[The Desktop Project is my way of clearing all the pretty pictures off my computer's desktop, by posting one per day until they're gone. I think this week is it - I'm almost out!]

Dark matter is funny stuff. We’ve known about its existence for many decades, and the more we look the better our evidence gets. We know it has mass, and therefore gravity, but we don’t know what it is! We do, however, know what it isn’t: normal matter of any kind, like cold gas, rogue planets, black holes, dead stars, or anything else made of protons, neutrons and the other types of particles we deal with in everyday life.

Since careful observations have shown clearly it can’t be any kind of normal matter, it therefore must be some sort of exotic flavor of matter, some kind of particle we haven’t yet seen.

One thing we’re pretty sure about it, though, is that it doesn’t interact with normal matter except through gravity. Dark matter can pass right through you and you’d never know it. But put enough of it in one spot, and its gravity will reveal its presence.

Which is why the galaxy cluster Abell 520 is such a mystery. Here’s the beauty shot:

Pretty, isn’t it? Abell 520 is a galaxy cluster about 2.4 billion light years away, and a mass of several trillion times our Sun’s — it’s made of galaxies, each with billions of stars in them. And a galaxy cluster is a collection of hundreds or even thousands of galaxies bound together by their gravity. In fact, Abell 520 is more than one cluster: it’s actually a collision between two or more clusters! As they move through space, clusters can collide, and actually quite a few of these cosmic train wrecks are known.

When clusters collide, a lot of things happen. The gas clouds in between galaxies in the two cluster slams into each other, heating up to millions of degrees and glowing in X-rays. In the picture above, that gas has been colored green so you can see it (invisible to the eye, the X-rays were detected by the Chandra Observatory). The orange glow is from stars in galaxies (as seen by the Canada-France-Hawaii and Subaru telescopes). The blue is actually a map of the dark matter made using Hubble observations. The gravity of dark matter distorts the light passing through from more distant galaxies, making it possible to map out the location of the otherwise invisible stuff (you can read about how that’s done here and here).

Since dark matter doesn’t interact with normal matter, we expect it to simply pass through the collision point, sailing on as if nothing had happened. That’s been seen in a half dozen other galaxy cluster collisions, including the Bullet Cluster — hailed as definitive proof of the existence of dark matter — as well as Abell 2744 aka Pandora’s cluster (seen here on the right), and the newly found Musketball cluster.

But Abell 520 isn’t like those others. The problem is, there’s a clear peak in the dark matter right in the middle of the cluster, not off to the sides as you might expect. It looks as if the dark matter slammed to halt in the middle of the collision instead of sailing on.

Here’s the thing: this does not mean dark matter doesn’t exist, or we’re wrong about it. The other clusters I mentioned above make it clear we do have a pretty good grip — so to speak — on the behavior of dark matter.

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April 24^th marks the 22^nd anniversary of Hubble’s launch into space. To celebrate it, NASA and ESA released this devastating panoramic view (also available here) of the mighty star-forming region 30 Doradus:

Yegads. [Click to embiggen, or get the 4000 x 3200 pixel version, or grab the ginormous 267 Mb 20,323 x 16,259 pixel version. There's also a way cool zoomable image too.]

30 Dor is a vast, sprawling, and chaotic region located in the Large Magellanic Cloud, a dwarf irregular galaxy that orbits our Milky Way. Even though it’s about 170,000 light years away it’s so bright it’s easily visible using binoculars (if you happen to live in the southern hemisphere or not far from the equator). The reason it’s so bright is that this stellar nursery is churning out thousands of stars, and some of them are the massive, hot, and blue type. These flood the surrounding gas with ultraviolet light which makes the gas glow.

In fact, those young stars are so luminous and energetic they’re eating away the cloud from the inside out! Those big cavities you see are where the light and fierce winds of subatomic particles blown from the stars are slamming onto the gas, pushing it outwards. The edges of the cavities are bright because that’s where gas piles up, and shines more brightly.

In fact, the folks at Chandra released a similar version of this image, except they added observations from that observatory, which detects X-rays (as well as an image using Spitzer which sees in infrared). X-rays are emitted from extremely hot gas, and as you can see in the image inset here (click to embiggen) the cavities are filled with X-ray emitting material (colored blue in the image). I wrote more about this in a post when a similar image was released.

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In 2003, NASA’s Spitzer Space Telescope launched into space to begin a mission to observe the heavens in infrared. That kind of light is emitted by warm objects, so its main imaging camera — called IRAC, for Infrared Array Camera — had to be cooled using liquid helium, or else the infrared light it gave off would interfere with its own observations!

This type of coolant leaks away slowly, and after about five and a half years — a much longer period of time than originally hoped, which was a bonus — the liquid helium was finally depleted. However, this didn’t end the mission; instead it marked the beginning of the "warm phase". Observations could still be made, though only with some of the detectors that weren’t so severely affected by the raised temperature.

That was in May 2009. Spitzer has now been running warm for 1000 days, and to celebrate that milestone the folks running the observatory released their favorite 10 Spitzer IRAC images. Over the years I’ve featured half these images on the blog (see the list below), but I have no idea how I missed this amazing shot:

Isn’t that cool? Well, so to speak. Haha. Because of the warm mission, you see. Ha ha.

