Supernova nightmares

Posted: Thursday, May 10, 2007 11:58 PM by Alan Boyle

Some astronomers would be delighted if a super-bright supernova blast like the one reported this week were to occur in our own galaxy. One says it could be "the best star-show in the history of modern civilization." But if the blast was pointed right at us from close quarters? Well, that would be bad.

How so? And how bad? For the answers to those questions, you can turn to some cool Web sites, a few good books ... and a couple of bad movies.

When astronomers reported that a new breed of supernova had been detected millions of light-years away, they also noted that the star involved in that explosion appeared to be similar to Eta Carinae one of the biggest and intrinsically brightest stars in our own Milky Way galaxy. Eta Carinae is a mere 7,500 light-years from Earth - and although astronomer Mario Livio said he didn't think the star posed any danger, MSNBC.com users naturally wanted more detail:

Harald Jensen, Guatemala City, Guatemala: "Would you please be so kind as to enlighten us with the calculations for if and when Eta Carinae explodes? I would like to know how much time we would have to 'run away' from the shock wave caused by such an explosion? Naturally, I’m not worried about running away, but I know that interstellar sightings are not as easy as 1, 2, 3. Can you explain (in kindergarten terms please), depending on what kind of telescope first saw the explosion, how fast the shock wave would expand, and therefore, when would we feel it at home, here on Earth? Or is it as easy as 7,500 years (and that is the answer), with the shock wave traveling at the speed of light, and it would not make much of a difference when we saw it?"

Mack: "If Eta Carinae went supernova, how long would it take for the effects to be seen and/or felt on Earth, specifically any gamma ray effects? Or, put another way, if it goes supernova, how long would it take the gamma rays to travel the 7,500 light years to Earth if the energetic jets emanating from the star were pointing our way?"

Kenneth: "Let's see if I understand this. Dear Eta is 7,500 light years away, so what we are observing occurred 7,500 years ago. So [Livio's observation that a supernova] "could happen tomorrow or it could happen 1,000 years from now" is in need of editorializing. What is technically more accurate, is that Eta may blow or has probably already blown its top. But it's so far away that the light and other energy particles of that event have not yet reached us. We wish that we might live to experience it in the next 7,499 years. I'd like to 'see' it. The riddle that may be answered in this unfolding epoch, is why, thankfully, the magnificent supernovae exist in light-wave form. And the distance being so far, and the size so massive, that it would appear to be of original ejecta from the first big bang. Is it such that the beginning of time is coming to an end? Godspeed, earthlings."

Harry and Leslie: "Could any supernova that occurs in space ever reach Earth, and what could the effects of such an event be on planet Earth?"

The questions about timing are easy to answer: When Livio was talking about "tomorrow," he meant that the effects of such a supernova could be observed in the sky starting tomorrow - which of course means that the event itself would have occurred 7,500 years ago back at Eta Carinae. That's a long time ago, and the supernova that was first observed last September in the constellation Perseus took place much, much longer ago - about 240 million years ago.

However, you couldn't say that represented the "original ejecta of the big bang." The big bang is thought to have occurred around 13.7 billion years ago - and that involved the explosive inflation of the space-time continuum itself, rather than an explosion in the supernova sense.

Mack correctly put his finger on the supernova's gamma-ray burst as the main thing to worry about. That would be the likeliest cause of a supernova nightmare, and there wouldn't be much we could do about it. That is, unless astronomers learn enough about stellar evolution to predict when a dangerously close star could blow up in spectacular fashion - giving us enough time to build, say, underground cities.

NASA

An artist's conception shows a gamma- ray burst sweeping over Earth's atmosphere, depleting ozone and creating smog in the process. In reality, the gamma radiation would be invisible.

A couple of years ago, astronomers took a close look at the potential effects of a nearby gamma-ray burst directed at Earth. The results weren't pretty. Research published in the Astrophysical Journal Letters proposed that a burst lasting just 10 seconds could have caused the Ordovician mass extinction, 440 million years ago.

As the invisible gamma radiation swept over Earth, it could have depleted half of our planet's protective ozone layer, leaving the surface vulnerable to the sun's deadly ultraviolet rays for five years or so. That could have killed off much of the life on land as well as plankton and other organisms near the ocean surface, disrupting the marine food chain in the process. Such a scenario matches up with what seems to have happened during the Ordovician extinction.

This NASA feature provides a summary of the research, as well as one animation that shows what a killer gamma-ray burst might look like from an earthly vantage point, and another animation that shows which areas of Earth would be affected the most.

