A Green Pea galaxy is a galaxy that is undergoing high rates of star formation. Astronomers believe that they might be a type of Blue Compact Galaxy. They are named Green Pea galaxies because they appear very small in size and they look greenish.
These galaxies where discovered in 2007, by a couple of volunteer astronomers. They were discovered at redshifts between 0.112 and 0.360. The galaxies are very compact and emit a lot of lines from oxygen. The biggest these galaxies get is around 16,000 light years or 5,000 pc. So the Milky Way is about 6.25 times as big as a Green Pea galaxy. Here is an image of a Green Pea galaxy: