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
One of my favorite things to do in the whole world is look at astronomical images. They are a source of great beauty, insight into our Universe, and wonder that we can understand them.
As it happens, I spent a solid chunk of my professional research career looking at supernovae remnants, the expanding debris after a star explodes. Everything about them is cool: the extraordinary energy released, the amazing beauty and symmetry they posses, the fact that many of the elements necessary for life are created in them.
So I’m pretty familiar with images of these things. Which is why I got a good surprise when the European Space Agency posted this picture of the supernova remnant G272.2-03.2, taken with the XMM-Newton observatory:
[Click to corecollapsenate.]
This is actually a composite image; the starry background is from an optical telescope, but the remnant itself is seen in X-rays by XMM-Newton. X-rays are emitted by very hot gas – heated to a million degrees or more – so you know right away this was an energetic event. I mean, duh, a star exploded.
The two colors (green and orange) tell you the gas is at two different temperatures. The outer rim is probably a thin shell of gas compressed as it slams into the very thin material between stars, while gas heated by shock waves fills the shell. My eye went right away to the bright bit at the right. That’s very common in objects like this when the expanding gas rams into a slightly denser part space (like some other floating cloud of gas) – you get a "dent" in the shell and it gets a bit brighter.
What surprised me most about this particular object is that I had never heard of it! That’s a little unusual; I try to keep up with such things. Then I found out this image was taken in 2001! So it’s not like I ever had a chance to see it. Weird.
So I did what I always do in these situations: looked for references to it in professional journal research papers. And what I found was… almost nothing. There’s a good paper analyzing it by my friend Ilana Harrus, but not much else. Her paper came out a few months before this XMM-Newton observation though, and I couldn’t find a paper with these observations in it.
So I don’t have a lot of information about it. It’s probably about 5000 years old, and may be somewhere between 6000 and 16,000 light years away; pinning down these numbers is very difficult. The star that blew up was probably 8 – 10 times the mass of the Sun, actually a bit of a lightweight for a supernova progenitor. The nebula itself is clearly a shell with hot gas in the interior, but it’s hard to know much more about it. From Ilana’s paper I read that it has some features that make it look old, others younger. But the lack of deep observations keeps this object something of a mystery. I’d love to see some long exposures from Hubble or the Very Large Telescope in Chile. There really aren’t very many good examples of moderate age supernova remnants, and this looks to be a pretty nice example of one.
But geez, next time, someone let me know before a decade passes, OK?
Image credit: XMM-Newton/ESA
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
When I picture an exploding star in my head — which I do unsurprisingly often — the imaginary mental detonation I picture is symmetric. That is, it expands like a sphere, getting bigger in all directions equally.
Supernovae are actually not like that though. Stars are messy affairs, and when massive ones explode they tend to have internal factors that distort that nice, smooth expansion. One big factor is that the actual point of explosion is off-center in the star, not at its exact heart. That can create a massively asymmetric explosion, blasting vast amounts of material and energy off to one side.
Mind you, the core itself in such a star still collapses to become a super-dense neutron star (or a black hole), but the sideways nature of the explosion can give a kick to the leftover ball of neutrons. Quite a kick. In fact, the energies are so titanic that an off-center supernova explosion can blast the neutron star in the other direction, screaming away from the explosion site like a shell out of the muzzle of a battleship gun.
And now astronomers may have found the most extreme example of this: what looks to be just such a neutron star barreling away from a supernova at high speed:
[Click to Chandrasekharenate.]
This image is a combination of observations from the XMM-Newton and Chandra X-ray observatories, the Digitized Sky Survey, and the 2MASS infrared survey. It shows the supernova remnant SNR MSH 11-16A, located about 30,000 light years away. The purple glow is from X-rays emitted by the gas superheated to millions of degrees by the exposion.
But look off to the right. See that comet-looking thing? I’ve put a close up of it here. You can see a dot at the head of the "comet": astronomers think that might be the runaway neutron star from the explosion that created SNR MSH 11-16A! It’s hard to know for sure, but a lot of things add up to make me think they’re right.
