New Risk to Earth Found in Supernova Explosions

Laura Kinoshita

Date: 08 January 2008 Time: 01:29 AM ET

An explosive star within our galaxy is showing signs of an impending eruption, at least in a cosmic time frame, and has for quite some time. From 1838 to 1858, the star called Eta Carinae brightened to rival the light of Sirius, the brightest star in the sky, and then faded to a dim star. Since 1940 it has been brightening again, and scientists think Eta Carinae will detonate in 10,000 to 20,000 years.

Fortunately, Eta Carinae is far away, at least 7,500 light-years from Earth. If it explodes, most of its energy will be scattered or absorbed in the vast emptiness of space. It also happens to be tilted about 45 degrees from the line of sight to Earth, so any type of gamma-ray burst, a high-energy outburst expected with this star's eventual eruption, would miss the Earth. Cosmic rays would be diffused by magnetic fields, and most of the damaging light would not affect life on Earth.

In general, threats to life on Earth from supernovae are extremely small, for all except the nearest explosions — those 30 light-years away or closer.

But what if a supernova were 100 times brighter than usual? Would there be any risk to life on Earth then?

Astronomers found such a record-breaking supernova last year, SN 2006gy.

Brightest ever

SN 2006gy was the brightest supernova ever recorded until an even brighter one was discovered in November.

Astronomers now know the progenitor of SN 2006gy was remarkably similar to Eta Carinae. They warn a superluminous supernova might explode right in our own galaxy.

Brian Thomas at Washburn University has been studying the effects of astronomical explosions at the Goddard Space Flight Center. He decided to investigate what would happen to Earth's protective ozone layer if Eta Carinae explodes with the brilliance of SN 2006gy.

It turns out that even though SN 2006gy was one of the brightest supernovae ever recorded, it did not generate a large amount of X-rays. Thomas and his team found most of the light, including damaging X-rays and cosmic rays, would scatter into space before ever reaching Earth.

So would there be any damage to Earth from such a spectacular event? Though Thomas found X-rays and cosmic rays would cause little damage, he also looked at optical light, particularly short-wavelength blue light (400 nanometers), where the spectrum of SN 2006gy peaked. No one had ever considered the effects of this light before, either from supernovae or any other type of event.

?The visible light could be significant,? Thomas says, ?But this depends a little bit on your definition of significant.?

Brighter than Venus

If Eta Carina were to explode like SN 2006gy, it would quickly become the brightest object in the sky other than the sun and the moon.

For those living where Eta Carina is always above the horizon (Antarctica, New Zealand and extreme southern regions of Australia and South America) the light would vastly outshine Venus, visible even during the day. The radiation would illuminate the evening sky with a bluish glow nearly strong enough to read by, and the effect would likely last for months — perhaps six or more.

The cumulative effects of long-duration exposure to blue-enhanced light would begin to interfere with life on Earth.

Those who study chronobiology, or the effects of biological timing, have found that low levels of blue light can strongly affect the endocrine systems of mammals by causing physiological and alerting responses. Blue-enhanced light is associated with reduced levels of melatonin production and affects circadian rhythms. For these reasons, it is sometimes prescribed to counteract seasonal affective disorder (SAD) or winter depression.

?This is not going to be an ?everything dies immediately? kind of event,? Thomas said. ?But with the risk factors associated with higher levels of this kind of light it?s certainly something that could be important in the longer run.?

New threshold

In a paper about to be published in the journal Astrobiology, Thomas explains that even short exposures to blue light can increase insomnia, reduce resistance to infection and is being studied as a possible risk of cancer.

Yet, in the case of Eta Carinae, the effect of these optical photons would be minimal. The scattering of photons by dust and gas is greatest at blue wavelengths (thus giving Earth its blue sky) and the sheer distance of Eta Carinae diminishes the optical intensity by about 20 percent.

But while damage from optical light is not a factor for Eta Carinae, the effects of this light should be considered to any risk assessment of supernovae. Based on his results, Thomas now estimates the biological threshold for supernovae to be about 100 light-years away. At that distance, life on Earth can expect some sort of supernova radiation about once every 20 million years.