Neutron Stars

What's a Neutron Star's Mass limit?

Also like white dwarfs, neutron stars have a maximum mass beyond which neutron degeneracy pressure cannot balance gravity. However, since the physics of neutrons stars is not well understood this mass limit is not well known. It is believed to be close to 3 solar masses. If this limit is exceeded, an object so dense that not even light can escape may form. This is of course a none other than a black hole.

What's Synchrotron Radiation?

As electrons are accelerated away from the magnetic poles of the neutron star, they spiral around the magnetic field lines producing a type of radio wave called synchrotron radiation. It is this that produces the two beams of radio waves emanating from the magnetic poles.

What's the lighthouse model?

As the neutron star rotates the radio beams sweep across the sky. When a beam briefly passes through our line of sight we see a pulse of radio. This is very similar to the rotating lamp in a lighthouse and hence we refer to this as the lighthouse model. The rate that a pulsar pulses is directly related to the rotation rate of the pulsar. The more rapidly it pulsates, the more rapidly it is rotating.

What's the relationship between pulsar and supernova remnant?

At the location of every pulsar there is a supernova remnant. However, at the location of every supernova remnant there is not a pulsar. One possible reason for this is if a neutron star is not formed either in the case of a Type Ia supernova or perhaps a black hole is formed instead. Another reason could be that a pulsar did indeed form but its orientation is such that its beams of synchrotron radiation are never pointed towards the Earth. For example, if you were hovering above a lighthouse in a helicopter at night you would not see any flashes of light.

What are pulsars?

Bell and her colleagues noticed regular pulses of radio waves coming from particular regions of the sky. These objects became known as pulsars.

What are glitches in the pulsars?

Even though pulsars are gradually slowing down, occasionally they are observed to suddenly speed up. These glitches as they are called are due to the star shrinking slightly as it rearranges its shape into a lower energy configuration. The slight decrease in size causes a corresponding increase in spin rate due to the conservation of angular momentum.

How does a pulsar 'die'?

Eventually the pulsar is spinning so slowly that it no longer emits any pulses and 'dies' by becoming invisible. However, the dark core of neutrons remains.

Neutron stars have no magnetic field since they are made entirely of neutral neutrons. T/F

False

What did Jocelyn Bell do?

In 1967, a young graduate student named Jocelyn Bell at the University of Cambridge in the UK helped construct a crude radio telescope made of an array of simple antennas covering four and half acres of English countryside. The goal was to survey the sky at radio wavelengths.

What's a Neutron Star's Mass and Radius like?

Like white dwarfs, neutrons stars hold themselves up with degeneracy pressure which does not depend on mass so they shrink in size as they become more massive.

What's a Neutron Star's rotation like?

Neutron stars are expected to be rotating very rapidly due to the conservation of angular momentum. Some neutron stars are spinning hundreds of times per second.

How high is a Neutron star's mass?

Neutron stars can have masses up to a few times that of the Sun and yet are packed into a sphere the size of a city.

What's a Neutron Star's density like? What's the result?

Neutron stars have extremely high densities, about a billion times higher than the density of a white dwarf. As a result, a tiny speck of neutron star material that would fit on the head of a pin would weigh as much as two battleships under the Earth's gravity. A thimbleful would weigh as much as a mountain!

What's a Neutron Star's surface like?

Neutron stars have solid, rigid crusts made most likely of iron. The gravities at their surfaces are so strong that if you were stand on one, the gravity would flatten you much thinner than a piece of paper! The interiors may consist of a superconducting "fluid" of neutrons which can move around without resistance.

How do Pulsars spin? Why?

Pulsars gradually spin slower and lose energy as they age. This is due to the friction between its magnetic field and the particles surrounding it. This also reduces the energy of the radiation emitted - older pulsars are less energetic and emit longer wavelengths than younger pulsars

Which are longer wavelengths? Pulse or radio?

Pulsars which are relatively young are also observed to pulse at shorter wavelengths than radio.

Pulsars are

Pulsating Neutron Stars

What range in the electromagnetic spectrum was being observed when the first pulsars were discovered?

Radio

What's the Neutron Star Hypothesis?

Stellar evolution models predict that rapidly spinning neutron stars should be found at the centers of supernova remnants so another reasonable hypothesis is that pulsars might be related to these.

When was the possibility that Neutron Stars could form in a supernova explosion first predicted theoretically?

The possibility that neutron stars could form in a supernova explosion was first predicted theoretically in the 1930's. However, very few astronomers believed that such bizarre objects could actually form and be detected.

What's the pulsar at the center of the crab nebula like?

The pulsar at the center of the crab nebula formed from the supernova explosion witnessed in 1054. It is a young pulsar rotating about 30 times per second. The crab pulsar is so young and energetic that not only is it is pulsing at radio wavelengths but also at visible wavelengths as well. The crab pulsar is also observed to pulse at X-ray wavelengths.

What's the Pulsar Model?

The pulsar model invokes a rapidly spinning neutron star in which the magnetic and rotational axes are not aligned. Surrounding the pulsar is its very powerful magnetic field. Charged particles are accelerated away from the magnetic poles at close to the speed of light producing two beams of electromagnetic radiation.

Were the pulses natural? What was the early hypothesis? How could it be tested?

The pulses were so regular that is was difficult to believe that they were natural. One early hypothesis was that pulsars were artificial signals from extraterrestrial civilizations! So how could signals from little green men and women from space be distinguished from natural sources? The extraterrestrial hypothesis is quite easy to test. If pulsars are indeed artificial signals from extraterrestrial civilizations then we would expect to find sun-like stars at the locations of pulsars. However, at the location of every single pulsar astronomers found not a solar-type star but a supernova remnant. Clearly, no aliens would be sending signals from the aftermath of a supernova explosion so the extraterrestrial hypothesis cannot be correct.

How are Neutron Stars charged?

The surfaces of neutron stars are charged due to the decay of neutrons into protons and electrons. The interiors may contain charged particles as well. The motion of these charged particles due to the rapid rotation generates very strong magnetic fields about a million times stronger than the strongest magnetic fields generated in labs on Earth.

What are the pulses hot spots like?

To explain visible and X-ray pulses from pulsars we have to invoke the presences of hotspots on the surface. The magnetic field tends to channel gas into a small area near the magnetic poles. This heats the gas creating a hot spot. As the star spins, the hot spot moves into and out of view producing the observed visible and X-ray pulses.

The pressure within a neutron star that opposes the inward force of gravity comes from

degenerate neutron pressure

The diameter of a typical neutron star of 1 solar mass is predicted to be approximately

that of an average city, about 30 km.