The space mystery of the “Great Eruption” of Eta Carinae, a supermassive star some 7,500 light years away, has puzzled astronomers for more than a century. But a study published Thursday in the journal Nature has pushed scientists closer to solving the mystery.

Eta Carinae (Chandra X-ray Observatory/Associated Press)

The “Great Eruption” happened in 1838, when the massive star suddenly lit up, becoming the second-brightest star in the night sky — and then stayed that way for a decade.

The explosion was profound, but it was not a supernova. And while Eta Carinae later dimmed and loss an incredible amount of mass, it never died. It is still believed to be 100 times the mass of our Sun.

Today, astronomers are able to observe the effects of Eta Carinae’s explosion through light echoes, which are produced after a sudden flash or burst of light reflects off a source, and then arrives at the viewer some time later, Ars Technica explains. Light from Eta Carinae, for example, continues to reach Earth long after the initial eruption faded.

A new study of those light echoes, performed by Armin Rest, an astrophysicist at the Space Telescope Science Institute in Baltimore, and published in the journal Nature, found that Eta Carinae was far colder than previously thought at the time of its brightening: A cool 8,540 degrees Fahrenheit.

That finding suggests the “Great Eruption” is not like other so-called “supernova impostors,” or events that resemble the explosive supernova deaths of stars, as was previously believed.

It also means that researchers still do not know what caused the Great Eruption. Rest told

“With Eta Carinae, we are dealing with a really extreme phenomenon, which drags us into unexplored territory as far as theory is concerned.”

“We do have some ideas — a collision between two stars in a binary system; an explosive but non-terminal thermonuclear burning event in the core of the star that might release energy — but these are really just ideas and have not been very well-developed, so we don't yet have clear predictions for the observed phenomena we expect to see.”

The scientists say they plan to look for more light echoes from Eta Carinae, to better understand how it erupted.

Unraveling the mystery of the explosion of Eta Carinae could help astronomers understand the end-stages of very massive stars, and distinguish between eruptions and supernova explosions.