The image above may look like a comet, but in fact the stream you see in this image taken by NASA ultra-violet space telescope GALEX is 13 light years long, and is comprised of gas thrown off the red giant star Mira over the last 30 000 years or so as it moves through space at a speed of about 130 km/s.
For more information read the press releases (here, and here with a really nice animation), which do a good job of explaining what is physically happening and how this discovery happened. Some of the images are quite impressive, even from a purely aesthetic point of view.
A BBC story is somewhat less informative, but it does includes more quotes from one of the primary scientists involved in this discovery< Mark Siebert (who just happened to be a grad student in the Astro Dept here at Johns Hopkins before moving out west. Hi Mark!).
Mira is a very well studied star and the archetype of a specific class of variable star (Mira variables), but this huge trail of cast-off gas was only discovered in GALEX images taken last year. This kind of discovery is yet another example that illustrates why having telescopes operating at wavelengths other than the optical is vital for advancing our understanding of the Universe. Hubble or ground-based telescopes like Keck and the VLT, with their primarily optical detectors and tiny fields of view, could never have made this discovery.
Almost as impressive as the scientific discovery itself is the effort JPL and GALEX PR team put into producing the press releases. You can see 6 different spins of the story one after the other. The work on the animations are also first class.