I saw this article hit the mainstream press today, “concerning the huge emission of gamma rays back in December by a neutron star on the other side of our galaxy”:http://news.bbc.co.uk/2/hi/science/nature/4278005.stm. If we dig a little bit on this, we soon find mention of the “SWIFT”:http://swift.gsfc.nasa.gov/docs/swift/swiftsc.html gamma-ray burst (GRB) probe. A quick peek at the page they’ve setup concerning this once-in-a-lifetime event reveals a “nice array of illustrations and information”:http://www.nasa.gov/vision/universe/watchtheskies/swift_nsu_0205.html.
Not too bad. I like the point that was made about being thankful for the distance of this event. Too close, and a spectacle like this one can lead to mass extinctions on a world like ours. Good thing there aren’t too many restless neutron stars is our unfashionable western arm of this spiral galaxy.
What is a neutron star? Take a star like ours, but make it 20 times more massive. When a star like that gets very old, it burns out most of the nuclear fuel that is the source of its abundant energy. There comes a time when the outward pressure of the nuclear reactions can no longer counterbalance the gravitational pressure of the massive star, and it collapses.
This star’s collapse will cease when the electrons in its hydrogen get so close that Fermi pressure, a purely quantum mechanical effect, overcomes gravity and holds the star from collapsing completely. It’s amazing to me that this Fermi pressure, exerted when two electrons get too close to being in the same quantum state, is capable of overcoming the sheer brute mass of such a star.
That isn’t the end of the story. The core of the star fuses into iron, and this continues until the core is so large that gravity again overwhelms the star. It becomes energetically preferable for inverse beta decay to now occur. This is the process by which electrons and protons interact and produce neutrinos and neutrons. The neutrinos, untethered to the neutrons, escape the star (and can reach us, telling us about the star that birthed them!) while the neutrons remain bound in the star remnant. Again, if the star wasn’t too much more massive than originally posited, the neutron Fermi pressure will counterbalance gravitational collapse and sustain the star as a dense ball of neutrons. This is our neutron star.
I thought a neat resource for information about this phenomenon was “Dr. M. Coleman Millers”:http://www.astro.umd.edu/~miller/nstar.html site.