Astronomy is arguably the most beautiful of the sciences. I'm biased, of course, but it's nearly impossible to gaze upon a picture of a galaxy, a moon, a nebula, and not see in it something compellingly artistic. Sometimes it's the color, sometimes the shape, and sometimes it's the knowledge that we can understand the subject of the picture itself. Science doesn't take away from the beauty of nature. It enhances it, multiplies it. There are so many incredible astronomical photographs released every year that picking ten as the most beautiful is a substantial task. But it becomes easier when you consider the science behind the image as well. Does this image tell us more than that one? Was the scientific result drawn from an image surprising, or did it firm up a previously considered hypothesis? Still, there's something to be said for a simple, drop dead gorgeous picture. So here I present my Top Ten Astronomy Pictures for 2007. A note: I tried to avoid repeating using images that were too similar to the ones I posted last year, so you may be surprised at some of my picks, or some pictures I didn't pick. You may even disagree with me, so if you have a favorite that missed being on this list, leave a link in the comments! I just bet ten images aren't enough for most people anyway. All of the pictures here are clickable, and link to higher resolution versions; some substantially so. Enjoy.

Number 10: A Comet Bursts Forth I remember when I first heard of Comet Holmes: I'd received an email in October from a BABloggee telling me about it, and I couldn't wait for the sky to darken so I could see it with my own eyes. Naked eye comets are rare, but a run-of-the-mill faint one to suddenly brighten by a factor of a million; well, that's a must-see. People all over the world took pictures of the comet's outburst (the cause of which is still something of a mystery). For a while, the outburst appeared as a circular shell surrounding the comet, though eventually a more tail-like appearance took shape... but it was still just plain ol' odd.

This picture was taken on November 29, 2007 by the gifted astrophotographer Tamas Ladanyi (he also has another picture taken on December 5 which shows the motion of the comet, too). This remarkable shot covers several degrees of sky; by this time the comet was getting difficult to see with the unaided eye (it got fainter as the debris cloud expanded), and at the time the cloud was bigger than the full Moon in the sky. I love the starry background in this picture, and the small open cluster of stars on the right. As I write this, Comet Holmes is still fading away. But will it brighten again? In 1892, just after it was first discovered, Holmes had an outburst very similar to this modern one. So even as it dims and moves into the outer solar system, is it gearing up for another exciting run?

I've written quite a bit about this comet, if you want the background info, and I even made a video.

Number 9: A Black Hole on Mars [Update (January 20, 2010: I was just informed I had the wrong link to the original image in my post! Oops. I fixed it below.] What is it about Mars that draws us in? Even through a big telescope it's small, distant, and fuzzy. But when you gaze up on it in the night sky it's a bright bloody beacon, a glaring eye staring back at you. No wonder the ancients thought it the god of war! We've been exploring the Red Planet for decades now, and we have learned so much about it. Sometime over the past few decades, Mars transformed from an astronomical object to a place, an actual location where we can go and look around. And even though terabytes of data have been taken, Mars still has a surprise or two up its sleeve.

Update: I had originally described this hole as a skylight to a cavern, but that turns out to be incorrect. I would normally keep the incorrect text here and strike it through so you could see my mistake, but the way I had written it makes that hard. So instead, I'll simply delete the incorrect text, admit my mistake, thank commenter AJ Milne for pointing it out, and post the correct description!

When I saw this image, my jaw dropped. Other images of the surface showed black spots suspected to be cavern entrances or pits, but they showed featureless black holes. But then I saw this picture, and once again Mars reinforced the idea that it's a place. The picture (taken with the HiRISE camera on board the Mars Reconnaissance Orbiter) shows a pit crater, a collapsed shaft going down into the surface of Mars. It's on the side of a volcano, and similar features are seen in Hawaiian volcanoes. Most likely they form when magma under the surface subsides, and the ground above it collapses. In this hi-res picture, with the Sun off to the side, we can see rocks and debris littering the floor. It looks like the pit leads to a tunnel off to the right, but that may be an illusion caused by the shadowing. It tricked me at first! But it's probable that this does not lead to an underground cavern. Still, I've stood at cavern entrances in Virginia and Kentucky (and a sinkhole, once) that aren't too different in appearance from this pit, bringing home the fact that Mars, like Earth, is an actual world. When I look at this picture my sense of exploration, of wanting to walk into that pit and poke around, is nearly overwhelming. I can't help thinking that Jules Verne would have felt at home standing at the lip of that crater.

