Image 1 - Light Echoes Allow Astronomers To Watch 'Great Eruption' Replay
February 15, 2012

Light Echoes Allow Astronomers To Watch ‘Great Eruption’ Replay

Astronomers recently got a rare opportunity to view a cosmic event that took place from 1837 to 1858 known as the "Great Eruption."

The outburst temporarily made Eta Carinae the second brightest star in the sky during that period.

Today's astronomers are able to view the event because some of the light from the eruption took an indirect path to Earth and is just now arriving.

The 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 towards Earth.

This effect, called a "light echo", delayed the light 170 years from reaching Earth, compared to the light that arrived directly.

The team recorded the explosion after this time delay and compared it to eyewitness reports of the brightness of the star from the 1800's to determine that the light echo was from the "Great Eruption."

"You are at the stadium, watching the game, and your team scores," Federica Bianco, a postdoctoral researcher at University of California - Santa Barbara  (UCSB) and Las Cumbres Observatory Global Telescope Network (LCOGT), said in a statement. "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 traveling in a different direction than the light seen in the 1840's."

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

Very massive stars are 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.

"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," Andy Howell, a staff scientist at LCOGT and adjunct faculty member at UCSB, said.

The team's observations are the first time astronomers have used spectroscopy to analyze a light echo from a star undergoing powerful recurring eruptions.

Spectroscopy captures a star's "fingerprints," which helps provide details about its behavior, such as temperature and speed of ejected material.

The replay of the "Great Eruption" is giving astronomers a unique look at the outburst, and turning up some surprises as well.

The star does not behave like other stars of its class, and Eta Carinae is a member of a stellar class called Luminous Blue Variables.  This class of star is extremely bright and prone to periodic outbursts.

The temperature of the outflow from Eta Carinae's central region is about 8,500 degrees Fahrenheit, which is cooler than other eruption stars of its class.

The team first spotted the light echo while comparing visible-light observations they took of the star in 2010 and 2011 with the U.S. National Optical Astronomy Observatory's Blanco 13-feet telescope at the Cerro Tololo Inter-American Observatory in Chile.

They said they will continue monitoring Eta Carinae because light from the outburst is still hitting Earth.

"We should see brightening again in six months from another increase in light that was seen in 1844," Rest said in a statement. "We hope to capture light from the outburst coming from different directions so that we can get a complete picture of the eruption."

The research was published in a February 16 letter to the journal Nature.


Image 1: Hubble Space Telescope image showing Eta Carinae and the bipolar Homunculus Nebula which surrounds the star. The Homunculus was partly created in an eruption of Eta Carinae, the light from which reached Earth in 1843. Eta Carinae itself appears as the white patch near the center of the image, where the 2 lobes of the Homunculus touch. Credit: NASA

Image 2: This is a Hubble Space Telescope image of the massive binary star system Eta Carinae taken in 1995. The pair of stars is hidden, but give off the white-blue glow at the center of the image. The hourglass shape is due to two huge lobes of gas expanding outward, a product of the mid-1800s Great Eruption that was studied as a light echo in Rest, et al. Credit: Jon Morse (University of Colorado), and NASA


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