Every year, this gets harder. Not that deciding what pictures to use in 2006, 2007, or 2008 was all that easy! But astronomy is such a beautiful science. Of course it has scientific appeal: the biggest questions fall squarely into its lap. Where did this all begin? How will it end? How did we get here? People used to look to the stars asking those questions, and coincidentally, for the most part, that's where the answers lie. And we'll be asking them for a long time to come. But astronomy is so visually appealing as well! Colorful stars, wispy, ethereal nebulae, galactic vistas sprawling out across our telescopes... it's art no matter how you look at it. And our techniques for viewing the heavens gets better every year; our telescopes get bigger, our cameras more sensitive, and our robotic probes visit distant realms, getting close-up shots that remind us that these are not just planets and moons; they're worlds. So every year the flood of imagery takes longer to sort through, and far longer to choose from. And the choices were really tough! This year leans a bit more toward planetary images than usual, but that's not surprising given how many spacecraft we have out there these days. I don't pick all these images for their sheer beauty; I consider what they mean, what we've learned from them, and their impact as well. But have no doubts that they are all magnificent examples of the intersection of art and science. At the bottom of each post is a link to the original source and to my original post on the topic, if there is one. If you disagree with my picks, or think I've missed something, put a link in the comments! All the pictures have descriptions, and are clickable to bring you to (in most cases) much higher resolution version. So embiggen away! And welcome to my annual Top Ten Astronomy Pictures post. Enjoy. ENTER THE TOP TEN GALLERY

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#10: Pandora's Galaxy

NGC 4522 is a spiral galaxy that lies 60 million light years away, a denizen of the Virgo Cluster, the nearest such collection of galaxies to our Milky Way. At that distance, you could wait a million years and not see any motion of the galaxy through the cluster. But don't be fooled; NGC 4522 is screaming through Virgo at 10 million kilometers per hour. You can see that for yourself: look above it! There is copious gas and dust flowing out of it from the top, as you can easily see in this picture from the Hubble Space Telescope. As the galaxy slams into the thin gas that exists in the cluster, that material exerts a pressure on the gas and dust inside the galaxy, blowing it out the back. This image of the galaxy shows that material streaming upwards, tendrils of dark dust and red gas flailing behind the galaxy. If you look carefully, you can see patches glowing blue, as new, massive, searingly hot stars are born out of the compressed material. But I love this picture not just because of that, but also because of the huge number of far more distant galaxies you can see littering the background. Most of them are hundreds of millions of light years away and more, but the illusion of them bursting out of NGC 4522 is fairly strong. Galaxies don't actually do that, of course, but I rather like the poetic image of babies erupting out of the galaxy like dandelion seeds on the intergalactic wind.

Original Hubble news storyBA Blog postGigantic version

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#9: A Computer's Spot in the Sun

Dark blemishes on the Sun have been known since antiquity; when the setting Sun's disk is dimmed by dust in the air, you can sometimes see sunspots against its reddened face. Those are monster spots, and very rare, but even the normal run-of-the-mill sunspots aren't well understood. But that's changing. In the 1850s came the first hints that sunspots were magnetic in nature, and over the past 15 decades we've made quite a bit of progress. We now know that sunspots are manifestations of the Sun's seething and complex magnetic field. Where the field lines erupt from the surface, the hot solar gas is constrained like a prisoner in a cell. The gas cools, but cannot sink back down beneath the surface. It stays put, looking dark against the Sun's otherwise blinding light. Magnetic fields are incredibly difficult to model mathematically, but as computers improve, so does our ability to understand magnetic behavior. That picture above looks just like a sunspot, but it isn't: it's a computer-generated simulation of the flow of gas on the Sun as it is squeezed by the solar magnetic fields. It's the first time a sunspot has been modeled in three-dimensions, and is a breakthrough for scientists studying how the Sun works. It's also a first for my Top Ten posts: a picture that's not of a real object! But how could I leave it off? It's so awesome! And this affects you too. In 1989 a giant magnetically-driven ejection of material from the Sun caused a massive power outage in Quebec in winter, and did billions of dollars of damage. Understanding how the Sun works in this way can help us predict when these events are likely to happen. That'll save money and lives. And we're expecting the Sun to start ramping up on sunspots again in just a few years...

