hole wind, and it’s hauling butt: the material is expanding at a speed upwards of 9300 km/sec — that’s 5800 miles per second, fast enough to cross the US in less than the tick of a watch. Want another unit? That means the wind is blowing at a brisk 0.03 times the speed of light!

I love black holes. They’re many things, but one they aren’t is subtle.

Another thing they are is ironic: although most people think of them as being able to suck down everything, including light, they power the most luminous objects in the Universe. This black hole probably is small, a few times the mass of the Sun. But much bigger ones exist, with millions or even billions of times the mass of the Sun. Those are in the centers of galaxies, and can have so much material falling into them and heating up that they can shine brighter than all the stars in the galaxy combined! It’s not the black hole itself that’s glowing, but it’s the center, the engine, behind that raw fury.

And that wind may be more than bright: there’s some evidence that the mighty gale from a galaxy’s central black hole affects the overall state of the galaxy itself. It may be tied to the way stars form in the galaxy, and even the size of the galaxy itself. Mind you, even a black hole with a billion times the Sun’s mass is still only as small fraction of a galaxy, which might have hundreds of billions of stars! So while you might think of something like that as a monster, it’s actually more amazing to me that something so tiny can be so influential on such a huge scale.

Illustration credit: NASA/CXC/M.Weiss


Related Posts:

- Star eaten by a black hole: still blasting away
- S marks the spot
- A tiny galaxy that hides a big secret
- Black hole erupts in nearby galaxy

CATEGORIZED UNDER: Astronomy, Pretty pictures

Followup on the star torn apart by a black hole: Hubble picture

By Phil Plait | April 7, 2011 8:31 am

I recently wrote about a mind-boggling event: astronomers capturing what are apparently the final moments in a star’s life as it was literally torn apart by a black hole.

Today, NASA has released some new pictures of the event, including this Hubble Space Telescope shot:

[Click to embiggen.]

I know, it may not look like much at first. But remember what you’re seeing: the violent death of a star ripped apart by the gravity of a black hole… and it’s happening 3.8 billion light years away! That’s about 40,000,000,000,000,000,000,000 kilometers, so the fact that we can see it at all is pretty amazing. And terrifying.

In this false-color Hubble image, the galaxy and explosion are marked. Pretty much everything you see in the picture is a distant galaxy, a billion of more light years away. Normally, the host galaxy itself would appear as a dot, at best with some small amount of fuzz around it, the glow of billions of stars reduced by the incredible distance. But the dying light of the star increased the galaxy’s brightness by a lot. A whole lot.

This image (click to greatly embiggen!) is a combination of visible light (white), ultraviolet (purple), and X-rays (yellow and red) from NASA’s Swift observatory, the satellite that first detected the explosion. While the spikes are not real — they’re just an optical effect from the telescope itself — it still speaks to the drama of what we’re seeing.

And so just what are we seeing?
Read More

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
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