Explosive stellar replay offers clue to bright star survival

The brightest star in our galaxy should have destroyed itself 170 years ago, but didn’t. Now astronomers think they know why. Ben Lewis reports.

Eta Carinae, the brightest star in the galaxy, going off with a bang.
Eta Carinae, the brightest star in the galaxy, going off with a bang.
J. Hester / Arizona State University, NASA / ESA

About 170 years ago astronomers watched as Eta Carinae, the brightest star in our galaxy, unleashed an almighty outburst that released almost as much energy as a typical supernova.

And yet, Eta Carinae survived the cataclysmic explosion.

Ever since, astronomers have been searching for an explanation for why the powerful blast wasn’t enough to obliterate the star. Now, a group of astronomers led by Nathan Smith from the University of Arizona in the US has been given a window into the past, allowing them to watch the explosion again.

The scientists were able to watch light from the event which had rebounded, or “echoed”, off interstellar dust and is just now arriving at Earth.

Their analysis, published in the Monthly Notices of the Royal Astronomical Society, revealed some startling facts about the outburst. The spectacular blast spewed material with a total mass equivalent to 10 suns across the galaxy at between 10,000 and 20,000 kilometres per second – the fastest speeds ever recorded by anything that wasn’t a supernova.

However, the question remained as to why such a massive explosion didn’t rip the star apart. The only explanation the researchers could come up with was that something dumped enough energy onto it to cause it to eject its outer layers, but not enough to completely annihilate itself.

They suggest that Eta Carinae, which is one half of a binary system, originally had a third companion – making a triple system with two stars close together and the third further out. At some point near the end of its life, the larger of the close-in stars began expanding and dumping energy onto its nearest neighbour.

The receiving star bulked up to around 100 times the mass of our sun and became extremely bright. The donor star was stripped of its hydrogen layers, shrinking to around 30 solar masses and exposing its hot helium core.

This change of mass changed the balance of gravity, pushing the helium-core star further away from its now gigantic sibling until it interacted with the outermost star, kicking it inwards. Sometime later, that star merged with the gigantic sibling, creating an outburst of material.

Initially, the ejected material was dense and expanded slowly as the two stars spiralled closer and closer. When the two inner stars finally combined, the explosion blasted material outwards 100 times more rapidly. The fast-moving stuff slammed into the back of the slower-moving earlier ejecta with a force that made it glow.

And that was the light seen by astronomers over a century and a half ago.

Following the immense, destructive merger, the triple system became a binary, with the smaller helium-core star in an elliptical orbit that passes through the giant star’s outer layers every 5.5 years.

However the mystery of Eta Carinae doesn’t end there – astronomers are keeping a close watch, with some speculating this explosion could have just been a warm up for a full supernova sooner rather than later.

  1. https://www.spacetelescope.org/images/opo9409a/
  2. https://academic.oup.com/mnras
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