Original news storyBA Blog post

Credit: Matthias Rempel, NCAR

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#8: Home

Rosetta

is an impressive European spacecraft on its way to the unlikely-named comet 67 P/Churyumov-Gerasimenko. Once it arrives in 2014 it will extensively map the comet's solid nucleus and, incredibly, even deploy a lander

which will take close-up pictures and sample the comet's material. Comets are on orbits that can make them difficult to reach, so the gravity of planets must be "borrowed" to get the probe to its destination. In November 2009, Rosetta swung by the Earth for its third and final assist. While it was still over 600,000 km (360,000 miles) away, it took this remarkable image of our home. I look at pictures of Earth from space every day, and it's easy to forget how special our world is. Pictures like this remind us how stunning and beautiful it is. And better yet, in my mind, the crescent shape is so similar to the way we think of the Moon and Venus that's it a strong reminder that we are but one world of many, a planet both mundane and special, ordinary and unique. Plus, I'm a sucker for home pictures.

Original news storyBA Blog postCredit: ESA ©2009 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

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#7: The Lunar abyss stares back

One of the biggest news stories of the year was the confirmation of large quantities of water on the Moon, stored in the endless dark of crater floors at the lunar poles. If the crater is deep enough sunlight can never reach the bottom, and the temperature stays chillingly cold. Water can come in the form of comets, for example, which impact the Moon and blast water molecules over the surface. When sunlight hits them the molecules quickly fall apart, forming hydrogen and hydroxyl ions. But if the water settles to the floor of a polar crater, it can stay intact. Over billions of years, it builds up. Several attempts to find these hypothetical ice fields resulted in disappointment until October 2009, when the LCROSS lunar impactor

slammed into the floor of the crater Cabeus, and water was positively detected in its plume. In the buildup to that event, the Lunar Reconnaissance Orbiter took detailed images of the lunar poles, returning this dramatically lit view of Erlanger, a crater 10 km (6 miles) across just 90 km (50 miles) from the Moon's north pole. The Sun is perpetually on the horizon as seen from the crater, and only the raised rim is ever lit. In this shot nearly the entire circle of the rim can be seen, poking into the sunlight and creating a ghostly halo at the Moon's pole. The crater has steep walls, and the bottom almost never sees sunlight. Is there ice lurking in this inky blackness as well?

Original news storyBA Blog postCredit: NASA/GSFC/Arizona State University

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#6: The Cosmic Hand of Destruction

Every now and again, an astronomy picture will make me stop and do a double-take. This one really threw me when I first saw it and made me laugh out loud. It's a hand! And it's reaching for something! Perhaps some sort of cosmic sausage! This eerie shot, from the Chandra X-ray Observatory, is actually showing extremely hot gas around a pulsar, the rapidly spinning core of a once-exploded star. The pulsar -- named PSR B1509-58 -- is buried in the bright spot in the "wrist" of this structure. As the pulsar spins, it generates extremely strong magnetic winds which blow out and interact with previously existing gas in the region. Dense knots of gas form, and those are at the tips of the "fingers" (if only they were holding a "laser"). The red gas above is probably expanding material from the supernova explosion that formed the pulsar in the first place, something like 1700 years ago. The jet of material stretching out to the lower left of the pulsar is being ejected from the pole of the star. It's moving at 200 million km/hr -- fast enough to get from the Earth to the Moon in 7 seconds -- and is hundreds of trillions of kilometers long. Mind you, the pulsar itself is perhaps only 10 km (6 miles) or so across. My home town was bigger than that. But my favorite thing about this: doesn't this image look like an actual X-ray of a hand and wrist? That's funny, because the image was in fact taken in X-rays! Chandra is sensitive to the high-energy light coming from astronomical objects, so in a weird twist of fate an X-ray image actually looks like, well, an X-ray image! See? Astronomy is humerus.

Original news storyBA Blog postCredit: NASA/CXC/SAO/P. Slane, et al.

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#5: Beautiful plumage!

