The European Space Agency's giant space telescope Herschel ran out of fuel last month and stopped working, as scientists had been expecting. But before then, the telescope was able to capture images of a rare cosmic event: the merger of two large, ancient galaxies into one super-sized galaxy. At first, astronomers thought that Herschel had accidentally captured two identical images of the same galaxy located 11 billion light years from Earth, but after performing follow-up studies on the data, they realized they were looking at two independent spiral galaxies — both the same shape as our own Milky Way galaxy — slowly but steadily fusing together across a bridge of gas. Eventually, scientists say that they will combine and change shape, becoming an elliptical galaxy.

"the most efficient star-forming factory ever found."

Astronomers turned to NASA's Hubble telescope and Spitzer telescope to learn more about the great galactic fusion, and what they found did not disappoint. It turns out that not only are the galaxies merging, but they are each giving birth to stars at an incredible rate. "This monster system of interacting galaxies is the most efficient star-forming factory ever found in the universe at a time when it was only 3 billion years old,” said Hai Fu, a research associate the University of California and the lead author of a paper on the find published this week in the journal Nature. NASA published a video simulation of the merging process sped up to cover 1.5 billion years in a matter of minutes.

The two galaxies are together producing about 2,000 new stars a year, compared to the single-digit number produced by the Milky Way each year. Astronomers also say that the discovery challenges the conventional theory that most elliptical galaxies are formed by absorbing smaller ones, instead suggesting many may have formed via a similar merging process to the one observed by Herschel.

Correction: This post originally stated that the Spitzer telescope was "ground-based," which is incorrect: It is an infrared space telescope.