The most obvious is that tail of gas pointing right back to the center of the supernova gas cloud. A hot, young neutron star blows out a high-energy wind of subatomic particles called a pulsar wind, and that pushes against gas floating out in space. As a runaway neutron star blasts through space, it would leave a glowing trail like that. The X-rays appear to be coming from a single, tiny point, just what you’d expect for a neutron star, and observations using optical and infrared don’t see it; again, just what you’d expect since neutron stars are tiny and don’t glow visibly. They’re brightest in X-rays due to their phenomenally strong magnetic fields whipping particles around at high energies.
The fainter tail to the side is something of a mystery, though. Apparently things like this have been seen before, but it’s not clear what’s causing it.
Knowing the distance to the supernova remnant we can get its size, which, together with its expansion rate, tells us that it’s something like 15,000 years old. If that dot is the ejected neutron star, it’s screaming away from the explosion site at a mind-numbing 10 million kilometers per hour (6 million mph) — fast enough to cross the entire United State in two seconds flat! Yegads.
Other runaway neutron stars have been seen moving away from supernovae at high speeds, but none this fast. Again, this has not been confirmed to be the neutron star in question, but if it is, it handily holds the speed record for such an event. Mind you, this star probably has about the mass of the Sun: over an octillion tons!
Can I get another "yegads" from the congregation?
Images like this show me that my mental images of some phenomena need to be updated. We like to simplify in our heads, and that’s fine if it helps us get a grip, a basic understanding, of a complex event. But we have to remember that the Universe is weird and complex and sometimes gleefully bent on crushing our preconceptions. That might make some people uncomfortable, but it fills me with joy. Who wants a boring Universe where we easily understand everything?
Weirdness is way more fun.
Image credit: X-ray: NASA/CXC/UC Berkeley/J.Tomsick et al & ESA/XMM-Newton, Optical: DSS; IR: 2MASS/UMass/IPAC-Caltech/NASA/NSF
Life must suck for HD 189733b.
It’s a planet orbiting the star HD 189733, about 63 light years from Earth. It’s similar to Jupiter, being slightly more massive and slightly bigger. Unlike our own big brother, though, HD 189733b is far closer to its parent star, orbiting just about 4 million kilometers (about 2.5 million miles) above its surface! That means the cloudtops of the planet are at a scorching 840°C (1500°F), so hot that the atmosphere of the planet is getting blown away by the star itself, creating a comet-like tail of gas escaping from the planet.
And now, adding insult to injury, astronomers have discovered that the star is prone to cosmic hissy fits — and this may actually blow torch even more air from the planet.
[Illustration by NASA, ESA, and L. Calçada]
This pretty nifty: the researchers targeted the star and planet using Hubble in 2010 and didn’t see anything amiss. In 2011, though, they observed it with Hubble again, but also used NASA’s Swift satellite, which is sensitive to high energy emission like extreme ultraviolet and X-rays. They happened to be looking when the star erupted in a massive flare, quickly quadrupling its brightness in X-rays alone. And because the planet is so close to the star, it took the full brunt of this event, its atmosphere puffing up and actually getting blasted away into space by the fierce light from the star!
Just a few hours later, as seen from Earth, the planet passed directly in front of its star (like Venus did during its transit earlier this month). The atmosphere by this point was really streaming away from the planet in the aftermath of the flare, and was also blocking a bit of the star’s light. In the Hubble data the astronomers not only detected that absorption, but they could measure it to see how much hydrogen the planet was losing. It turns out something like 1000 tons of hydrogen was screaming away from the planet every second! And that’s a lower limit; they could only detect neutral hydrogen — that is, atoms that still held on to their electrons. There was probably plenty of ionized hydrogen they couldn’t detect.
NASA put out a short video that explains this as well:
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
NASA has released the final update on the UARS bird that burned up in re-entry last week: it came down in the Pacific, west of the US.
The Earth’s atmosphere is not a lid over us, but gets thinner with height, so it’s hard to define exactly what it means to say that the satellite burned up at such-and-such a spot. However, at 04:01 UTC on September 24th, the satellite’s motion became dominated by the Earth’s atmosphere, and for all intents and purposes that can be called the point where it came back in… or at least, where it started. The forward motion of the satellite took the pieces along a track 500 – 1300 km (300 to 800 miles) long, which is still safely out in the ocean.
Thus ends the UARS tale.
… but we’re not quite done yet. The venerable German astronomical satellite ROSAT is due to come back down in about a month or so. Smaller than UARS — a little over 2 tons, as opposed to over 6 — ROSAT will probably have more pieces survive the ride down because its mirrors had to be shielded from heat to operate. That means the odds of it hitting someone will be slightly higher than from UARS, about 1 in 2000. Bear in mind that’s still really small odds! The chance of a specific individual getting hit are still something like only 1 in 14 trillion.
ROSAT is an X-ray satellite, designed to study high-energy radiation from astronomical sources. Years ago, I looked briefly at ROSAT data of a supernova remnant while putting together an educational activity about exploding stars. I don’t feel the same connection to the satellite as I do to, say, Hubble, but still, it’s a little sad to see it come down. However, it did provide years of outstanding service to the astronomical community, and gathered a vast amount of data about the high-energy Universe around us.
Image credits: NASA; German Aerospace Center (DLR)
The Carina nebula is a sprawling, monstrous complex of gas located a mere 7500 light years from Earth. Hundreds of light years across, it’s massive enough to create thousands of stars like the Sun. Tens of thousands.
And churn out stars it does. Embedded in the nebula are several clusters of newborn stars, and many of these stars are so massive they’re nearly at the limit of how big a star can be without tearing itself apart. Stars that big explode as supernovae, and a new mosaic by the orbiting Chandra X-ray Observatory indicate they’ve been popping off in the nebula for quite some time:
[Click to enchandrasekharlimitenate.]
This image is pretty amazing: it’s a mosaic of 22 separate images by Chandra, covering 1.4 square degrees (seven times the area of the full Moon on the sky), and represents an exposure time of 1.2 million seconds! Since it shows X-rays coming from astronomical objects, it’s false color: red is from lower energy X-rays, green is medium energy, and blue from the highest energy photons.
The diffuse glow is from two sources: the stellar winds from those massive stars slamming into surrounding ambient gas at high speed, and from the shock waves generated when supernovae explode. Both are extremely high-energy events, and produce copious amounts of X-rays. That long, horizontal arc is probably the edge of a bubble, a shell of gas piled up from the winds of stars and supernovae like snow piled up in front of a snowplow.
That’s evidence right there that Carina has been cranking out supernovae over the past few million years. Interestingly, it’s what’s missing that provides more proof. Read More
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?
Y’know, I should never deal in superlatives. I said Thierry Legault’s shot of the ISS during the solar eclipse last week was the best picture of it, but now, as amazing as that picture is, I think we’ve found something to tie it: the Japanese solar observing satellite Hinode took this jaw-dropping video:
OK, I’ll say it: Holy Haleakala!
Hinode (pronounced HEEN-oh-day, which I’m telling you because I always say HI-node in my head when I see it) orbits the Earth, and has a near-continuous view of the Sun. When the Moon slipped between us and our star on January 4, Hinode had what might have been the best view. This video was made using images from the X-Ray Telescope, or XRT, and is sensitive to objects at temperatures of millions of degrees — the Sun’s magnetic field routinely generates such energies. You can see the looping material on the Sun, following the arcing lines of magnetism. The Moon is dark at these wavelengths, so it appears black in the video.
The other cool thing is the size difference between the Sun and the Moon. The Sun is roughly 400x bigger than the Moon and 400x farther away, so they look about the same size in the sky. But the Moon orbits the Earth in an ellipse, and can change its distance to us by quite a bit, well over 10% — that means its apparent diameter as seen on Earth can change by 10% too.