NEUTRON STARS

A neutron star is a type of remnant (remains) that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova. Neutron stars are small highly compact stars with densities comparable to that inside nuclei. They consist predominantly of Neutrons and a slight percent of Protons and Electrons. These huge neutron-rich nucleuses are bound by gravitation and require a minimum neutron star mass of ~0.1 Solar Mass. Above a maximum (Chandrasekhar Limit) mass of order 2–3 Solar Mass neutron stars are unstable and undergo a gravitational collapse to form Black Holes.

Neutron stars are formed in type II or Ib Supernova explosions when massive stars (M ≥ 10 x Solar Mass) run out of nuclear fuel after burning for millions of years. When the iron core in the center of the aging stars exceeds its Chandrasekhar mass, ~1.5 Solar Mass, the star undergoes gravitational collapse in just seconds and suffers violent death. Gravitational and kinetic energy of the order ~1053 ergs is released mainly by neutrino emission that blows off the outer layers. Only ~1% of the energy is actually seen in a brilliant burst, the supernova.

The gravitational field at the neutron star's surface is about 2 × 1011 times stronger than on Earth. The escape velocity is about 100,000 km/s (1 Lakh km/s), which is about one third the speed of light. Such a strong gravitational field acts as a kind of gravitational lens and bends the radiation emitted by the star such that parts of the normally invisible rear surface become visible. The temperature inside a newly born neutron star is from around 1011 to 1012 Kelvin. However, the extremely huge number of neutrinos it emits carries away so much energy that the temperature falls within a few years to around 106 Kelvin. Even at 1000,000 (1 Million) Kelvin, most of the light generated by a neutron star is in X-rays.

The nearest known neutron star, RXJ185635-3754, located in the southern constellation Corona Australis, was imaged by the Hubble Space Telescope in 2001. Its distance is estimated by parallax at only 200 light years. From its black body spectrum, a surface temperature of half a million Kelvin is found, and from its flux a radius of order 10 km is inferred. Circumstantial evidence indicates that it was born in a supernova explosion a million years ago.

Questions to Ponder

• How is a Pulsar different from a Neutron Star?
• Why don’t all Neutron Stars become Pulsars?