On April 24, 1990, the Space Shuttle Discovery roared into orbit, carrying with it the most famous observatory of all time: the Hubble Space Telescope. To commemorate this 19th anniversary, the Hubble folks have released this wonderful image of interacting galaxies called Arp 194.
Click to embiggen, or go here to hugely brobdingnangify. There's quite a bit going on here! First, take a look at the upper spiral galaxy, located 600 million light years from Earth. If you look at the top spiral arm, you'll see another galaxy nucleus lying along it; it's the bright orange patch above and to the right of the big spiral's center. It looks like that's the remains of a galaxy that is in the process of colliding with the big spiral (the other bright galaxy -- the compact spiral directly to the right of the big spiral's center -- appears to be a background galaxy, coincidentally seen nearby). This collision has drawn out a vast 100,000 light-year-long streamer of gas from the big spiral, which is fairly obvious as the blue ribbon in the middle of the image. This long tongue of gas has collapsed to form millions of stars; the most massive and most luminous are blue, which is why the streamer is that color. Eventually these stars will explode, and then that arm will really light up!
Interestingly, there is a galaxy below the interacting pair. Is it associated with them? It seems like it should be, since that blue stream of gas and stars looks like it goes right into that galaxy. But in fact you have to be careful here. In the zoomed image on the right, you can see the lower galaxy and the bottom of the streamer. I've indicated three spots in the image; see how they're redder and darker? That's because there's tons of dust there, material which absorbs and reddens the light from stars behind it. Dust is associated with young stars, so it's clear this dust must be part of that stretched arm of gas and stars from the upper galaxy. But the dust is seen in front of the lower galaxy. If the lower galaxy were interacting with the ones above it, you might expect that dust to be mixed in with the galaxy. So it's possible the lower galaxy is somewhat farther away, unaffiliated with the other galaxies, and untouched by the cosmic disaster. But it's also possible it's just a wee bit farther away, still part of the group, but has still managed to remain unscathed. Oh, one more thing: check out the top of that smaller image. See that one star that's glowing an evil red? That's probably an evolved star, a red supergiant. If it is, that means it may very well be the next star to go in this scene; red supergiants are stars at the very ends of their lives. It may have less than a million years before it detonates and becomes a supernova, and will outshine the combined light of the galaxies around it. Or it may just be a very distant background galaxy. I doubt it; the positioning of it right in the middle of that streamer is awfully suspicious. But stranger things have happened! After nearly two decades in orbit around the Earth, Hubble is still in a position to amaze me. I worked on three different Hubble cameras for a total of a decade, and I have seen literally thousands of images returned by the spaceborne observatory. Yet, even with all that, it still delivers pictures of the Universe that surprise, fascinate, and enthrall me. It's amazing to think of how much space is out there, how many incredible objects swim in its vastness, and how well we can probe the nature of them. Hubble will be serviced one final time by the Shuttle Atlantis, due to lift off May 12. It will almost certainly continue for several more years after that, but inevitably the time will come to shut the old bird down. By then, though, we'll have new telescopes, like the James Webb Space Telescope, ready to step in. What wonders will they show us?