Stars after the Star

After seeing Halle Berry I hung around Harvard Square and then went to the Monthly Observatory Night at the Harvard-Smithsonian Center for Astrophysics. The speaker was Bryan Gaensler who spoke about "The Brightest Explosion in History". It was a great talk.

On December 27, 2004, satellites detected the brightest explosion we've ever seen. It was a thousand times brighter than the Milky Way and lasted a half a second. it ionized the earth's upper atmosphere. We actually saw it twice because it echoed off the moon and hit us again. Why don't you remember seeing it? It wasn't in visible light but rather in gamma rays, which are filtered out by the atmosphere. But man has only seen two things brighter: the sun and the Great Comet of 1882. Satellites detected it and in fact were overloaded by it. In addition to the main explosion, there were (basically) aftershocks on a 7.56 second cycle.

Using data from several satellites that were affected, and knowing their position and the exact time it hit them, astronomers triangulated the explosion to a the magnetar called SGR 1806-20 located in Sagitarious. SGR 1806-20 rotates every 7.56 seconds so this matched up nicely with the aftershocks. This magnetar is 50,000 light-years away, on the far side of our Milky Way galaxy.

In case you were wondering, the International Space Station was fortunately on the other side of the earth so it was protected. If it weren't the occupants would have gotten the equivalent of a dental x-ray. Which is pretty impressive as it came from 50,000 light-years away (that's about 3,000,000,000,000,000,000 miles).

So what's a magnetar? Basically it's a rare type of neutron star (the remnants of a supernova) with a very strong magnetic field. There are 10 known in the Milky Way. They were first theorized in 1992 and the first one was observed in 1998, coincidently it was SGR 1806-20, so to us they're quite new. It's about 15 miles in diameter but has 1.5 times the mass of our sun. Made of neutrons, they are the densest stars in the universe. They are very hot, a million degrees on the surface, (our sun is only 6 000 degrees on its surface). Given their name, you might guess that they are the strongest known magnets, with a magnetic field a thousand million times that of the earth.

So what was the explosion? The surface of our sun has many magnetic feld arcs moving about and when they touch they release debris and energy. These are called solar flares. We think this explosion was the result of something similar on the magnetar. But with it's much greater magnetic field, it was much larger. The magnetar released more energy in one-tenth of a second than our sun has released in 100,000 years. It killed off everything within 10 light-years. If earth were that close it would have burned off the ozone layer. The magnetar lost about 1% of its energy but it's still there, spinning every 7.56 seconds.

This NASA site has some interesting animations you can watch. People have wondered if this event could have cause the tsunami at about the same time. Nope, the tsunami was 2 days prior. Someone had sent him a picture they took on a vacation in Rome. It had some flares on it and they wondered if it was the magnetar. Nope.

Afterwards we went to the roof for some observation. I'll never get tired of seeing Saturn through a telescope. The we also looked at the moon and mars. We looked at a double star in the Big Dipper called Mizar and at M44 The Beehive Cluster. One of the graduate students was giving her tour of stellar evolution. Stars form from clouds of gas called nebula and then spread out as they are affected by other stellar objects. We looked at three objects. First was M42, the Orion Nebula where you can see stars and the cloud that formed them. Then we saw M45, the Pleiades a young open cluster with young blue stars but with none of the original nebula visible. Last we looked at the Double Cluster in Perseus which while young has some stars old enough to have evolved into red-giants. It was a great night.