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Friday, 17 February 2012

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!"

Tuesday, 7 February 2012

Why Energy Efficiency Isn't All It's Cracked Up to Be

When New Yorker writer David Owen moved his family from Manhattan to a small town in northwestern Connecticut in 1985, it seemed like a green decision. Their tree-shaded house had been built in the 1700s and sat across from a nature preserve. Deer, wild turkeys and even bears could be seen in their yard; woods surrounded their neighborhood. It was a bucolic country existence, something out of a nature poem.
Yet for the global environment, the move was a minidisaster. The Owens' electricity consumption went up more than sevenfold, and the lack of both public transportation and dense housing that's typical of Connecticut (and much of the rest of the U.S.) meant the family had to buy several cars. And those cars got driven — a lot. Owen notes that he and his wife now put some 30,000 miles a year on their odometers, burning carbon with every gallon. Access to trees and wildlife and cleaner air in Connecticut was great, but for the climate, it's dense and efficient Manhattan — where cars are optional and living space is much tighter — that does less damage per capita.
To Owen, the move was a lesson: what looks environmentally friendly isn't always the case. That's an idea he explores in his new book The Conundrum, which argues that energy efficiency, scientific innovation and even good green intentions are actually making our climate and environmental problems worse. While we rush to buy a Prius hybrid or fetishize local organic food, we're doing little to actually reduce the carbon emissions that are warming the planet — and we may even be going backward. "We're not actually making the problem better, we're making it worse," says Owen.
He centers his argument on energy efficiency, which simply means reducing waste and getting more economic output per unit of energy, and is one of the few environmental-policy options that nearly everyone can agree on. Democrat or Republican, climate scientist or climate skeptic, you'd be hard-pressed to find anyone who'd be against reducing wasted energy. That's why paeans to efficiency find their way into every energy stump speech, including those of President Obama, who noted in the State of the Union speech that "the easiest way to save money is to waste less energy." One of the White House's most heralded green accomplishments this term has been an increase in automobile fuel-efficiency standards, tightening them from 25 m.p.g. now to 54.5 m.p.g. for cars and light trucks by 2025.
But Owen notes improved efficiency doesn't always translate to reduced energy use, thanks to something called the "rebound effect." It's pretty simple: as we become more efficient at using energy, we can save money — which then allows us to use more of that energy than we did before. Picture it this way: you trade in your gas-guzzling SUV for a new efficient hybrid, end up paying less per mile for gasoline, and use some of the savings to drive more than you did with the SUV. The efficiency has rebounded.
It's not clear how big the rebound effect really is. Efficiency advocates say that the effect, when it exists, is limited. Amory Lovins, the head of the Rocky Mountain Institute and an efficiency evangelist, has written that "no matter how efficient your house or washing machine becomes, you won't heat your house to sauna temperatures, or rewash clean clothes." There's a limit to how much more I would drive after buying a hybrid even as my gas bill shrinks. 
But Owen argues that the rebound effect is much broader than a one-to-one relationship. I might drive a little bit more using the savings from my more efficient car, but I might also take the rest of those savings and spend it on something else — perhaps a vacation flight, or a new television. And nearly everything we buy and consume today requires energy, from appliances to holidays. Perhaps that's the reason American electricity production grew 66% between 1984 and 2005 even as the economy overall became much more efficient. And things are likely to be even worse in a rapidly growing nation like China, where a lot of people are acquiring consumer goods and other luxuries for the first time. "Energy efficiency by itself is not a sufficient green strategy," says Owen.
Hopes that we might simply run out of fossil fuels before we've cooked the sky now seem unfounded, thanks to the discovery of new unconventional supplies like oil sands or shale natural gas. We can decarbonize the energy we use by replacing fossil fuels with solar, nuclear and other alternatives, but that will take decades at best, and we're moving far too slowly.
So if we want to bring down carbon emissions, we have to use less energy — even very efficient energy — and that likely means we'd have to live with less growth. Good luck trying to explain that to voters, though. "There is a fundamental conflict between the idea of propagating growth and the idea of reducing carbon emissions," says Owen. "But if you're in a public-policy position, it's almost impossible to say that." 
That doesn't mean energy efficiency can't be a useful environmental tool — it can, provided it's coupled with policies that effectively increase the cost of energy, so savings from efficiency are conserved rather than being spent on additional consumption. We can also change policy to promote sustainable, dense urban living. (Hong Kongers are well-off, but they use only one-third as much energy as Americans largely because they live in one of the densest cities on the planet.) And we can focus on the environmental policies that really matter. Buying local food is fine, but what matters much more is how far you drove to get to the market — or whether you needed to drive at all. "What we need to do is make more big cities like Manhattan," says Owen. "But that's a tough sell."
Indeed it is. Owen notes that he likes living in small-town Connecticut, even if it isn't great for the planet. And while everything from highway construction to zoning regulations seem designed to induce sprawl, I suspect many Americans simply don't want to live in New York City, just as they don't really care about climate change enough to accept more expensive energy or slower economic growth. "We already know what we need to do and we have for a long time," Owen writes. "We just don't like the answers." That's the conundrum — and the solution won't be easy.