Supernova ‘Impostor’ Eta Carinae has a Violent History, Scientists Find

The giant star system Eta Carinae has baffled astronomers for a long time. Scientists have discovered the giant star system Eta Carinae’s strange and violent past, which reveals critical knowledge about how extremely massive stars die.

Astronomers studying the night sky in the Southern Hemisphere during the mid-1800s have observed that the previously inconspicuous star Eta Carinae had grown brighter, almost outshining the other stars in the sky, only to fade again over the next 10 years, too dim to be seen by the naked eye.

The phenomenon led scientists to believe they had been observing a rare supernova. The massive outburst of the star during the 1800s, called the "Great Eruption," had formed the Homonculus nebula, an hourglass-shaped cloud of gas and dust surrounding the star.

However, scientists from the University of Arizona have discovered that the so-called Great Eruption was only the latest of a series of massive explosion created by Eta Carinae since the 13th century. This discovery compounded the mystery behind the star system.

"Eta Carinae is what we call a supernova impostor," Megan Kiminki, a doctoral student in the University of Arizona's Department of Astronomy and Steward Observatory, said in a statement. "The star became very bright as it blew off a lot of material, but it was still there."

Supernova explosions occur in two ways. When giant stars run out of fresh nuclear fuel, which means there is no more pressure to sustain them against their own weight, the central part of the star collapses. Another is when matter piles up on the compressed core of a white dwarf or a dead star, which could trigger an explosion.

Eta Carinae may have exploded and died but it did not stay dead. According to scientists, it got brighter once, brightened again and even doubled in brightness for a time.

In a new paper published in the Monthly Notices of the Royal Astronomical Society, the researchers analyzed the images of Eta Carinae taken with NASA's Hubble Space Telescope and found that there had been two earlier eruptions. The researchers discovered that the gas that was further away from the Homonculus nebula was moving more slowly than the debris, which is closer to the nebula. The scientists hypothesized that earlier eruptions had occurred during the 13th and the 16th centuries.

The scientists were measuring the motions of the fast streams of matter ejected by young stars forming in the Carina Nebula. But they found out that the same approach could be used to measure the motion of debris coming from Eta Carinae itself. They measured the movement of about 800 blobs of gas by aligning different images of the star system taken at different time periods, which allowed them to calculate a possible timeframe for the eruptions.

The researchers also discovered that Eta Carinae is a binary system of two extremely massive stars that orbit each other every 5.5 years, which are bigger than the sun with one of them approaching death.

While the cause of the Great Eruption during the 1800s remains a mystery, the eruptions offer a unique insight about the last "unstable" phases of the life of an extremely massive star. The discovery also identified a new subclass of supernova outbursts where a star suffers violent eruptions shortly before the final great explosion.