Saturday, July 14, 2007

Star with Mystery Partner?

When stars are more massive than about 8 times the Sun, they end their lives in a spectacular explosion called a supernova.

The outer layers of the star are hurtled out into space at thousands of miles an hour, leaving a debris field of gas and dust. Where the star once was located, a small, incredibly dense object called a neutron star is often found. While only 10 miles or so across, the tightly packed neutrons in such a star contain more mass than the entire Sun.
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A new X-ray image shows the 2,000 year-old-remnant of such a cosmic explosion, known as RCW 103, which occurred about 10,000 light years from Earth. In Chandra's image, the colours of red, green, and blue are mapped to low, medium, and high-energy X-rays. At the center, the bright blue dot is likely the neutron star that astronomers believe formed when the star exploded.

For several years astronomers have struggled to understand the behaviour of this object, which exhibits unusually large variations in its X-ray emission over a period of years. New evidence from Chandra implies that the neutron star near the center is rotating once every 6.7 hours, confirming recent work from XMM-Newton. This is much slower than a neutron star of its age should be spinning.

One possible solution to this mystery is that the massive progenitor star to RCW 103 may not have exploded in isolation. Rather, a low-mass star that is too dim to see directly may be orbiting around the neutron star. Gas flowing from the unseen neighbour onto the neutron star might be powering its X-ray emission, and the interaction of the magnetic field of the two stars could have caused the neutron star to slow its rotation.

RCW 103: A Star with a Mystery Partner?
Credit: NASA/CXC/Penn State/G.Garmire et al

Neutron Star
For a sufficiently massive star, an iron core is formed and still the gravitational collapse has enough energy to heat it up to a high enough temperature to either fuse or fission iron. Either in the aftermath of a supernova or in just a collapsing massive star, the energy gets high enough to break down the iron into alpha particles and other smaller units, and still the pressure continues to build.

When it reaches the threshold of energy necessary to force the combining of electrons and protons to form neutrons, the electron degeneracy limit has been passed and the collapse continues until it is stopped by neutron degeneracy. At this point it appears that the collapse will stop for stars with mass less than two or three solar masses, and the resulting collection of neutrons is called a neutron star. Pulsars are thought to be neutron stars.

If the mass exceeds about three solar masses, then even neutron degeneracy will not stop the collapse, and the core shrinks toward the black hole condition.

This neutron degeneracy radius is about 20 km for a solar mass, compared to about earth size for a solar mass white dwarf. The density is quoted as about a billion tons per teaspoonful compared to 5 tons per teaspoonful for the white dwarf.

Neutron stars may be crystalline with crusts on the order of 100 meters thick and an atmosphere a few centimeters thick. They may have 10 to the 11 times the earth's gravity and a powerful magnetic field. A neutron star might have an atmosphere a few centimeters thick and mountain ranges poking up a few centimeters through the atmosphere. A neutron star is thought to be about 1/100,000 the diameter of the Sun, and a nucleus is on the order of 100,000 times smaller than an atom.

First Light from The Canarias Telescope by Stefan @ BackReaction
Supernova theory strengthened by new observations - ESO release

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