Technology & Science

'Light echoes' replay long-ago star show

A star's astonishing "Great Eruption" during the 19th century has been re-observed through "echoes" visible today.


The new research, published this week in the journal Nature, provides a new window into the life of a star just before it goes supernova, said Doug Welch, a physics professor at McMaster University in Hamilton who co-authored the study.

Eta Carinae, a double-star 120 times the mass of the sun that is only visible from Earth's southern hemisphere, became the second-brightest star in the sky during its "Great Eruption" from 1837 to 1858.

"That's pretty impressive, given that it is 7500 light years away," Welch said. "It was hard to miss."

The brightest star in the sky, Sirius, is just 8.6 light years from Earth.

Astronomers in the 19th century measured Eta Carinae's brightness and reported its colour during the Great Eruption, but weren't able to photograph it. Eventually, the star faded from view.

However, not all the light from the Great Eruption travelled directly to Earth. Some of it was initially angled away from the Earth when it  left the star, then later bounced off interstellar dust and was deflected back towards the Earth. Ultimately, that detour made its path to Earth much longer than the direct one.

"And it took an extra 150 years to take that path, for the light to reach us," Welch told CBC's Quirks & Quarks in an interview set to air Saturday. "We get to see the outburst in the reflection."

Welch said the reflected light or "light echoes" are much fainter than the light that travelled to Earth directly. The echoes were captured using powerful modern telescopes, including the Magellan and du Pont telescopes at Las Campanas Observatories in Chile.

Despite the light's faintness, it contained a great deal of information, including the chemical composition, temperature, and velocity of the blast of material ejected from the star during the Great Eruption. They also provided a rare opportunity to observe the event from a "different perspective."

The event was thought to be one of a series of stellar "burps" that happen during the unstable period in the 50,000 years before a star goes supernova — a mere blink on the astronomical scale.

"This is spectacular object that's clearly unusual and it's one of a very rare class of objects, where you get only one or two or three per galaxy and they last very brief periods of time," Welch said.

The data showed that the event was much cooler than scientists previously thought — about 5000 C instead of 7000 C — and wasn't consistent with existing scientific explanations for  events in nearby galaxies that had been previously thought to be similar to the Great Eruption. The findings suggest that the material billowing out from the star during the event had been pumped out mechanically.

"It's like somebody inside lifted the outer surface of the star," Welch said.

Eta Carinae is expected to end its life in a supernova within the next 50,000 years, and is sure to impress at that time, Welch said.

"It will be the brightest object in the sky by far, and probably brighter than the full moon."