"Long before it's in the papers"
February 18, 2012

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In night sky, a delayed replay of cataclysm seen in 1830s

Feb. 18, 2012
Courtesy of the Space Telescope Science Institute
and World Science staff

As­tro­no­mers say they’re watch­ing a de­layed re­play, hid­den deep in space, of an ex­plo­sion that had jaws drop­ping on Earth at the dawn of the steam­boat era.

The cataclysm orig­i­nat­ed in a gi­ant, un­sta­ble double-star sys­tem called Eta Cari­nae. Dubbed the “Great Erup­tion,” it first at­tracted not­ice in 1837 and was ob­served through 1858. Cam­er­as had barely been in­vented then and as­tro­no­mers lacked tools to ac­cu­rately rec­ord the star sys­tem’s pet­u­lant ac­ti­vity.

Light from a mas­sive stel­lar out­burst in the Ca­ri­na Neb­u­la re­flect­ing off dust clouds sur­round­ing a be­he­moth double-star sys­tem. The col­or im­age at left shows the Ca­ri­na Neb­u­la, a star-forming re­gion lo­cat­ed 7,500 light-years from Earth. The mas­sive double-star sys­tem Eta Ca­ri­nae re­sides near the top of the im­age. The star sys­tem, about 120 times more mas­sive than the Sun, pro­duced a spec­tac­u­lar out­burst that was seen on Earth from 1837 to 1858. But some of the light from the erup­tion took an in­di­rect path and is just now reach­ing our plan­et. The light bounced off dust clouds (the boxed re­gion about 100 light-years away at the bot­tom of the im­age) and was re­rout­ed to Earth, a phe­nom­e­non called a light ech­o. The im­age was tak­en in Feb­ru­ary 2000 by the U.S. Na­tion­al Op­ti­cal As­tron­o­my Ob­ser­va­to­ry Cur­tis Schmidt Tel­e­scope at the Cerro Tololo In­ter-A­mer­i­can Ob­serv­a­to­ry (CTIO) in Chil­e. The three black-and-white im­ages at right show light from the erup­tion il­lu­mi­nati dust clouds near the doomed star sys­tem as it moves through them. The ef­fect is like shin­ing a flash­light on dif­fer­ent re­gions of a vast cav­ern. The im­ages were tak­en over an eight-year span by the U.S. Na­tion­al Op­ti­cal As­tron­o­my Ob­ser­va­to­ry Blanco 4-meter tel­e­scope at the CTIO. (Cred­it: NA­SA, NOAO, and A. Rest (Space Tel­e­scope Sci­ence In­sti­tute, Bal­ti­more, Md.))

Astronomers say they’re watching a delayed broadcast of a spectacular outburst initially seen on Earth nearly 170 years ago. The event originated in a giant, unstable double-star system called Eta Carinae. Dubbed the “Great Eruption,” the outburst came to the world’s attention in 1837 and was observed through 1858. Cameras had barely been invented then and astronomers lacked tools to accurately record the star system’s petulant activity. Luckily for today’s astronomers, some of them report, some of the light from the eruption took an indirect path to Earth and is just arriving now—an opportunity to analyze the outburst in detail. The wayward light was heading in a different direction, away from our planet, when it bounced off dust clouds 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—but the way we see it change fits with the way it was seen to evolve back then, scientists say, so they can also predict what it will do next. The observations are providing new insight into the behavior of powerful massive stars on the brink of detonation. The views of the nearby erupting star reveal some unexpected results, which will force astronomers to modify physical models of the outburst. “Everything astronomers have known to date about Eta Carinae’s outburst is from eyewitness accounts,” said the study’s leader, Armin Rest of the Space Telescope Science Institute in Baltimore, Md. “Modern observations with science instruments were made years after the eruption actually happened. It’s as if nature has left behind a surveillance tape of the event, which we are now just beginning to watch. We can trace it year by year to see how the outburst changed.” The team’s paper appeared Feb. 16 as a letter to the journal Nature. Located 7,500 light-years from Earth—a light year is the distance light travels in a year—Eta Carinae is one of the largest and brightest star systems in our Milky Way galaxy. Although the chaotic duo of stars is known for its petulant outbursts, the Great Eruption was the biggest ever observed. During the 20-year episode, Eta Carinae threw off the equivalent of about 20 Suns by weight and became the second brightest star in the sky. Some of the outflow formed a pair of twin giant lobes that adorn the system. Before the epic event, the stellar pair was 140 times heftier than our Sun. Because Eta Carinae is relatively nearby, astronomers have used a variety of telescopes, including the Hubble Space Telescope, to document its escapades. The team’s study involved a mix of visible-light and spectroscopic observations, which examine different components of the light, from ground-based telescopes. The delayed broadcast is giving astronomers a unique look at the outburst and turning up some surprises. The turbulent star doesn’t act like others of its type. Eta Carinae is classified as a Luminous Blue Variable, a type of large, extremely bright star prone to periodic outbursts. But the temperature of the outflow from Eta Carinae’s central region, for example, is about 8,500 degrees Fahrenheit (5,000 Kelvin), which is much cooler than that of other erupting stars. “This star really seems to be an oddball,” Rest said. “Now we have to go back to the models and see what has to change to actually produce what we are measuring.” Rest’s team first spotted the light echo while comparing visible-light observations he took of the star in 2010 and 2011 with the U.S. National Optical Astronomy Observatory’s Blanco 4-meter telescope at the Cerro Tololo Inter American Observatory in Chile. He obtained more data from this observatory captured in 2003 by astronomer Nathan Smith of the University of Arizona in Tucson, which helped him piece together the whole 20-year outburst. The images revealed light that seemed to dart through and illuminate a canyon of dust surrounding the doomed star system. “I was jumping up and down when I saw the light echo,” said Rest. “We knew it probably wasn’t material moving through space. To see something this close move across space would take decades of observations. We, however, saw the movement over a year’s time. That’s why we thought it was probably a light echo.” Although the light in the images appears to move over time, he said, it’s really an optical illusion. Each flash of light is reaching Earth at a different time, like a person’s voice echoing off the walls of a canyon. The team followed up its study with spectroscopic observations, using the Carnegie Institution of Washington’s Magellan and du Pont telescopes at Las Campanas Observatory in Chile. That study helped the astronomers decode the light, revealing the outflow’s speed and temperature. The observations indicated that ejected material was moving at roughly 445,000 miles an hour (more than 700,000 kilometers an hour), which matches predictions. Rest’s group monitored changes in the intensity of the light echo using the Las Cumbres Observatory Global Telescope Network’s Faulkes Telescope South in Siding Spring, Australia. The team then compared those measurements with a plot astronomers in the 1800s made of the light brightening and dimming over the course of the 20-year eruption. The new measurements matched the signature of the 1843 peak in brightness. The team will continue to follow Eta Carinae because light from the outburst is still streaming to Earth. “We should see brightening again in six months from another increase in light that was seen in 1844,” Rest said. “We hope to capture light from the outburst coming from different directions so that we can get a complete picture of the eruption.”