Ironically, black holes aren’t too hard to find. They’re bright.
Here’s a bunch of ‘em:
Every dot of light in that image is a black hole, hundreds of millions and even billions of light years away. Before you say, "Say wha?", this will take some explanation.
The black holes themselves are black (duh). But as matter falls into them, it can settle into a disk, called an accretion disk. If you remember your college physics — you did take college physics, right? — as something falls into a black hole, it acquires a huge amount of kinetic energy (for you pedants, it actually converts gravitational potential energy to kinetic energy). Think of it this way: when you hold a rock up over the ground, it has potential energy– the potential to move due to gravity. When you let go, that potential energy becomes kinetic energy– the energy of motion. If you don’t think it has energy, then let it hit your toe. The kinetic energy will be converted to a loud crunching sound, and you will potentially have to go to the hospital.
So matter falling into gravity can gather energy, and matter falling into a black hole can get a lot of energy. This is converted to motion and heat, and as the matter piles up into the accretion disk it gets terribly, terribly hot: as hot as millions of degrees. There are also associated magnetic fields and other forces which can make the matter in the disk light up, getting it extremely bright. The bigger the black hole, the brighter this disk can get.
Astronomers think that in the center of every big galaxy there is black hole with millions or even billions of times the Sun’s mass. Guess how bright they can get?
Answer: pretty damn bright. In fact, as long as they are actively feeding, black holes like this are the brightest sustained objects it the Universe. We call them "active galaxies". They’re so bright they they can be spotted when they are billions of light years away. And hey, didn’t I say that the spots in the image above were at that distance?
Yes, good! You’re paying attention. The image at the top of this entry is from the orbiting Chandra X-ray Observatory — it’s only one part of a bigger image revealing 1300 black holes at the centers of galaxies.
When matter gets heated to millions of degrees, it gives off X-rays, so Chandra is a great telescope to spot black holes, especially the supermassive monsters in the centers of galaxies. We’re still trying to figure out just how many galaxies are active, and how many are quiescent like the Milky Way (our central black hole has 4 million times the mass of the Sun, but is not currently feeding, so it’s not active).
Also, we’re not completely sure what the structure of the accretion disk is like near a hole. The current theory is that near the black hole it’s very flat and thin, but farther out it puffs up into a torus or doughnut (or bagel if you’re from New York City). But think about this: imagine putting a pea in the center of a donut hole. From most viewing angles, the pea is hidden. If you view it face-on you can see the pea, but at an angle the donut blocks your view. From edge-on you’re looking through a lot of doughnut and can’t see the pea at all.
This model explains a lot about what we see with these active galaxies, but is it right?
Maybe. Maybe not.
The new observations from Chandra are very interesting. If we see a black hole torus face-on, we expect to see X-rays of all energies, since they are free to get to us. But if we see one edge-on, only the highest-energy X-rays can penetrate the obscuring torus, so we’d expect to see only those high-energy X-rays and no low energy ones. So, looking at 1300 black holes as Chandra did, you’d expect to see a few that are face-on, a few edge-on, and most ranging in between. In other words, the observations should show most black holes sending out a mix of high and low energy X-rays.
Oops. They didn’t. They reveal lots of high-energy-X-ray-emitting-galaxies, and lots of low-energy-X-ray-emitting-galaxies, but very few in between, the opposite of what the model predicted.
Does this mean the model is completely wrong? No, because in fact the model does very well predicting what we see from black holes in a whole bunch of other obsrevations, hundreds and even thousands of them. So what these new data really mean is that the details of the model need to be worked on more. Maybe in active galaxies the torus doesn’t puff up as much. Maybe the disks are bigger than we thought, or there isn’t as much dust in the torus, or a hundred other reasons.
The devil, that rascal, is always in the details. And if there is any actual place in the Universe that could be described as Hell, it’s the gaping maw of a black hole and the swirling maelstrom surrounding it. So there will always be devilish details to hammer out.