Number 8: The Wonderful The red giant star Mira is called "The Wonderful" because it brightens and dims noticeably to the naked eye, sometimes going from quite bright to total invisibility in just a few weeks. Mira is a star on its death bed; it is in the final stages of sloughing off its outer layers, and in a few hundred thousand years the entire envelope of the star will have been ejected, leaving only a naked and very hot white dwarf star. It's been studied extensively for decades, centuries-- but when the Galex spacecraft pointed its ultraviolet camera at the sky near Mira, astronomers got quite a surprise.

Mira is like a comet! But a lot, lot bigger: the tail of Mira is 13 light years (130 trillion kilometers/80 trillion miles) long. The tail is really the gas ejected by the star as it dies. As Mira moves through space, the gas between the stars slows the ejected material, forming the long streamer. It reminds me of The Little Astronomer's hair flowing behind her as she runs. But Mira is moving substantially faster than my daughter can run; it's tooling along at 140 kilometers/second (90 miles/second). It's taken the star 30,000 years to move the length of its tail. In the picture, the motion is left to right, so the left-hand side of the tail is the oldest. On the right you can actually see the bow shock as the star plows through the gas between stars. This image isn't the highest resolution, or the prettiest of the lot. But it has so much to say. Mira is much like the way the Sun will be in a few billion years when its time runs out, for one, so by studying this image we are perhaps peering into our own future. But it also tells us that even a familiar object can surprise us, when we look at it with different eyes.

For far more detail on the story of this remarkable object, read my blog entry on it.

Number 7: The Lover's Embrace of Arp 87 Galaxies seem like immutable giants of the cosmos; serene, majestic, and unmoving. But that's an illusion. Everything in space is in motion, including galaxies. As you can imagine, when you take a collection of several hundred billion stars and set it in motion, it can be pretty hard to stop it. Galaxies move through space at numbing speeds, and the forces built up are mighty. But then, sometimes, another galaxy gets in the way. Then those vast forces come into play. Gravity twists and pulls at the galaxies as they dance past each other. And, as if they resist the inevitable recession of their partner, they reach out to one another in what looks like a tender embrace, but is in reality a stark (if lovely) portent of the destruction wrought on both galaxies.

Arp 87 is the name given to the system of two galaxies, NGC 3808A and NGC 3808B. They passed each other just as the age of dinosaurs was starting to get going on Earth, 200 million years ago. The gravity of B (the cigar-shaped galaxy on the left) drew out a long tentacle from the much larger A (the spiral on the right), and it appears as if the passage also wrapped the tendril around B, perhaps more than once. It's also possible the tendril flared out, separating into streams that only appear to entwine the smaller galaxy. The pair is 300 million light years away, and the power of the Hubble Space Telescope is clear when that forbidding distance is taken in. We can see reddish gas clouds collapsing to form stars in the galaxies, and we can (just barely) tell that NGC 3808B is looking a little disturbed. Will the two galaxies continue to separate, or will gravity eventually win, drawing them together? Perhaps the latter. In a few hundred million years the merged remnant will settle down and look like a normal galaxy once again. And while this may look like a totally alien tableau, keep in mind that the Milky Way Galaxy has suffered through such collisions in the past... and will again: the Andromeda Galaxy is bearing down on us. In a couple of billion years, the fireworks here will begin.

My original blog entry about Arp 87 is here.

Number 6: Lightning at Weikerscheim Observatory Astronomy only plays a tangential role in this picture... you can see stars in the sky, and the observatory is pretty obvious. But it's the terrestrial drama that steals the show.

Jens Hackmann took this stunning picture of a lightning storm near the Weikerscheim Observatory; the 300 second exposure is enough to see the stars streak and the observatory lit up by ambient light. Sometimes, when it's cloudy, observing is difficult... but you can still get incredible pictures.

Number 5: A Meeting of the Moons Jupiter is fantastically massive, and its fearsome gravity holds thrall over a retinue of moons that might otherwise be called planets in their on right. By far, the most interesting of its family are the moons Europa and Io. Europa is an ice world, covered in a thick sheet of ice that might reach down to depths of over perhaps several kilometers. It's nearly a dead certainty that underneath that forbidding icecap is an ocean of water, kept liquid from energy input by gravitational stress as Europa passes by her sister moons. Of all the real estate in the solar system, many astronomers have their money on Europa as the best place to look for alien life. Io, on the other hand, is perhaps the worst place for life. It has an incredibly high sulfur content, for one thing. For another, the same gravitational heating that keeps Europa's ocean liquid also keeps Io's interior molten, but it gives the moon a cosmic case of indigestion. Io is wracked with volcanoes. They are almost constantly erupting, spewing molten sulfur over a kilometer high in the low gravity, and plumes of dust and gas blast hundreds of kilometers off the surface. This activity was first discovered when the Voyager 1 probe passed the moon in 1979, but subsequent space probes have gotten even more detailed images. When the New Horizons Pluto probe passed Jupiter in March 2007 for a gravity boost, it snapped a beautiful picture of the sisters.

