In 2007, Dutch school teacher Hanny van Arkel participated in the Galaxy Zoo project. She discover this strange green blob on an image, and it was named Hanny’s Voorwerp. Hanny’s Voorwerp is Dutch for Hanny’s object. It lies approximately 700 million light-years away from Earth near the spiral galaxy IC 2497 in the constellation Leo Minor.

The strange green blob floating near the spiral galaxy begs the question: “What is it?” It turns out that IC 2497 is what astronomers call an active galactic nucleus or quasar. The quasar is powered by a massive black hole at the center of the galaxy. Right now, the quasar is no longer active. In other words, it is now turned off, which makes it a transient phenomena. The radiation from the AGN has illuminated a gas cloud trailing behind the galaxy, and is triggering the formation of new stars. This illuminated section of the gas cloud is Hanny’s Voorwerp. What we are seeing in Hanny’s Voorwerp is a light echo. It is an echo of the radiation that came from the now inactive quasar.

via : Hubble Zooms in on a Space Oddity

You can see a graphic explaining this after the break.