But what is it? Just off the top of the picture is a young star. It’s a newborn, a mere baby, probably less than a million years old, and like human babies it tends to spew matter out of both ends. In this case, the star’s rapid spin coupled with its intense magnetic field create two powerful jets of material that blast away from its poles at speeds of up to 100 kilometers per second! What you’re seeing here is one of those jets as it plows through a cold cloud of gas and dust. The shape may be due to the material in the jet following the twisted magnetic field lines, or it may be formed as the shock waves emanating from the interaction become unstable, a bit like breaking waves from a ship ramming through the water at high speed. Either way, it looks for all the world — the galaxy! — like a rainbow tornado.

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So on Twitter, my pal Carolyn Porco — imaging team leader for the phenomenal Cassini Saturn spacecraft — announced that a bunch of raw (that is, unprocessed) images were just released from Cassini. Taken on April 14, these new shots show the moons Tethys and Enceladus. I’ve written about Enceladus about thirty two bajillion times, because it’s fascinating, and photogenic as heck. Geysers of water are erupting from its south pole, reaching heights of hundreds of kilometers, with some particles leaving the tiny moon altogether.

And, unsurprisingly, the new pictures do not disappoint, especially this one of Enceladus!

I know. [Click to encronosenate.]

It almost looks like a solar eclipse picture, but the dark circle is actually the night side of Enceladus, with the sunlight coming from the lower left. You can see quite a few geyser plumes, blasting up from the surface to such heights that they’re illuminated by the Sun and we can see them around the edge of the moon.

As I looked at this shot, I suddenly realized something neat: you can see (at least) three dark arcs across the geysers — I’ve used arrows to point them out in the inset picture. These arcs are actually shadows being cast by the moon itself! The curved edge of the moon’s horizon casts a shadow into space, and where it falls across the geyser plumes it reveals itself as a dark, curved line.

What’s so very cool is that you can see the arc of the shadow edge on the upper left — what I’ll call the first arc — is actually higher off the limb of the moon than the one lower on the right (the third one is closer to the limb of the moon under the first arc).

Why is this cool? Because it tells us where those shadows are falling on the geysers. In other words, it’s 3D info on the geysers for free!

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Right now as I write this, the Sun is settling down after a minor flare tripped a flippin’ huge and spectacular prominence: a looping tower of plasma hundreds of thousands of kilometers high! Using Helioviewer.org, I created a short movie of the eruption, and you just have to see it. Make sure you have the resolution set to hi-def!

Isn’t that amazing? The flare that triggered this event was no big deal, about an M1.7, which is nothing to worry about at all. We had far bigger ones in March! But that arc of plasma — ionized gas — is astonishing. Flares happen when the magnetic field lines of the Sun get tangled, and suddenly release their vast, vast stored energy. The erupting plasma follows those field lines up and away from the Sun. Some escapes forever, and some falls back to the surface. You can easily see it flowing in these videos.

These views show the eruption in two different wavelengths, though both are in the ultraviolet, where the magnetic activity is easiest to see. My friends at NASA Goddard put up some fantastic pictures of it, like the one above [click to embiggen]. They have some video there, too.

I’ll note that the active region shown here is on the side of the Sun rotating toward the Earth right now, so if there are more eruptions in the next few days we may see some affect on Earth, like aurorae at northern latitudes. As usual, you don’t need to panic about this stuff. The worst it can realistically do here on Earth is cause blackouts if a particularly big storm overloads our power grid. And while that would be irritating, it’s unlikely. So sit back and enjoy the show!

[Edited to add: Some folks are asking how long this event took; it went from start to finish in just a few hours. Another common question is how big the Earth would be compared to this, and the answer is: really really teeny. Check out the curve of the Sun's edge, and remember that the Sun is over 100 times the diameter of the Earth!]

Credits: NASA/SDO/helioviewer.org. Music: “Feral Chase” by Kevin MacLeod.

Spiral galaxies are among the most beautiful objects in the Universe. Their grand, majestic nature is sweeping on a scale of hundreds of thousands of light years; their delicate arms are composed of a hundred billion stars blurred into a milky stream; and as for their cores… well, that’s a different story.

Let me present to you two surpassingly beautiful galaxies, each with a dark secret in its heart.

First is NGC 4698, as seen by Adam Block using the 0.8 meter Schulman Telescope at Mt. Lemmon in Arizona:

[Click to galactinate.]

NGC 4698 is relatively close, at a distance of 60 million light years. This image is lovely, with the faint outer arms clearly visible, the inner arms lined with clouds of dust like black pearls on a string. The core looks odd, though, which I noticed right away. It’s brighter than I would’ve expected, and appearing almost as if it’s popping right out of the plane of the galaxy.

And here’s a second spiral, M77, one with which I’m fairly familiar — I spent a long evening photographing it for an observational astronomy class in grad school. This remarkable image, however, was constructed by Andre vd Hoeven, who downloaded dozens of Hubble images of M77 from the online archive, and painstakingly assembled them into this amazing shot:

[Click to embiggen.]

Wee see M77 a bit more face-on than NGC 4698, and by coincidence it’s also roughly 60 million light years away. The red glow dotting the arms is indicative of star formation; those are vast gas clouds glowing from the heat of young, hot stars embedded in them. It too is thick with dust, and like NGC 4698 the core looks… odd. Too bright, too compact. In the high-res version you can also see a greenish glow off to the left of the core, like a searchlight shining in that direction.

So both of these galaxies look normal at a perfunctory glance, but clearly have something else going on, something not obvious that makes them special. A secret, if you will. (more…)

[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