Supernova Countdown: Giant Star Could Explode Any Day Now

About 165 years ago, Eta Carinae mysteriously became the second brightest star in the sky. In 20 years, after ejecting more mass than our sun, it unexpectedly faded

When the sun finally dies some 5 billion years from now, the end will come quietly, the conclusion of a long, uneventful life. Our star will, in a sense, go flabby, swelling first, releasing its outer layers into space and finally shrinking into the stellar corpse known as a white dwarf.
Things will play out quite differently for a supermassive star like Eta Carinae, which lies 7,500 light-years from Earth. Weighing at least a hundred times as much as our sun, it will go out more like an adolescent suicide bomber, blazing through its nuclear fuel in a mere couple of million years and exploding as a supernova, a blast so violent that its flash will briefly outshine the entire Milky Way. The corpse this kind of cosmic detonation leaves behind is a black hole.
For Eta Carinae, that violent end might not be long in coming, according to a report in the latest Nature. "We know it's close to the end of its life," says astronomer Armin Rest of the Space Telescope Science Institute and the lead author of the paper. "It could explode in a thousand years, or it could happen tomorrow." In astronomical terms, a thousand years might as well be tomorrow; as for a supernova blowing up literally tomorrow, well, that's almost unheard of.


In 1843 Eta Carinae gave a hint that the end might be near when the hitherto nondescript body flared up to become the second brightest star in the sky, after Sirius. It stayed that way for 20 years or so, then faded and left behind a majestic, billowing cloud of gas known as the Homunculus Nebula. Eta Carinae lost some 10% of its substance in this event, which astronomers now call a "supernova impostor," after which it has returned to relative quiet — or what passes for quiet in such an unstable object.
Astronomers back in the day did the best they could to observe the 20-year flare, but without modern instruments, they couldn't really learn much. That has frustrated investigators now just as it did then, since studying Eta Carinae in detail could tell them a lot about what caused the outburst and maybe even help them figure out when the inevitable supernova explosion is going to occur.


But as the Nature report makes clear, that understanding may now be at hand. Using a fiendishly clever new observing technique, Rest and his colleagues have been able to take readings of the original blast in real time. "We can look directly at the eruption," says Princeton astrophysicist Jose Prieto, a co-author of the report, "as it's never been seen before."
To understand how they did that, start with the basic fact that light from the outburst sped away from Eta Carinae in all directions. Some of it headed straight toward Earth to wow 19th century astronomers. But some of it took a detour, reflecting off dust clouds in interstellar space in what astronomers call a "light echo." At least a bit of that echo was redirected toward Earth. The dust clouds were so far from the star that the long-delayed light is only now reaching us, and unlike in 1843, we now have the instruments to study it.


It gets even better. The 1843 flare-up played out over 20 years, which means the light-echo version will do the same. "We took observations nine months ago," says Rest, "and we were looking at 1843. Now we're looking at 1844. It's like a movie. It's really cool." (Of course, the images are from 7,500 years before 1843 and '44, since that's when the stellar event occurred; it just took 7½ millennia for the light to reach us.) Better still, astronomers can see light echoes from a variety of dust clouds, at varying distances from the star. That creates detours of varying lengths, so they can see different phases of the eruption all at once.
"The big puzzle," says Prieto, "is what caused the outburst. This star has been studied to death with all sorts of telescopes, but no one theory has ever been able to tell us what happened." It might have been some sort of instability deep within the star itself, or the blast might have been triggered by matter dumped on Eta Carinae by a stellar companion.