Europa is the crescent on the lower left, and Io (obviously) is the one on the upper right. The plume you see is from the volcano Tvashtar, which has been active for quite some time now. If you look right at the bottom of the plume, you can see molten sulfur glowing red. Two other volcanoes appear to be making some noise as well. While they appear to be close together, the two moons were actually nearly 800,000 kilometers apart when this picture was taken; Io was on one side of Jupiter and Europa on the other, but from the spacecraft's perspective they were next to each other in the sky. This picture is actually a composite of two images; one was greyscale and had high resolution, and the other was in color but had lower resolution. By merging the two, we can see more details than we could from the color image alone, and we get the benefit of having the colors enhance the scene. When I first saw this image, I knew right away the two moons were not close together at all. My secret? I saw that the dark side of Europa was truly dark, but Io's dark side was lighter. That meant that Io was positioned such that Jupiter was illuminating its otherwise dark half, while Europa must have been on the other side of Jupiter, where it was dark. Sometimes, you can tell a lot just by looking at a image and picturing the geometry in your head. Funny: I almost picked a movie sequence of Tvashtar erupting for this Top Ten list, but I had to cull it due to the other images I liked better. Plus, the animation is 752 kb, and I didn't want to choke my server. But you can see it on Emily Lakdawalla's Planetary Society Blog. It's worth watching!

Number 4: Dark Matter Makes an Appearance It's baffling, and a little humbling, to think that what we can see, taste, and feel makes up only a tiny fraction of the Universe. Normal matter only constitutes 4% of the cosmic budget, with dark energy eating up a whopping 73%. The remaining 23% belongs to the mysterious dark matter: some exotic form of material that does not emit light, but does exert a gravitational force. Astronomers knew it was out there for decades; it pulled on galaxies and clusters and galaxies, changing their speeds. We just couldn't see it! But the gravity of dark matter has a subtle effect: it acts like a lens, bending the path of light coming from objects behind it. We can use telescopes to map out the location of normal matter, and then measure the distortion of background objects from the intervening dark matter. When that is done, spectacular results emerge.

In this Hubble image, the white dots are entire galaxies. They are part of a cluster called CL0024+1652, which is a whopping 5 billion light years away. The blue glow is the location of the dark matter, revealed by its distortion of the shapes of more distant galaxies behind it. The dark matter is in the shape of a ring surrounding the cluster, which indicates that a long time ago, CL0024 suffered a mighty blow, colliding head-on with another cluster. The dark matter from the two clusters passed right through each other, and their gravity caused the material to form the ring shape. We're seeing this right down the barrel of the collision. This image is a stunning confirmation of the existence of dark matter, and our understanding of how it works. Many people -- who don't understand the science -- claim dark matter doesn't exist, and that astronomers are making it all up. Well, there's a giant smoke ring in the sky indicating they are quite wrong. However, that's not to say we understand everything about such events. In a picture I almost picked for the Top Ten, we see evidence that there are holes in our knowledge.

In this composite image of the galaxy cluster Abell 520 using Chandra, CFHT, and the Subaru telescope, red is normal matter heated to millions of degrees, and blue shows the location of the dark matter. The problem is, where the normal matter is densest the dark matter is least dense, and vice-versa. That's the opposite of what's expected here. Perhaps dark matter particles interact with each other differently than we think, or there was some odd factor in the cluster collision that's throwing a monkey wrench into the works. There's a lot more about dark matter to learn, and images like these will help us solve these mysteries. Right now there a lot we don't know... but we'll figure it out. That's why astronomy is so much fun.

Number 3: Chaos in Vela I have long been a fan of Davide De Martin from Sky Factory. He takes images from professional observatories and stitches them together to make images of indescribable beauty, elegance, and wonder. His work is, simply, breathtaking.

This image shows the devastation wrought when a star explodes. The Vela Supernova Remnant formed when a massive star 800 light years away blew up 11,000 years ago. Expanding at a ferocious velocity, it is now 8 degrees across in the sky -- 16 times the apparent width of the Moon, and about the size of your outstretched fist! David's mosaic shows a stunning amount of detail, tracing the variety of shapes and patterns the expanding gas makes as it slams into the interstellar junk floating around it. And if that's not enough, the full-size image he has on his site is well over a billion pixels in size. Think about that the next time you brag about your digital camera.

