Not only are there diamonds in the sky, some of them are emerald-cut sparklers, according to astronomers who've found an unusual rectangular galaxy.

The cosmic oddball, dubbed LEDA 074886, is a dwarf galaxy 70 million light-years away in the constellation Eridanus, the River.

Most galaxies exist in one of three forms: a disk with spiral arms (such as our Milky Way), a football-shaped ellipsoid, or an irregular, lumpy blob.

But LEDA 074886 is a remarkably symmetrical rectangle, akin to an emerald-cut gem.

Astronomer Alister Graham, an associate professor at Swinburne University of Technology in Australia, was among the first to spot the weird galaxy in an image taken for an unrelated project with the Subaru Telescope in Hawaii.

When Graham first saw the boxy shape, "I thought it was a mistake," he said. "And then I couldn't stop smiling."

Diamond Galaxy Really a Cylinder?

The emerald-cut dwarf was spotted along the edge of a much more massive elliptical galaxy known as NGC 1407.

The smaller, faint galaxy has 50 times fewer stars than the Milky Way, which helps explain why the odd object went undetected for so long.

Graham and colleagues think the galaxy is actually shaped like a short cylinder—"like a case of CDs"—that we happen to see from the side.

The galaxy was probably formed by a merger of two spiral galaxies that had their axes pointed in opposite directions.

When the two galaxies collided, the axis of each galaxy formed the "corners" of LEDA 074886, the astronomers speculate.

A Hybrid Galaxy Merger

A widely held theory is that galaxies grow via mergers, most often as bigger galaxies consume their smaller companions. In fact, our Milky Way galaxy shows signs of recent snacking on one of its dwarf companions.

The rectangular galaxy is something of a hybrid, though, because it shows characteristics of two established types of galaxy mergers.

Elliptical galaxies tend to lack the ingredients for making new stars, so computer simulations show that when two ellipsoids merge, the galaxies form similar—if less extreme—boxy shapes and have little star-forming activity.

By contrast, the models show that galaxies with lots of star-forming gases don't become rectangular after they merge, but they do exhibit fresh rounds of star formation.

The Subaru image, however, revealed that LEDA 074886 has both a geometric outline and an inner disk of star formation. The unusual dwarf galaxy may therefore help astronomers model more complex types of galaxy mergers.

Ben Moore, a theoretical physicist at the University of Zurich and a study co-author, already had plans to model the formation of LEDA 074886 on a supercomputer later this year.

Those simulations will help determine, for example, how long the galaxy will keep its boxy shape, Graham said.

In space, diamonds aren't forever: Unless LEDA 074886 merges with yet another perfectly aligned galaxy, it may lose its well-defined corners over the next billion years.