A Neutron star [Pulsar] is the collapsed inner core of a star which has lost most of its mass in a nova explosion. The density of the neutron star is the density of the atomic nucleus.
The best known neutron star is at the center of the Crab nebula. The nebula is the expanding outer shell of stellar explosion recorded by Chinese astronomers in 1054. Now 10 light-years across, the nebula is expanding at over 1,000 kilometers per second. Flipping between two images made nearly 30 years apart, this animation clearly demonstrates the expansion.
A typical neutron star has a mass of at least 1.4 solar masses packed into a radius of 10 km, spinning with a period of milliseconds because it has retained the angular momentum of the core of the star from which it was formed.... with a density close to that of the atomic nucleus!
Matter orbiting a neutron star can have a period as short as a millisecond and matter crashing into the surface can have speeds as high as one third of the speed of light! Neutron stars have no nuclear fuel but they do radiate energy as both radio waves and as high energy photons.
Some neutron stars actually gain energy by accreting matter which falls to the surface releasing approximately Gm/R Joules in potential energy: a remarkable 200 MeV per proton: far in excess of the 7 MeV per proton available from nuclear fission. An accreting neutron star has a typical surface temperature of some 10 million K with peak black body radiation in the x-ray region of the spectrum.
Recent (2001-2) x-ray images of the central portion of the crab nebula show an expanding ring structure and polar jets.
The Gum Nebula is the nova remnant. closest to Earth. The nebula spans 40 degrees across the sky, and is so large and faint that it is easily lost in the bright and complex background. The Gum Nebula [highlighted nicely in this wide angle photograph], is so close that we are much nearer the front edge than the back edge, [450 and 1500 light years respectively]. The complex nebula lies in the direction of the constellations of Puppis and Vela. Oddly, much remains unknown about the Gum Nebula, including the timing and even number of explosions that formed it.
The Chandrasekhar limit (1.4 solar masses) is the maximum mass of a white dwarf. The Tolman -Oppenheimer -Volkoff (TOV) limit (first proposed in 1939) is the maximum mass of a neutron star. Modern estimates range from ~1.5 to ~3.0 solar masses. If The original star that forms both the expanding nebula and the neutron star, has a mass that is ~20 times the mass of the Sun the neutron star is heavier than the TOV limit itand it collapses immediately to a black hole.
Two types of black holes are now recognized. Stellar black holes detected as companions in binary star systems (with masses up to ~10 solar masses) and huge accreting black holes thought to exist at the centers of galaxies with a mass of one million to one thousand million solar masses. The supermassive black hole at the center of our Galaxy has a mass of ~7.4x1036 kg. The mass of OJ287, the largest measured supermassive black hole to date (2007), is 3.6 x1040 kg. That is an amazing 1.8x1010 times the mass of the Sun.
Light cannot leave a black hole across the event horizon - defined as the radius for which the escape velocity is the velocity of light - but an accretion disc of matter can radiate electromatic waves from points outside the event horizon.
The event horizon
The event horizon (the maximum radius of a black hole of mass m) is found by making the escape velocity the velocity of light in the equation....
Inserting the mass of the Sun 2x1030 kg and the values of G (6.67x10-11) and c (3x108m/s) gives an event horizon of ~ 3 km for a hole of four solar masses.
A remarkable demonstration of the extraordinary density of a black hole is afforded by calculating the radius of the event horizon for a hypothetical black hole with the mass of the Earth. There is no reason to suppose that holes of this size exist, but if they did, they would be the size of a bird's egg!
Gamma ray bursts
Gamma ray bursts, about twice a day, outshine for 10-20 seconds, even the 1000 Watts per square meter that we receive from the Sun in total photon energy received per second. Gamma ray bursts have been a mystery since their discovery in the 1970's, but have recently been shown to be the signature of the birth of a black hole as the core of a massive star that exceeds about 4 solar masses collapses to a black hole in some distant galaxy.
Some gamma ray bursts have a complex structure, with scattered x-ray bursts up to half an hour after the main event. In these cases the initial black hole is thought to be accreting remnants left behind as the outer star explodes.
Gravity waves?
The merger of two black holes is expected to generate very rapid changes in the local gravitational field which is expected to propagate energy through space as gravity waves. To date (2006) gravity waves have not been detected but efforts are intensifying. NASA has generated computer simulations of the gravity waves expected to radiate from a black hole merger. It may be some time before their existence is confirmed with experimental evidence.
Quasars - super luminous 'quasi stellar' (ie. small) objects - believed to be at cosmological distances - are now thought to be powered by the release of gravitational potential energy as matter is drawn towards a super-massive black hole at the center of a galaxy in the very distant past. The diameter of a quasar is estimated [from the relatively short time of output fluctuations] to be just a few light hours. The output of an entire galaxy appears to be emitted from a region of space no larger than the solar system!