This story is a few days old, but it’s still an interesting one. The following picture was taken by NASA’s Galaxy Evolution Explorer space telescope (Galex for short) in ultraviolet:
Now, one of the first ideas this might conjure up is a comet, though if you’ve seen pictures of comets, you’d know that this was not quite right. Indeed it is not a comet, but something much bigger and further away.
It is a luminous trail of gas 13 light years long being ejected by a moving star, which is at the center of the “head” of this trail, on the right side of the image. What makes this even more interesting is the fact that this tail is only “visible” in ultraviolet light (making it technically invisible to us). Here’s a comparison of the ultraviolet imagery and the visible imagery:
As you can see, there’s no trace of the tail. So what it is? Well, Mira is a red giant, an older star much like what the sun is predicted to be in a few billion years. It’s spent most of its hydrogen reserve and is now fusing helium, which makes it more unstable and causes it to eject hot debris. As it moves through the galaxy at a supersonic rate, the debris leaves a trail, which is visible to us only in ultraviolet. For a better, more detailed explanation see NASA’s page on the subject and Phil Plait’s Bad Astronomy Blog post.
[Update] There is another good post on the topic by Julianne of Cosmic Variance.