The good news is that the light-echo observations will give theorists a trove of information to work with — and in the next few years, says Rest, "we'll be getting more observations, and they'll keep getting better."
If Eta Carinae is going to blow imminently, the obvious question is whether Earth is in mortal danger. Fortunately, the answer is no. At 7,500 light-years, the intense radiation from even a powerful supernova would lose its punch by the time it reaches us. All we'll experience is the most spectacular light show in many centuries. The last confirmed supernova explosion in the Milky Way happened in 1604, a teasingly close five years before Galileo pointed his first, primitive telescope skyward.
It is, in short, about time for another big blast, and even though the theorists haven't weighed in, Rest has reason for hope. "There was one of these 'supernova imposters' in another galaxy," he says — something similar to Eta Carinae's 1843 outburst. "And then, a few years later ... kaboom!"

A Telescope as Sharp as Hubble — But On the Ground

 
Most people think that the Hubble Space Telescope (HST) is the most powerful stargazing system in the world. It's understandable, given the astounding images and spectacular science the instrument has been delivering since it went into full operation back in 1993.

In fact, though, the Hubble is relatively puny. The mirror at its heart is just 7.9 ft. (2.4 m) across, which gives it only about one-seventeenth of the light-gathering power of the 33-ft. (10 m) twin Keck telescopes, atop Hawaii's Mauna Kea. The HST's big selling point is its super-sharp vision. Hubble orbits high above the Earth's atmosphere, a turbulent sea of constantly roiling air that makes the stars twinkle and blurs the vision of ground-based telescopes. A Keck in space would be the ideal solution — if it wouldn't be impossibly expensive to build, and if there were any way of wrestling such a gigantic piece of hardware into orbit. 
But there's another way: a technology known as adaptive optics (AO) can de-blur the vision of a ground-based telescope — and astronomers at the 26-ish-foot (8-ish-meter) Gemini South telescope in Chile have debuted the most powerful AO system to date. According to astronomer Francois Rigaut, who led the team that built the new hardware, its images rival the Hubble's for sharpness, and in a press release, Matt Mountain, director of the Hubble's home base, the Space Telescope Science Institute, called the image quality "incredible."
Adaptive optics itself isn't a new concept, but that doesn't make it any less remarkable. The idea is to counteract distortions in the atmosphere by bouncing light off a mirror that puts an opposite distortion on what you're looking at, thus canceling out the original blur. It's as though you were looking at a funhouse mirror via an anti-funhouse mirror with exactly the opposite shape. 
But things are actually trickier than that. Since shifting air pockets cause the atmospheric blur to change constantly, this "opposite distortion" has to change as well. So the mirror isn't rigid: it's thin, and backed by tiny adjusters that change its shape as conditions change, many times a second. The mirror knows what adjustments to make because a small telescope nearby is always watching a bright star in the bigger telescope's field of view: since a star should always look like a perfect pinpoint of light, any deviation from that perfection tells the AO system how the atmosphere is jiggling at that moment, and the deformable mirror adjusts instantly to counter it.
Unfortunately, you can't always count on finding a bright star in the direction you want to look, so AO designers now make their own: they shine a laser to bounce off sodium atoms in the upper atmosphere, creating an artificial guide star you can place pretty much anywhere.
That solves one of the problems with AO. The other is that the deformable mirror corrects only blurriness relatively close to the reference star (or laser point), since atmospheric turbulence is very localized. Rigaut's team solved this one by creating five different laser guide stars and using multiple deformable mirrors in a single AO system. It's kind of Rube Goldbergian in its complexity, but it works. 
Impressive as AO technology is, this doesn't mean NASA should stop work on the James Webb Space Telescope, the Hubble's successor, which is slated to go into orbit in 2018. The new system sharpens only a small amount of Gemini's field of view, while JWST will have perfect vision across its entire 21-foot mirror. The Webb is also designed to work in infrared light, ideal for seeing the most distant galaxies and for spotting newborn planets around nearby stars. Infrared light doesn't get through the atmosphere very well, and no amount of de-blurring will change that.
Still, the notion of producing space-sharp images from the surface of the Earth is something astronomers could barely imagine back when the Hubble first went into orbit. And for those astronomers who have to wait patiently for their turn at the oversubscribed space telescope, it's a welcome alternative.