“Death Star Gamma-Ray Gun Pointed Straight at Earth”

So read the headline on Fox News, March 5, 2008. This was no joke. In fact, it was based on nine years of observations on WR104, a Wolf Rayet star about 8,000 light years away in the constellation Sagittarius. It was first discovered in 1999 by University of Sydney astronomer Peter Tuthill.

Wolf Rayet stars, named after two 19th Century French astronomers, Charles Wolf and Georges Rayet, are the most massive stars, typically with greater than 20 solar masses.  Like Achilles of Greek Mythology, they have a short but glorious life. They live fast and die hard. They are the hottest, the brightest, the biggest and the most short-lived of all stars. And when they die, it is by a huge supernova usually resulting in a black hole.

Normally, a supernova 8,000 light years away from us probably wouldn’t pose a threat, but Wolf Rayet stars are special. The collapse to a black hole by a rotating, massive star is a good candidate for a gamma ray burster. The core collapse, followed by the reflection of the shock wave of the outer shell collapse is believed to lead to the brightest explosions in the known universe.

The rotation adds asymmetry. When the collapse proceeds, the shell of exploded gas is least dense along the polar axis, which acts to funnel the x-rays, gamma rays and high energy particles into a concentrated beam. If it points towards us, we see the supernova as a gamma ray burst, such as the more than 300 already observed. If it points away from us, we just see it as a massive supernova, like SN2008D in the spiral galaxy NGC2770, most likely the death of a Wolf Rayet star.

WR104 is rotating. Like more than 60% of the other 230 Wolf Rayet stars identified in our galaxy, WR104 is in a binary system, with a rotation period of about 240 days. A key property of these massive, hot stars is that the radiation pressure is so high, the outer shells of the star are constantly being blown off into the local solar wind. In WR104, the combination of rotation in the binary system and huge solar wind creates an impressive pinwheel spiral.

Now comes the scary part. Observations released in Astrophysical Journal on March 1, 2008 indicated an orbital inclination of between 0 and 16 degrees of the spin axis of the system to earth. In other words, it’s as though we are looking right down the spin axis of WR104. When it goes supernova, expected within the next 100,000 years, we will be provided a “unique vantage point” to the supernova explosion.

Dr Tuthill, the discoverer, said, “I used to appreciate this spiral for its beautiful form, but now I can’t help a twinge of feeling that it is uncannily like looking down a rifle barrel. If such a gamma-ray burst happens, we really do not want Earth to be in the way.”

And here we have stood for almost nine years, keeping our fingers crossed that we would not be at the bull’s eye when WR104 went off, until the AAS meeting on Jan 7, 2009. Grant Hill, an astronomer at Keck, refined the spectroscopic measurements of the double star and was able to see some motion of the stars toward and away from us. This means we can’t be on the spin axis. His recent observations suggest the system is inclined about 30 -40 degrees away from us.

We probably won’t be in the beam when the gamma ray burst goes off. Now we can all sleep a little easier.

For more information check out:
http://www.physics.usyd.edu.au/~gekko/wr104.html
http://blog.professorastronomy.com/2009/01/one-less-catastrophe-to-worry-about.html
http://www.universetoday.com/2009/01/07/wr-104-wont-kill-us-after-all/

Published in Feb 2008 Cosmic Messenger