Astronomers stunned again by Eta Carinae – the star that will not die

Eta Carinae, one of the brightest, most massive stars in the Milky Way, famously erupted 170 years ago in a blast that somehow left the star intact. Astronomers may have found an explanation. Image: NASA, ESA, and J. Hester (Arizona State University)

About 170 years ago, Eta Carinae, one of the brightest, most massive stars in the Milky Way, erupted with a titanic blast, releasing almost as much energy as a supernova explosion and becoming at one point the second brightest star in the night sky. Somehow, the star survived the “Great Eruption”, providing an intriguing mystery for astronomers.

Now, studying light from the outburst that rebounded, or “echoed,” off interstellar dust and has only now reached Earth, researchers have found the original explosion created a huge 10-solar-mass cloud of debris expanding 20 times faster than expected – more than 32 million kilometres per hour (20 million mph) – fast enough to travel from Earth to Pluto in just a few days.

Velocities that high are seen in the aftermath of supernova explosions, but not in events that leave a star intact.

“We see these really high velocities in a star that seems to have had a powerful explosion, but somehow the star survived,” said Nathan Smith of the University of Arizona. “The easiest way to do this is with a shock wave that exits the star and accelerates material to very high speeds.”

Smith and Armin Rest of the Space Telescope Science Institute in Baltimore wrote a pair of papers in The Monthly Notices of the Royal Astronomical Society describing the recent observations and offering a possible explanation.

Researchers first detected the light echoes from Eta Carinae in 2003, initially using telescopes at the Cerro Tololo Inter-American Observatory in Chile. Using the larger Magellan telescopes and the Gemini South Observatory, also in Chile, the researchers collected spectra to determine the velocity of the expanding debris.

The data did not jibe with accepted ideas about stellar evolution.

Six steps to a cosmic eruption. Image: NASA, ESA, and A. Feild (STScI)

Massive stars normally die when their cores run out of nuclear fuel. When the outward pressure of fusion-generated energy suddenly stops, gravity takes over and the core collapses, generating a tremendous shock wave that blows the outer layers of the star into space. Depending on the original mass, the core becomes a compact neutron star or a black hole.

In the case of Eta Carinae’s eruption, some process must have produced a supernova-like shockwave that came just shy of the energy needed to destroy the star. What might have happened?

Smith and Armin argue Eta Carinae likely started out as a triple star system with two massive stars orbiting close together and a third star farther away. When the more massive of the two closely orbiting suns neared the end of its life, it began to expand, allowing the slightly less massive star to suck in an enormous amount of material.

That star could swell to about 100 solar masses, in the process stripping away the dying sun’s outer atmosphere and leaving an exposed helium core about 30 times more massive than the Sun.

“From stellar evolution, there’s a pretty firm understanding that more massive stars live their lives more quickly and less massive stars have longer lifetimes,” Rest said. “So the hot companion star seems to be further along in its evolution, even though it is now a much less massive star than the one it is orbiting. That doesn’t make sense without a transfer of mass.”

That mass transfer would have changed the gravitational architecture of the system, allowing the helium-core star to move away from its huge partner, so far, in fact, that it eventually interacted with the outer third star, kicking it inward. That star finally crashed into the supermassive star at the heart of the triple system in a cataclysmic merger.

In its initial stages, ejected material moved relatively slowly as the two stars spiralled closer and closer together. When the stars finally merged, debris was blown away 100 times faster, catching up and ramming into the slower-moving material and generating the light seen in Eta Carinae’s eruption.

The helium-core star, meanwhile, ended up in an elliptical orbit that carries it through the giant central star’s outer atmosphere every five-and-a-half years, generating X-rays and shock waves.

The now-binary star system shines five million times brighter than the Sun with two enormous, symmetrical lobes of expanding debris expanding to either side. The large star will likely exhaust its nuclear fuel in the near future, astronomically speaking, and explode as a supernova.