‘Light echoes’ allow astronomers to spot instant replay of ‘Great Eruption’

Washington -  Astronomers are watching the astronomical equivalent of an instant replay of a powerful outburst from the unstable, behemoth double-star system Eta Carinae, which was  initially spotted on Earth nearly 170 years ago.

Dubbed the “Great Eruption,” the outburst lasted from 1837 to 1858 and temporarily made Eta Carinae the second brightest star in the sky.

But luckily for today’s astronomers, some of the light from the eruption took an indirect path to Earth and is just arriving now.

The wayward light was heading in a different direction, away from our planet, when it bounced off dust lingering far from the turbulent stars and was rerouted to Earth, an effect called a “light  echo.”

Because of its longer path, the light reached Earth 170 years later than the light that arrived directly.

Armin Rest of the Space Telescope Science Institute in Baltimore, Md., initially discovered the light echoes, and was the leader of the study.

Federica Bianco, a postdoctoral researcher at UC Santa Barbara and Las Cumbres Observatory Global Telescope Network (LCOGT), used LCOGT’s Faulkes Telescope South in Siding  Spring, Australia, to record the explosion after this time delay and compare it to eyewitness reports of the brightness of the star from the 1800’s to determine that the light echo was indeed  from the “Great Eruption.”

“You are at the stadium, watching the game, and your team scores,” said Bianco.

“But you are too far away to get the details of the action, or in the case of Eta Carinae’s great eruption, you do not have modern instruments, detectors, and spectrographs to study it.

“Now we are getting a replay –– an up-close, detailed view of our cosmic eruption. And just like with the replay, we get to see the outburst from a different point of view, as the light that we  see now was originally travelling in a different direction than the light seen in the 1840’s.”

Andy Howell, a staff scientist at LCOGT and adjunct faculty member at UCSB, added that they finally “got to watch firsthand the event that created the iconic Hubble Space Telescope images  of giant lobes of gas expanding away from Eta Carinae.”

“It turns out it was an explosion –– and a strange, cool one. It just blows my mind that we can go back in time and watch one of astronomy’s greatest hits, and greatest mysteries, with a cosmic  instant replay.”

The observations are providing new insight into the behaviour of powerful massive stars on the brink of detonation.

Very massive stars, especially a pair of them, are exceedingly rare, and little is known about how they lose mass as they die.

The views of the nearby erupting star revealed some unexpected results, which will force astronomers to modify physical models of the outburst.

Located 8,000 light-years from Earth, Eta Carinae is one of the largest and brightest star systems in our Milky Way galaxy. Although the chaotic duo is known for its petulant outbursts, the  Great Eruption was the biggest ever observed.

During the 20-year episode, Eta Carinae shed some 20 solar masses. Some of the outflow formed the system’s twin giant lobes. Before the epic event, the stellar pair was 140 times heftier  than our Sun.

The team’s study involved a mix of visible-light and spectroscopic observations from ground-based telescopes.

These observations mark the first time astronomers have used spectroscopy to analyze a light echo from a star undergoing powerful recurring eruptions, though they have measured this  unique phenomenon around exploding stars called supernovae.

Spectroscopy captures a star’s “fingerprints,” providing details about its behavior, including the temperature and speed of the ejected material.

The replay is giving astronomers a unique look at the outburst, and turning up some surprises. The turbulent star does not behave like other stars of its class. Eta Carinae is a member of a  stellar class called Luminous Blue Variables –– large, extremely bright stars that are prone to periodic outbursts.

The study has been published in the journal Nature.