I wrote more about David and his Vela image on my blog.

Number 2: STEREO Eclipse Technically, this one is not a picture, but I had to include it anyway. It's just so cool! Studying the Sun seems like a pretty good idea; as the major source of light and heat for our planet, it's a good thing that we try to understand it. And the Sun is a star, with all that implies: it's huge in size, and frightening in its energy production. To better understand it and the complicated nature of its surface activity, NASA launched a pair of satellites that can take pictures of the Sun simultaneously from different angles, providing a 3D view of our nearest star. They're called the Solar Terrestrial Relations Observatory, or (haha) STEREO. One satellite orbits the Sun ahead of the Earth, and the other behind. They stay in the same orbital plane as the Earth does in its travels around the Sun, and that means that sometimes you get interesting geometric situations which arise. The STEREO team realized that there would be a time when the Moon would pass in front of the Sun as seen by one of the spacecraft. Solar eclipses are rare on Earth, but from STEREO's vantage it would be even more unusual. From the Earth, the Sun is about 400 times farther away than the Moon, and is also about 400 times the Moon's physical size. These two characteristics cancel out, meaning the Sun and Moon are about the same apparent size in the sky, so every solar eclipse has the Moon slipping in front of the Sun, with the dark disk of the Moon just barely (if even) covering the bright disk of the Sun. That is, every solar eclipse as seen from the Earth. But STEREO was receding from the Earth and Moon. To it, the Moon appeared much smaller than we see it, stuck as we are on the surface of our planet. The Sun, however, was still at about the same distance, and therefore looked the same size as we see it. This means that, to STEREO, the Moon appeared far smaller than the Sun. Not long after launch, the situation arose that the Moon would pass directly in front of the Sun as seen from one of the spacecraft. STEREO turned its eye that way, and recorded what may be the most remarkable footage of a solar eclipse ever taken. This sequence shows something we can never see from the Earth: a shrunken Moon passing in front of the Sun. Technically, it's not even really an eclipse; it's a transit (when something small crosses in front of something large). That video, only 8 seconds long, is incredible. We see the Sun and Moon literally every day, of course, and it totally shook me to see them look so different. That's why I chose this sequence. I love seeing the familiar become unfamiliar, grasping a new perspective on the everyday, the ordinary. You see? There is something new under the Sun.

By the way, STEREO has taken some truly excellent images of the sky, including an astonishing sequence of Comet McNaught.

And the Number One Astronomy Picture of 2007 is...

The Beautiful Face of IC 342

I am a sucker for spiral galaxies, especially ones that are face-on. They are sweeping, majestic, chillingly beautiful, and utterly mesmerizing. There are many such grand design spirals in the sky, and astronomers are almost all familiar with the roster: M81, M51, M101, and others. But there is one that is a bit less seen, less well known. That's because it hides itself, tucked away in a part of the sky where stars and dust are thick, obscured by the fog of our own Galaxy. But it is no less gorgeous for being demure.

This creature is IC 342

, a nearby spiral lying only 11 million light years away, very close on a galactic scale. If IC342 were located in a region of the sky where our own Milky Way didn't interfere with the view so much, it would be one of the most celebrated objects in the sky, and would most likely sport a proper name and not just a numerical designation. But because we see it through so much murk, it remains mostly unknown. What a misfortune! It is a spectacular object

. The power of the Mayall 4-meter telescope at Kitt Peak in Arizona reveals the true beauty of this galaxy. Almost all the stars in the image are actually in our Galaxy, much closer than IC 342 (imagine looking out a dirty window to see a bird sitting on a fence to get an idea of what's going on). But look at it! The arms trace out the births of countless stars, punctuated by glowing red gas as the young stars heat their cocoons. Dark dust lanes hide the stars behind them, mimicking the way the Milky Way dust hides IC 342 itself. You can trace the magnificent arms all the way in to the center, where they meet in a pinwheel of colossal proportions. In the highest resolution images (click here if you dare!

) the picture is even more boggling: you can see far more distant galaxies in the background, some edge-on spirals, and even one ring galaxy. Huge clumps of star-forming regions in IC 342 reveal themselves, and you can spot places where the spiral arms split into separate spurs. Even the foreground stars, so much closer to home, provide a polychromatically spangled vista against which the much more regal spiral lies. All in all, a devastating picture. The colors, the features, the composition... and it shows us that sometimes, beauty and grandeur can be missed, even when it's in your own back yard. And that's why it's my Number One Astronomy Picture of 2007.