Here is an updated Hubble image of the galactic merger NGC 6240 (image description below) from an image in 2008. The image was taken with the Wide Field Camera 3 and Advanced Camera for Surveys which gives us a more detailed look at the center of the galaxies than in 2008. Click the image above to see the difference.
When I say galactic merger I don’t want to imply this merger is complete – far from it. What isn’t seen here are two black holes at the center of the merger only 3000 light-years apart and that is close enough for their fate to be set. The two black holes are feeling their mutual gravitational attraction and are slowly spiraling towards each other and will eventually merge into a single black hole.
We do have X-ray evidence of the two black holes in this image from the Chandra X-ray Observatory taken in 2002.
NGC 6240 is located 400 million light-years away in the constellation Ophiuchus, that is so far away, who knows perhaps the merger has already taken place, the cosmic look back time in action.
A side note: You would think there would be stars colliding in such mergers, but this is not the case. The distances between stars is so large such collisions are unlikely at least in any widespread way.
From Hubble (and you can get desktop versions of the image at the link):
Not all galaxies are neatly shaped, as this new NASA/ESA Hubble Space Telescope image of NGC 6240 clearly demonstrates. Hubble previously released an image of this galaxy back in 2008, but the knotted region, shown here in a pinky-red hue at the centre of the galaxies, was only revealed in these new observations from Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys.
NGC 6240 lies 400 million light-years away in the constellation of Ophiuchus (The Serpent Holder). This galaxy has an elongated shape with branching wisps, loops and tails. This mess of gas, dust and stars bears more than a passing resemblance to a butterfly and, though perhaps less conventionally beautiful, a lobster.
This bizarrely-shaped galaxy did not begin its life looking like this; its distorted appearance is a result of a galactic merger that occurred when two galaxies drifted too close to one another. This merger sparked bursts of new star formation and triggered many hot young stars to explode as supernovae. A new supernova was discovered in this galaxy in 2013, named SN 2013dc. It is not visible in this image, but its location is indicated here.
At the centre of NGC 6240 an even more interesting phenomenon is taking place. When the two galaxies came together, their central black holes did so too. There are two supermassive black holes within this jumble, spiralling closer and closer to one another. They are currently only some 3000 light-years apart, incredibly close given that the galaxy itself spans 300 000 light-years. This proximity secures their fate as they are now too close to escape each other and will soon form a single immense black hole.