The Cassini spacecraft

has been orbiting Saturn since 2004, and has been returning a virtual firehose of incredible images and science ever since. I'd argue it's one of the most dramatically successful space probes of all time. It's certainly revolutionized our view of the ringed planet over and again. But it's been the views of Saturn's fleet of moons where Cassini really shines. Some are small, some huge, but they are all worlds, as unique and individual as people. But even in this diverse retinue of satellites, Enceladus stands out. At first you might not think so: it's an icy ball just 500 km (300 miles) across. But close-up images from Cassini show deep grooves (called tiger stripes) on the south pole of the moon, and also detected massive water geysers erupting from them! Ever since that discovery, Enceladus has been a major target for close fly-bys from Cassini. On November 21, 2009, Cassini flew

within 100 kilometers (60 miles) of the moon's pole, taking a series of images

stitched together into this incredible mosaic

by The Planetary Society Blog's Emily Lakdawalla. It shows many erupting geysers, spewing out tons of water into space at hundreds of kilometers per hour. Seen near the day/night dividing line, the plumes stretch into space and into the sunlight, making them visible even over the dark surface. It's unclear if the interior of Enceladus is a moon-circling ocean, or if these geysers are from local patches of water heated by natural radioactivity and the tidal flexing of the moon from the mighty gravity of Saturn. But every time Cassini passes near and even through the plumes, Enceladus reveals a little more about itself and we learn a little bit more. Oh, one more thing: the moon's name comes from a Greek myth; Enceladus was one of the giant sons of Saturn. He was fatally wounded in a battle, and buried under Mt. Etna. Amazingly, in the mythology, eruptions from that volcano are said to be due to Enceladus turning over to relieve stress from his injured side! I think the ancient Greek storytellers might be amused to know that their myths are reflected in the geysers of a distant world.

Original news story (includes an amazing animated flyby of the plumes!) BA Blog postCredit: NASA / JPL / SSI / mosaic by Emily Lakdawalla

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#4: Hubble's Metamorphosis

In May 2009, the Space Shuttle Atlantis roared into orbit to rendezvous with the ailing Hubble Space Telescope. After several exciting spacewalks, astronauts succeeded in making many upgrades to the venerable observatory

, including installing new high-resolution cameras, fixing old shorted-out cameras, and replacing needed hardware like computer parts and gyroscopes. We all waited anxiously, and finally, in September, new images from the refurbished 'scope were released. Although at the time I didn't post about this image specifically, this shot of NGC 6302, the Butterfly Nebula, is a jaw-dropper. It's a planetary nebula, the gas emitted from a star not unlike the Sun as it finally dies after billions of years. The core of the star undergoes various paroxysms, and those are reflected in the outer layers, which expand and contract over millions of years. A gust of material blows away from the dying star like a super solar wind, and as the star heats up the gas glows. In this case -- in most cases, really -- the gas doesn't expand in a sphere, but instead like an oblate spheroidal shell, like a beach ball someone is sitting on. Faster material blows out the top and bottom of the shell, creating these vast, delicate-looking structures. This image, taken with the newly installed Wide Field Camera 3, shows hydrogen (red), oxygen (blue and green), helium (blue-green), and nitrogen and sulfur (also red) in the gas. Someday, about 6 billion years from now, our own Sun will undergo a similar transformation. It may not be quite hot enough during the process to light up in such a dramatic and pretty way, but when I see images like this I often wonder if humans, or our nearest equivalent at the time, will be watching as it happens. Hopefully they'll be peering out the rear view window of their spaceships as they find another system to settle in.

Original Hubble news storyBA Blog postCredit: NASA, ESA, and the Hubble SM4 ERO Team

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#3: Eternally Stargazing

I normally wouldn't associate Easter Island with astronomy. Of course, the skies there must be incredibly dark; located about 3000 km west of Chile in the Pacific Ocean, you might wonder how much more black the nights could be, and of course the answer is none. None more black. The island, called Rapa Nui by its inhabitants (and yes, more than 3000 people live there), is most famous for its moai, or statues of heads (and the model for Squidward's house, of course). When I was a kid, Easter Island was a big deal in the ancient astronauts nonsense, but even today as a hardened skeptic I see the mystical appeal of the statues. All of them vaguely expressionless, staring upwards, and waiting, always waiting... And wait they have. Carved over five centuries ago, they still sit on the island, motionless beneath the canopy of the sky above. Posed this way, the moai Piro Piro was captured by the gifted astrophotographer Stéphane Guisard in this stunning image. It's truly marvelous: you can see the Large Magellanic Cloud (one of several satellite galaxies orbiting the Milky Way) to the right, the bright star Canopus to its lower left, and, on the other side of the moai, a rainbow. But not just a rainbow: it's a moonbow, using the Moon's illumination instead of the Sun's. Note too the clouds in the picture. Normally that would spoil the view for an astrophotographer, but in this case they add to the moody atmosphere of the picture, compounding the ethereal feeling to the scene.