One final note: these active galaxies can pour out gamma rays as well– gamma rays have even higher energy than X-rays. NASA and the Department of Energy are building GLAST, an observatory whose main mission is to investigate these supermassive black holes (I’ve written about GLAST several times). It’s due for launch in November, so by this time next year we’ll have a lot more data, and a lot more answers. But we’ll have a lot more questions, too! This is truly the game that never ends, which is one reason it’s so much fun.
March 12th, 2007 at 12:32 pm
What do the colors in the xray image indicate?
March 12th, 2007 at 12:47 pm
What a coincidence! This image is on my page-a-day Astronomy calender for today. You don’t happen to have one of those calendars do you?
Thanks for the great post, your explanation is a huge improvement over the small blurb on my calendar.
March 12th, 2007 at 1:41 pm
would be nice if they would make a roll-over gif so we can see the visible side
March 12th, 2007 at 1:46 pm
also, from reading the original article on the chandra site
“In this image, the red represents low-energy X-rays, green shows the medium range, and blue the higher energy X-rays.”
March 12th, 2007 at 2:22 pm
Michael, this image was just released today, so I don’t think it’s the same thing. They have released other images in the past that look pretty much the same (lots of colored dots
but this one is a little different, since it indicates the obscuration of the black holes.
March 12th, 2007 at 2:34 pm
Wow! That’s almost enough black holes to fill the Albert Hall!
March 12th, 2007 at 2:36 pm
“you did take college physics, right? ”
Ummm, no.
Is that a bad thing?
I still think this is cool. Who’d have thought that finding a big black hole, we’d look at something that IS visible? Awesome.
March 12th, 2007 at 3:20 pm
Cool picture
And again I learned something.. 
Your search-link for GLAST is not quite accurate - you gave the admin-side search link. You probably meant http://www.badastronomy.com/index.php?s=glast.
March 12th, 2007 at 3:21 pm
Another utterly fascinating article, BA. Yet another example of why your page is one-stop shopping for fascinating Astronomy and should be checked at least thrice-daily. Keep up the good work!
PS: The image also makes a nice desktop
March 12th, 2007 at 3:52 pm
I remember the time when black holes were only a hypothesis. And now look: 1300 in a single picture!
March 12th, 2007 at 6:10 pm
That’s pretty amazing. The amount of energy and mass involved is just mind boggling. And that 1300 is just one tiny chunk of the sky.
I saw Joe Rogan (of BA moon hoax debate fame) in Denver the other day. It was a regular stand up comedy act. He did a bit about how people feel insignificant looking at the Grand Canyon. Joe gestured wildly up at the sky (ceiling…); “Look at ALL that up there! You can take a 6 hour donkey ride to the bottom of the Grand Canyon, it’s just a ditch!”
Here’s a Q and BA question: What would it look like on Earth if the Milky Way’s black hole were active? And how good of a tan would I get standing outside?
March 12th, 2007 at 7:00 pm
I saw a calculation done several years ago that indicated we could potentially retrieve up to 50% of the energy in any mass by spiraling it into a rotating black hole. What wasn’t particularly clear to me was how do you determine(or create) a rotating black hole. From what I recall of my black hole physics, it can really have only two qualities. Mass and charge. So how can we even know if it’s rotating?
Way cool pics, by the way,,,looking forward to GLAST,,,
GAry 7
March 12th, 2007 at 8:55 pm
Isn’t the angular momentum of the collapsing object also conserved as it becomes a black hole? I seem to remember that there was some amount of surprize from physicists when a soloution to the general relativity equations was found that described a rotating object becoming a black hole.
March 12th, 2007 at 10:13 pm
“…black hole … have only two qualities. Mass and charge.”
=====
Make that THREE qualities … mass, charge and spin.
March 13th, 2007 at 1:11 am
Gary, if your feeling really brave look up “Kerr Metric” on Wikipedia.
March 13th, 2007 at 3:12 am
csrster, I did as you suggested to Gary because I was feeling brave but when I faced the enemy I turned & ran, screaming
March 13th, 2007 at 5:53 am
What would our sky look like at night if our galaxy’s black hole was currently feeding?
March 13th, 2007 at 6:13 am
What are the chances that “dark matter” is just a bunch of black holes and neutron stars that aren’t munching anything at the moment? Too scattered to do coherent (and thus recognizable) gravitational lensing, too many to affect planetary orbits like a single black hole in the Oort cloud or Kuiper belt might, but certainly massive enough to invalidate our mass models.
