‘GUEST STARS’
http://www.time.com/time/health/article/0,8599,2106904,00.html
by Michael D. Lemonick / Feb. 16, 2012

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!”

SECOND SUN
http://io9.com/5738542/earth-may-soon-have-a-second-sun
Earth may soon have a second sun
by Alasdair Wilkens / Jan 20, 2011

The red supergiant star Betelgeuse is getting ready to go supernova, and when it does Earth will have a front-row seat. The explosion will be so bright that Earth will briefly seem to have two suns in the sky.

The star is located in the Orion constellation, about 640 light-years away from Earth. It’s one of the brightest and biggest stars in our galactic neighborhood – if you dropped it in our Solar System, it would extend all the way out to Jupiter, leaving Earth completely engulfed. In stellar terms, it’s predicted to explode in the very near future. Of course, the conversion from stellar to human terms is pretty extreme, as Betelgeuse is predicted to explode anytime in the next million years. But still, whether the explosion occurs in 2011 or 1002011 (give or take 640 years for the light to reach Earth), it’s going to make for one of the most unforgettable light shows in our planet’s history. For a few weeks, the supernova will be so bright that there will appear to be two stars in the sky, and night will be indistinguishable from day for much of that time. So don’t count on getting a lot of sleep when Betelgeuse explodes, because the only sensible thing for the world to do will be to throw a weeks-long global supernova party.

Physicist Brad Carter explains what Earth (and hopefully humanity) can look forward to:

“This is the final hurrah for the star. It goes bang, it explodes, it lights up – we’ll have incredible brightness for a brief period of time for a couple of weeks and then over the coming months it begins to fade and then eventually it will be very hard to see at all.”

Although there’ll be no missing the explosion, Carter points out that the vast majority of material shot out from the supernova will pass by Earth completely unnoticed:

“When a star goes bang, the first we will observe of it is a rain of tiny particles called neutrinos. They will flood through the Earth and bizarrely enough, even though the supernova we see visually will light up the night sky, 99 per cent of the energy in the supernova is released in these particles that will come through our bodies and through the Earth with absolutely no harm whatsoever.”

Indeed, just in case anyone is concerned, Betelgeuse is way too far away from Earth to do us any damage. There’s been some doomsday speculation of late around the eventual supernova – which might not happen for a million years, it bears repeating – but, as with pretty much all doomsday speculation, you can just ignore it. In any event, the Betelgeuse explosion will likely be the most dramatic supernova Earth ever witnesses – well, unless our Sun eventually explodes and destroys our planet, which would probably leave Betelgeuse the runner-up. Either way, it isn’t the first, as history has recorded the appearance of several so-called “guest stars.” Most of these just looked like short-lived stars in the night sky, but some were bright enough to be seen in the day.

The first supernova that history records is thought to have occurred in 185 CE, when a star 8,200 light-years away exploded. Chinese astronomers make explicit note of the sudden appearance of a star and its subsequent disappearance several months later, and the Romans may also have made more cryptic references to it. Astronomers have since located the remnants of the exploded star, confirming the accuracy of the ancient accounts.

The two most dramatic supernova explosions occurred in the 11th century. A supernova in 1006 – you can see its modern remnant above – is the brightest star ever recorded, appearing in the records of China, Egypt, Iraq, Italy, Japan, and Switzerland. There’s even some thought that a rock painting by the Hohokam, a Native American tribe in what is now Arizona, represents the first recorded sighting of a supernova in the Americas. Here’s the petroglyph in question, which might well record the presence of an unexpected bright light in the sky:

The various observations even allow us to pinpoint what specific type of supernova it was. In all likelihood, it was a Type Ia supernova, which for a few weeks burn as brightly as five billion suns. Astronomer Frank Winkler explains that we can work out from that supposition:

“By knowing this distance and the standard luminosity of Ia supernovae, we can calculate, in retrospect, just how bright the star must have appeared to 11th century observers. On the magnitude scale used by astronomers, it was about minus 7.5, which puts its brightness a little less than halfway between that of Venus and that of the full Moon. And all that light would have been concentrated in a single star, which must have been twinkling like crazy. There’s no doubt that it would have been a truly dazzling sight. In the spring of 1006, people could probably have read manuscripts at midnight by its light.”

The supernova of 1054 wasn’t quite as dramatic, and it seemed to go almost entirely unrecorded in Europe, although there’s some thought that records of the new star made by Irish monks got corrupted into allegorical accounts of the Antichrist. Still, the rest of the world saw it just fine, with records popping up in China, Japan, Korea, Persia, and the Americas. Astronomers of the time period wrote that it could be seen in daylight for over three weeks and remained visible in the night sky for nearly two years.

A pair of supernovas in 1572 and 1604 were extensively studied by two generations of legendary astronomers, Tycho Brahe and Johannes Kepler. Since then, the Milky Way hasn’t had any supernovas visible from Earth, and so our night sky has remained rather tediously ordinary.

There’s about sixteen known candidates in our galaxy for a future supernova explosion, and quite a few of them would have a dramatic effect on our skies. But Betelgeuse is by far one of the closest, and its huge size means its explosion will be particularly dramatic. This is one cosmic disaster that we actually want to see happen sooner than later, because there may never be a sight quite like this ever again.