Astronomers May Finally Know Why Eta Carinae Erupted 170 Years Ago

Astronomers May Finally Know Why Eta Carinae Erupted 170 Years Ago

The Eta Carinae system has fascinated astronomers for decades, and we may be one step closer to understanding this bizarre solar system today. Eta Carinae consists of at least two stars with a combined luminance of more than 5 million times that of our sun. Between 1837 and 1858, a massive eruption from Eta Carinae made it one of the brightest objects in the sky. A team of international astronomers thinks they’ve worked out what caused the eruption: an almost unfathomable transfer of mass between stars.

The new Eta Carinae hypothesis is laid out in a pair of studies led by Nathan Smith of the University of Arizona’s Steward Observatory. In the first, Smith and his team describe how light echoes from the event some 7,500 light years away have only recently reached Earth. After bouncing off interstellar dust, the light arrived at Earth allowing astronomers to track the luminance of Eta Carinae as well as the speed of ejecta from the eruption. This ejecta would become the distinctive bi-lobed Homunculus Nebula.

Usually, stars brightening abruptly is a sign of a supernova, but Eta Carinae still exists. Indeed, the team found that ejecta streamed outward from the stars more than 20 times faster than any known supernova — about 20 million miles per hour (32 million kilometers per hour). They also found that Eta Carinae had smaller light spikes as far back as 1845 and leveled off until 1858 when it dropped off.

Based on this data, the team developed a hypothesis for Eta Carinae’s eruption and published a second paper. They suggest that the binary system was trinary before the eruption. At some point prior to the eruption, one star was reaching the end of its life cycle and expanded too close to its companion. The companion star siphoned off so much stellar material that it grew to more than 100 solar masses. The other star was left with just 30 masses worth of super-hot helium and drifted farther away from its companion.

Astronomers May Finally Know Why Eta Carinae Erupted 170 Years Ago

Some material would have been ejected from Eta Carinae during this mass transfer, but the main event was still off in the future. As the 30-mass helium drifted away, it encountered the smallest of the three stars, which was probably still tens of solar masses. These two objects merged, causing the main eruption event that made Eta Carinae visible in the night sky on Earth. The material thrown off during this phase would have caught up with the previous ejecta, speeding it up by a factor of 100. The new merged star settled into an elliptical orbit of the larger one, passing through its outer layers every 5.5 Earth years and generating an x-ray blast.

This series of events doesn’t explain everything about this unusual star system, but it does offer a plausible explanation for how Eta Carinae produced a massive explosion without a supernova.

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