Or would we be seeing too many short occultations or lensing events?
March 13th, 2007 at 7:04 am
That’s really cool! (Err, I suppose it’s actually quite hot, but it’s still interesting.
)
But, uh, I never even took gradeschool physics. Still working on my Associates, then I’ll go for the BS and take physics as an elective (I’m going into a computer field). I’m not even sure what is offered, but I want to take astronomy, physics and biology. And it’s all your fault I’m taking astronomy.
March 13th, 2007 at 8:16 am
Hmmm, Kerr Vacuum. Still doesn’t tell how to know the dang thing is rotating. The only effect that might be conclusive is in the detection of temporal effects, ie, as in the temporal wormhole as a real world solution for lovers of time travel stories(myself included. I just love paradoxes.)
Yes, I just had to read it for myself. That’s sort of the very definition of a mystic,,,”Please mom, I’d rather do it myself,,,”,,,(Never trust a priest who hasn’t been there to tell you about ultimate realities,,,)
I’m not enough of a mathematician to claim full understanding of the equations, mainly because I’m too dang lazy to work them out however, I do have a very visual mind, and the imagery SEEMS fairly clear,,,
Ah well, perhaps I just don’t realize what I don’t know but it was an interesting article,,,
Thanks, csrster for the reference,,,
GAry 7
March 13th, 2007 at 8:52 am
[…] Chandra took a bunch of pictures of active galaxies. Bad Astronomy has a write up about the observation of black holes and what this means for modeling them. Read on […]
March 13th, 2007 at 10:51 am
Awesome reference Blondin, made me laugh
That image is just too damn cool. I actually finished writing the exam for my first college physics course a few hours ago. I’m studying computer science and it was about the physics of sensors. Alas, no accretion disks were mentioned.
Is it only the black holes in galactic centers that send out electromagnetic radiation in the x-ray and gamma ranges? I guess none of this is related to Hawking radiation - that’s thermal radiation, right?
*goes to sign up for more physics courses*
March 13th, 2007 at 4:52 pm
The physics class I took at college level for non-physics majors did not cover black holes. It did involve equations, but of the classical physics kind, and certainly not the difficult more dense ones (such as Maxwell’s, etc.) Also, unless one is a physics major (or just very smart with a great memory), a lot of people forget what they learn apart from their major. Heck, we studied the American Revolution and the Constitution how many times throughout school life and people STILL can’t get their facts straight!
Just sayin’ have mercy on the non-physics majors.
March 13th, 2007 at 4:54 pm
That was good.
March 13th, 2007 at 7:40 pm
[…] - 1300 black holes “Ironically, black holes aren’t too hard to find. They’re bright. Here’s a bunch of […]
March 13th, 2007 at 8:31 pm
When BA mentioned black holes feeding and not feeding, it somehow made me thing of Richard Dreyfuss saying “Boys… He’s back for his noon feeding…” We’re gonna need a bigger telescope!
March 14th, 2007 at 5:59 pm
Actually, Black Holes have 4 major measurable characteristics;
Mass
Charge
Spin
and the lesser known poopie pants factor.
As in, you get close enough and that is one Scarey Mother! :o)
(I hope you folks dont mind my space-warped sense of humor)
March 15th, 2007 at 9:04 am
[…] Also via Paul: who would have thought that black holes are bright? […]
April 2nd, 2007 at 10:22 am
[…] Original post by The Bad Astronomer […]
April 7th, 2007 at 5:44 pm
[…] buyuk boyutunu deÄŸiÅŸik formatlarda buradan indirebilir, bu yazı ilginizi çektiyse devamını buradan okuyabilirsiniz, buradan karadelik nedir sorusunun cevabını […]
July 4th, 2008 at 4:00 am
i thought mmmmm..m.m very goood no you very smart yet you ave not explained what the other galaxyes are have you now? have you found kashyykk yet are their wookies is luke really straight or si it an act he seems friendly involved with chewy peter puppa kiss his bum and all he could tast was puperdum trerima kashi pak