Eta Carinae, one the galaxy's biggest stars, and the nebula formed by its enigmatic, 19th century Great Eruption. Astronomers are searching for clues in the light echoes from the eruption. ( Jon Morse/NASA)
Eta Carinae, one the galaxy's biggest stars, and the nebula formed by its enigmatic, 19th century Great Eruption. Astronomers are searching for clues in the light echoes from the eruption. ( Jon Morse/NASA)

In Light Echoes, a Glimpse of the Cosmic Past

On Earth, echoes are produced when sound waves bounce around like pinballs. In space, echoes are produced when light does the bouncing.

Just as sounds can echo, so, too, can cosmic light. But instead of ricocheting off damp cavern walls, light traveling through the universe bounces off soft, dusty clouds.

Sometimes, this happens after an explosive event such a supernova. On Earth, most of the light we’d see from one of these exploding stars would have come directly here. But supernovas explode in three dimensions, sending light in all directions. Not all of that light is aimed toward Earth. If the geometry is right, some of the light zooming away from Earth might run into a cloud and end up being redirected toward us – kind of like a billiard ball bouncing off the cushion and rolling cleanly into the pocket. Because it took a more circuitous route through the cosmos, this rerouted light arrives at Earth after the light from the original explosion. Often, it’s delayed by centuries, arriving long after the supernova’s embers have been extinguished.

Called light echoes, these ancient reverberations bear the original signatures written into light by the explosion.

In other words, light echoes act as astronomical time machines or portals to the past. In some echoes, astronomers have deciphered details that reveal which kinds of stars exploded in the 16th and 17th centuries – information that would otherwise be lost to the cosmos, since telescopes of the time didn’t have the spectrometers needed to read the chemical inscriptions in the light.

Other echoes are being used to solve the mystery of Eta Carinae, one of the weirdest and most tempestuous binary star systems in the galaxy.

Located 7,500 light-years away, Eta Carinae lives in the Carina nebula, in the southern sky.

One of its two stars is, as described in last week’s episode of Cosmos: A Spacetime Odyssey, “pushing the upper limit of what a star can be.” The star is a bulging, nuclear behemoth more than 100 times as massive than the sun and more than a million times as bright. Circling that crazy huge star is a giant companion, but it wasn’t until 1996 that a Brazilian astronomer named Augusto Damineli realized it was there.

For much of recorded astronomical history, Eta Carinae looked like just another star.

But in the early 1800’s, it began to erupt. The outburst lasted for two decades, from 1837 to 1858. For a turn, Eta Carinae became the second-brightest star in the sky; astronomer John Herschel (William’s son), observed the first brightening in late 1837 from South Africa, and kept track of the flare-up.

“What origin can we ascribe to these sudden flashes and relapses? What conclusions are we to draw as to the comfort or habitability of a system depending for its supply of light and heat on so uncertain a source?” Herschel would write in 1847.

Pulses of light produced by the stellar eruptions wafted off into space, passing through the burgeoning clouds of dust and gas that were gathering around the spasming star.

Light curves recorded by Herschel and others at the time showed peaks in 1837, 1843, and 1845. After the outbursts subsided, the star had changed. It lost more than 10 solar masses of material, and was shrouded by a double-lobed, organic-looking structure called the Homunculus Nebula – the cosmic consequence of the giant star’s spectacular tantrums, and the chaotic shroud that hid its companion until recently.

For years, astronomers considered Eta Carinae the prototype for a class of explosions known as “supernova impostors.” Rather than destroying the star, as any self-respecting supernova would, these impostor novae shine as brightly as their counterparts yet leave their stars intact.

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Light echoes from Eta Carinae. (NASA/NOAO/A.Rest)

But light echoes from Eta Carinae’s 19th-century outbursts are challenging its status as the impostor nova poster child. Now, these echoes are bouncing off dust clouds near the star system. Since 2011, a team of astronomers has been monitoring and measuring these light echoes, the first of which was accidentally captured in an image from 2003.

“Eta Car’s great eruption is so long,” says astronomer Armin Rest of the Space Telescope Science Institute in Baltimore, MD. “It seems now to be a much more complicated thing, that we have found in the light echoes.”

The echoes reveal that the temperature of Eta Carinae’s Great Eruption was much cooler than expected – about 5,000 Kelvin, or 2,000 degrees cooler than anticipated. That relatively frigid temperature suggests the mechanism of the eruption is nothing like what scientists had surmised; instead of being the result of an energetic stellar wind, the explosion could have been triggered by a blast from the star’s surface or by an interaction with its binary companion.

Most recently, in a paper uploaded to the arXiv on April 15, Rest and his colleagues report the presence of nitrogen-rich molecules in the light echoes – cyanide, in fact, which contains a carbon and a nitrogen atom. “You need cool temperatures for that,” Rest says. “If you have hotter temperatures, the molecules break apart.”

Over time, Rest says, the characteristics of the light echoes are changing, indicating the expulsion of material and dense amounts of dust. Essentially, “what we are seeing is the formation of the Homunculus Nebula,” he says.

But the team is no longer sure which of the Great Eruption peaks they’re seeing. Initially, scientists suspected they were watching the star’s 1843 outburst. The shape of the light curve from the echoes fit the shape of the historical light curve, which rose over a few hundred days, plateaued, and then dropped. The only trouble is, the team isn’t seeing the next peak in the eruption, which happened in 1845. “It has stayed down for more than two years,” Rest says, of the light echo curve. He thinks they could be seeing echoes from the earlier, 1837 outburst that Herschel noticed, or perhaps the reverberations from a previous explosion that went unnoticed.

“It’s truly a mystery,” Rest says. “I would’ve bet $1000 bucks that we were seeing the ’43 peak, but I was wrong.”

So, astronomers aren’t yet sure what’s going on with this star and its giant eruptions. But Eta Carinae’s massive size and repeated outbursts portend the death of the system, which promises to be a spectacular cataclysm – a hypernova, or superdupernova – well worth the price of a ticket to the southern hemisphere to check it out when it does blow.

Eta Carinae isn’t the only complex stellar object whose secrets hide in these aging glimmers. Light from supernovas that shined in Earth’s skies centuries ago is also bouncing off dust clouds. These fading echoes are like time capsules that preserve information about the temperature and chemical spectrum of the original explosion; by reading the clues inscribed into the echoes, astronomers have been able to identify exactly what kind of stellar explosion produced long burnt-out remnants.

Rest and his colleagues used spectral lines to classify one of the remnants, in the Large Magellanic Cloud, as not only the result of a type Ia supernova (produced by an exploding white dwarf star), but a subtype of that class of explosion. Known as an overluminous type Ia, the explosion that produced remnant SNR 0509-67.5 likely resulted from the merger of two white dwarfs.

Teams have also looked to light echoes for information about Tycho’s supernova (a normal type 1a), which exploded in 1572, and for Cassiopeia A, a type IIb core-collapse supernova that exploded in 1680 and left a beautiful remnant in the Milky Way. Other light echoes have been identified in the Large Magellanic Cloud that correspond to explosions that aren’t as well known, as well as the echoes from supernova 1987a, which form concentric rings around the explosion site.

Light echoes can be fleeting and illusive, as demonstrated by the series of images below from another famous nova, known as V838 Monocerotis (watch a video here). What looks like an expanding cloud is actually the light from the explosion traveling outward and being reflected by the already existing cloud’s shells and cavities.

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Light echoes from V838 Mon create the illusion of an expanding dust cloud. (NASA/ESA/H.E.Bond)