Folks, the green light you seen in this image is... well, we don't know exactly. It is, by far, one of the most bizarre celestial objects in existence. Furthermore, it is FREAKING HUGE, about the size of the Milky Way actually. However, its origin is unknown.
Hanny's Voorwerp (which is Dutch for "Hanny's object") is the wispy green figure located just below the spiral galaxy IC 2497, which is about 650 million light-years from Earth in the constellation of Leo Minor. One of the leading theories about its formation suggests that the enormous figure was created by an active quasar looming near the black hole at IC 297's galactic center. This quasar was deactivated some time ago, leaving behind a tidal tail of ionized oxygen atoms in a voorwerp. Or perhaps the larger galaxy tore apart a gas-rich dwarf-galaxy, whose combined x-ray and ultraviolet radiation provided the spark that set the gas alight.
Upon further inspection, astronomers found several clusters of young stars scattered throughout the intricate filaments, saying “The star clusters are localized, confined to an area that is over a few thousand light-years wide,” explains astronomer William Keel, who is the leader of the Hubble Study from the University of Alabama in Tuscaloosa. “The region may have been churning out stars for several million years. They are so dim that they have previously been lost in the brilliant light of the surrounding gas.”
Furthermore, if the voorwerp did prove to have come from a no-longer active quasar in IC 297's central core, it could have some interesting implications for our models which explain how quasars form and are ultimately deactivated. Say the quasar shot a particularly powerful beam of light into interstellar space, illuminating the ionized oxygen, hypothetically, this suggests that quasars, which are mysterious and still poorly understood, might actually flicker on and off, intermittently casting a shadow on the blob. Said shadow could be responsible for the illusion of the gaping hole that's about 20,000 light-years wide. Yet, this "gaping hole" is still only a small fraction of the object itself, which is actually a twisted rope of gas about 300,000 light-years long that wraps around the galaxy.
Lastly, Hanny's Voorwerp was actually discovered by Hanny van Arkel, a dutch school teacher through the Galaxy Zoo project, which gives members of the public an opportunity to participate in assessing millions of galaxies cataloged in the Sloan Digital Sky Survey as well as the Hubble Space Telescope's archives.
If you have a bit of time to kill (and dream of having a big, ghastly space blob named after you), here's more information on the Galaxy Zoo project, where you can join the fun!