The Ordovician extinction may have had different causes, of course. Some have pointed to climate change as the killing blow - an unusual kind of "icehouse effect."  But even then, there could be a link between a gamma-ray burst, ozone depletion and global cooling.

Phil Plait, who is a gamma-ray astronomer as well as the author of "Bad Astronomy" and an upcoming book tentatively titled "Death From the Skies," shed more light on the subject in an e-mail:

"I'm researching this for my next book. There are two bad things that happen if you get enough gamma rays smacking into you:

"1) They dissociate ozone molecules. Bad. Worse, they also zap nitrogen molecules, which then go out and zap ozone molecules. Either way, a lot of ozone goes away. It depends on how close the supernova or gamma-ray burst is, of course, but some studies have shown that a gamma-ray burst ... could eradicate 30 percent of ozone globally, with some local places dropping by more than 50 percent. In technical terms, that would suck.

"2) Those nitrogen atoms go on and make NO₂ molecules, which is a reddish brown toxic substance. Not enough would be made, most likely, to hurt folks, but it's dark and absorbs sunlight, so they can contribute to global cooling. The Ordovician event may have been from a nearby supernova or gamma-ray burst, as there is evidence of increased UVB [ultraviolet light B] hitting phytoplanktons and also cooling at the same time.

"There is a third thing: cosmic rays, atomic nuclei accelerated to relativistic speeds, may also be sent our way by supernovae or gamma-ray bursts. No one is really sure. But there is a lot of evidence (and this shocked me) that the cosmic rays affect our weather by seeding clouds (I am unclear how this works in detail but I'll know better as I read more). More clouds means more cooling, so more cosmic rays could trip an ice age. Seriously.

"All of this depends on how close a supernova gets, and there is evidence that in the past few million years a few have exploded within a few dozen parsecs. Fe60 is a radioactive isotope of iron, and is created in supernova explosions. An excess of Fe60 has been found in ocean floor samples dated to a few million years ago. Cool, huh?

"The big SN 2006gy was 240 million light years away, so it won't hurt us. But Eta Carinae is only 7,500 light years away..."

Fortunately, as Livio pointed out during this week's news conference, the poles of Eta Carinae's hourglass-shaped structure appear to be pointed away from Earth. That has led astronomers to assume that the highly focused flash of a gamma-ray burst would also miss our planet.

"Right now, there are no supernovas close enough to hurt us," Plait told me in a follow-up phone call. "There are no gamma-ray sources close enough to hurt us. We think."

One possible cause of gamma-ray bursts is the merger of two dense celestial objects - say, neutron stars or black holes. And we might never see that coming.

If all that's not enough to throw a scare into you, there's a large pile of movies and books that address the supernova nightmare scenario. On the Astronomical Society of the Pacific's Web site, Foothill College astronomer Andrew Fraknoi lists seven supernova-themed tales that reflect "good astronomy and physics." Two are particularly worthy of note:

• "Supernova" by Roger Allen and Eric Kotani. Plait says Kotani is the pen name of a well-known astrophysicist - and although the premise of the book is purely fictional, there's some good science backing up the story.
• "The Twilight of Briareus" by Richard Cowper. This tale, which delves into supernova-caused climate change as well as some weirder fictional effects, comes with a strong recommendation from Paul Gilster, the blogger behind Centauri Dreams. "This guy could flat-out write," Gilster said.

Both books appear to be out of print - but used copies are available online, and you might find them as well at your local library or used-book shop. That qualifies the two books as a dual selection for the Cosmic Log Used Book Club, which highlights books with cosmic themes that aren't necessarily brand-new.

Unfortunately, supernova stories on the silver screen haven't fared as well. There was a "Supernova" movie starring James Spader and Angela Bassett, as well as a "Supernova" made-for-TV movie with Tia Carrere and Luke Perry - but neither made much of a scientific or a critical splash.

In the semi-documentary category, the Sci-Fi Channel's "Countdown to Doomsday" addressed the gamma-ray threat, with an assist from Plait. Gamma-ray flares also play a role in the "Nova" documentary "Monster of the Milky Way," but that show (which you can watch online) has more to do with black holes than supernovae.

So I'll have to turn to you for guidance: If you have any recommendations for gripping supernova tales, or even deliciously bad movies about supernova nightmares, by all means pass them along in the comments section below.

Update for 10 p.m. ET May 17: I added a reference to Fraknoi as the author of the excellent book recommendation list on the ASP Web site, in the interest of giving credit where credit is due.

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