A Type I supernova will be blown to bits, and will not leave behind a dense central remnant. A Type II supernova, however, (one triggered by the collapse of a massive star) will leave behind an ultradense relic, with a density of 100 million tons per cubic centimeter. If the object's mass is less than about 3 Msun, it forms a stable object known as a neutron star.
It is a neutron star rather
than a proton star because during the collapse
of the degenerate iron core (see previous lecture for details),
electrons and protons are squeezed together to form neutrons. The
relevant reaction is this:
e + p --> n + neutrino
It is this reaction which is the source of the tremendous burst
of neutrinos which carries away 99% of a supernova's energy.
A neutron star can be thought of as a single humongous atomic nucleus, containing 1057 neutrons packed into a single sphere 10 kilometers in radius. In addition to being amazingly dense, neutron stars have other amazing properties:
The surface of a neutron star is not anyplace you would want to visit. The gravitational acceleration is 100 billion g's (that is, 100 billion times the gravitational acceleration at the Earth's surface). The escape velocity at the surface of a neutron star is half the speed of light (that is, 150,000 km/sec, versus a paltry 11 km/sec for the Earth). On the surface of a neutron star, you'd be simultaneously vaporized by the intense heat and squashed flat by the intense gravitational force.
Neutrons, like electrons, must follow the laws of quantum mechanics. In particular, they must obey the Pauli exclusion principle, as outlined in a previous lecture
At a density of 3 tons/cm3, electrons are degenerate; the energetic electrons which are compelled to occupy the highest energy levels provide electron degeneracy pressure.
At a density of 100 million tons/cm3, neutrons are finally degenerate; the energetic neutrons which are compelled to occupy the highest energy levels provide neutron degeneracy pressure.
The interior structure of a neutron star is
highly uncertain. (We don't know a lot about how matter behaves
at these amazingly high densities.) One proposed model looks like
this:
How do we detect neutron stars if they are so tiny? Well, they may be small, but they are also hot.
At a temperature of 1,000,000 Kelvin, the wavelength of maximum emission is at 3 nanometers -- in the X-ray range. We can hunt for hot neutron stars by looking for X-ray sources.
Some neutron stars, however, also emit electromagnetic radiation by another mechanism. Read on to learn about pulsars.
Some neutron stars appear to emit regular pulses of electromagnetic radiation. When we are able to detect these pulses here on Earth, we refer to the neutron star which emits them as a pulsar. The first pulsar discovery occurred in 1967, when astronomers in England, while making a survey of the sky at radio wavelengths, discovered pulses coming from one particular position. The pulses happened at regular intervals of 1.33730119 seconds. The pulsar was first given the whimsical name ``LGM 1'', with the acronym LGM standing for Little Green Men. Perhaps, astronomers thought, a pulse so regular in its timing was an artificial beacon. Unfortunately for science fiction fans everywhere, pulsars turned out to be rotating neutron stars, not artifacts of extraterrestrial intelligence.
How do we know that pulsars are neutron stars?
One bit of circumstantial evidence is that some pulsars are in
the middle of supernova remnants, where you would expect to find
a neutron star. For instance, the Crab Nebula (the remains of the
1054 A.D. supernova) contains a pulsar with a period of 0.0033
seconds.
Additional evidence that pulsars are neutron stars comes from the brevity of the pulses. The pulses of light coming from the Crab Nebula pulsar, for instance, are only about 1 millisecond long. This means they are coming from an object less than 1 light-millisecond (300 kilometers) across -- otherwise, the pulses would be spread out over a longer time interval. The upper limits of 300 kilometers on the size of a pulsar is too small for them to be white dwarfs or main sequence stars, but comfortably accommodates neutron stars.
The fact that pulsars emit pulses of light is
explained by the Lighthouse Theory. The strong
magnetic field at the surface of a neutron star rips away
electrons from the surface and accelerates them to high speeds
(very nearly equal to the speed of light). The energetic
electrons emit some of their energy in the form of photons. Since
the magnetic field channels the electrons to the north and south
magnetic poles of the neutron star, the photons emitted by the
electrons emerge in two separate beams, one from
the north magnetic pole, and one from the south magnetic pole. A
model of the rotating neutron star, and the two beams of
radiation which it emits, is given below:
Since the magnetic poles are not aligned with the rotational poles (just as on Earth, where the north magnetic pole is somewhere near Ellesmere Island), as the neutron star rotates, the beams of radiation sweep around and around. If the beam of radiation sweeps across the Earth, we see a flash, or pulse of radiation. (Just as you would see a flash of light when the rotating beam of light from a lighthouse swept across you.)
About 600 pulsars have been cataloged in our galaxy. However, we will not see a neutron star pulsing unless one of its beams of radiation happens to sweep across us. Every pulsar is a neutron star, but not every neutron star is a pulsar as seen from Earth.
Neutron stars are potentially powerful energy sources. Drop 1 kilogram of hydrogen onto a 2 Msun neutron star. When it goes ``splat'' on the surface, it will release 30,000 trillion joules of energy - more than 40 times the amount of energy you would release by fusing the same hydrogen into helium.
Neutron stars in close binary systems have matter dumped onto them by their normal stellar companion. Hence, the neutron star which is dumped on is a very strong X-ray source.
Just as white dwarfs have an upper mass limit of 1.4 Msun (the Chandrasekhar limit), neutron stars have an analogous upper limit on their mass, above which neutron degeneracy pressure is insufficient to support the neutron star against gravitational collapse. The upper limit for a neutron star is a bit uncertain, but the best available calculations indicate that it is 3 Msun. A dense core which is more massive than this is doomed to collapse into a BLACK HOLE, an object so compact that even light cannot escape from its gravitational attraction.
Just for fun, why not take a Virtual Trip to a Neutron Star or Black Hole?