Original pageCredit: Stéphane Guisard, with permission.

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#2: The Detail's in the Devil

It's easy to think of Mars as a dead world. Cold, small, and dry, it seems unchanging through our telescopes. Almost. Mars has an atmosphere, though it's thin: about 1% of Earth's atmospheric pressure, it doesn't seem capable of doing much. But when you have a robotic probe like the Mars Reconnaissance Orbiter and its 50-cm resolution orbiting the Red Planet, you see far, far more, and learn that the air of Mars can create beauty. The image above shows a region of Mars near its mid-lower northern latitudes. It's a close-up of the bed of a crater, and you can see the ripples of sand dunes, endemic on the Martian surface. The sand is similar to beach sand here on Earth, but is dark in color because it's made of basalt, a greyish rock. Then why is Mars so red? It's because of much finer-grain dust, which is reddish in hue. The dust lies on top of the sand, making everything look red. But then there's that thin Martian air. Rising heat from the plains can blow through cooler air above, forming vortices like mini-tornadoes called dust devils. These then roll across the surface, picking up the lighter red dust but leaving behind the heavier, darker sand grains. What remains, as seen from above, are these gorgeous swirls, the fingerprints of the geology and weather of Mars. This image has a resolution of about a meter per pixel, and is only a half kilometer (.3 miles) across. The detail in it is phenomenal! It makes me wonder what it would look like to actually soar above the surface of Mars in a glider, tipped and tilted by the vagaries of the Martian winds. But then, it probably looks just like this. With less airsickness.

Original news story (with muchly embiggened version) BA Blog postCredit: NASA/JPL/University of Arizona

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And the number one Astronomy Picture of 2009 is... <Drum roll> #1: That First Small Step

At first, you might think I'm nuts. But if you haven't seen this image before, take a good, close look. See that shiny white thing in the upper left? That's where humans first slipped the surly bonds of Earth, and walked on another world. I was too young at the time for me to now remember Neil Armstrong stepping on the surface of the Moon in July, 1969. I do remember later missions, including watching from Cape Canaveral as Apollo 15 thundered off its launch pad. The excitement of those times was palpable, and is still fresh in my mind all these years later. Still, there's nothing like seeing it again, and having all those memories pour back... And this image released the floodgate. Taken by the Lunar Reconnaissance Orbiter, this high-resolution picture clearly shows the landing site of the Eagle, just a few meters from West Crater. Large boulders and rubble are strewn to the left of the crater; the very same debris that caused Armstrong to take control of the Lunar Module from the computer and find a safe, flat place to land. He made it with mere seconds left in the fuel reserves, showing just why it's sometimes critical to have a human at the wheel.

And there the lander sits, not even a half kilometer from certain destruction, showing how the history of humanity sometimes rests on a razor's edge. In the zoomed section of the image, you can see the lander, its four footpads, and even the darker material around it as the astronauts' bootprints stirred the lunar dust for the first time in perhaps millions of years. The arrow marks the position of the ladder affixed to the lander's leg, the very place where Armstrong left the manmade vehicle and stepped into completely unknown territory. For two and a half hours after that, two men from Earth scampered, played, measured, sampled, and photographed the environment, and in doing so drew a line right through history. Forever more, there will be a time before humans walked on another world, and a time after. And that, of course, is why I picked this image as my Top Astronomy Picture of 2009. It may not be the prettiest, but its import is inarguable. This image reminds us of how far we've gone, and when I look at it I'm reminded of how far we have yet to go. It's been nearly 40 years since a human walked on the Moon, and if we want to set our sights even farther, we'll still have to cover that old ground again before taking another giant leap.

Original news storyBA Blog postCredit: NASA/GSFC/Arizona State University

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