Star trails from the La Silla Observatory in Chile.  These are the paths traced out by stars over several hours as the Earth rotates on its axis.  You can find more information here.

3d-printed Zoetrope of a tumbling hypercube.  I find it quite mesmerizing.

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Hubble telescope photos of Saturn's southern aurora

Image of an Eta Aquarid meteor over Mount Bromo, captured by Justin Ng.

A coronal mass ejection from the surface of our sun on August 31, 2012 as captured by NASA.  A full HD video can be found here.

Geminid Meteors over Chile by Yuri Beletsky, courtesy of NASA's Astronomy Picture of the Day

Celestial Arch by Stephane Vetter, a beautiful photo of the aurora borealis above a glacial lake in Iceland. 

Shot of the Milky Way above the Himalayas by Anton Jankovoy

Two possibilities exist: Either we are alone in the universe, or we are not. Both are equally terrifying.

Arthur C. Clarke

Segment of the Orion Nebula, with its full spectrum of colors in visible light.

This is the dark side of the moon, not to be confused with the Pink Floyd album of the same name.  It's not actually dark, it's just turned away from us all the time.  It's interesting how this pattern of craters seems completely alien to us, even though it's on the nearest solid body in the universe.

This image was taken by NASA's Apollo 16 crew.

This little thing is called the Cubli, and it's a wonderful piece of technology that takes full advantage of rotational inertia to make a cube move or stand in amusing ways.  Stuff like this reminds me why science is so cool.

Image of four moons of Saturn along with its rings, taken by Cassini.  From biggest to smallest, the moons in this shot are Titan, Dione, Pandora, and Pan.

Another Geminid Meteor photo composite, this time from less than a week ago over the Teide Volcano.  More information here.

Image of the Bubble Nebula, courtesy of NASA and J-P Metsävainio.

I know it's not really a bubble, but I find the likeness very amusing, as well as the concept of a giant bubble just floating out there in space.

This is an image composed of 365 slices of photos of the same spot in the same forest taken over the span of a year, one photo a day.  It's amazing how much variety one location can have from season to season; it's easy to forget when we only see one day at a time.

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Lenticular clouds hanging out over the coastline at sunset