Homework #11

Examine this graph of a pulsar's period of variation versus time, which is evidence of planetary companions. What is the orbital period of the suspected planet that is closest to the pulsar?

0.18 year

If neutron stars have hot surface temperatures, why aren't they very luminous?

Although neutron stars are hot, they are very small and have little surface area from which to radiate, so their luminosity is low.

How can mass transfer into a compact object produce jets of high-speed gas?

Although not completely understood, jets seemed to be produced by "twisting" magnetic fields, Soft gamma-ray repeaters are thought to come from magnetars, Both neutron stars and black holes produce jets of radiation, and A magnetar is a pulsar with a much stronger than usual magnetic field.

What is the source of the continuous X-rays emitted by a close binary system that contains a compact object?

An accretion disk around the compact object is heated by friction.

Why does the short length of pulsar pulses eliminate normal stars as possible pulsars?

An object cannot emit pulses that are shorter than the time it takes light to cross its diameter.

(a)How can mass transfer into a compact object produce jets of high-speed gas? (b)How can mass transfer into a compact object produce X-ray bursts? (c)How can mass transfer into a compact object produce gamma-ray bursts?

Gas in an accretion disk is accelerated to high speeds and can interact with the compact object's magnetic field creating powerful jets of excited gas. Gas that flows from the accretion disk of a neutron star down to the surface of the neutron star accumulates in a dense layer that becomes degenerate until it ignites helium fusion to produce a burst of X-rays. The bursts repeat every time a large enough layer of degenerate fuel accumulates. Mass transferred to a neutron star as the result of a merger between it and another neutron star or a black hole can cause a violent explosion resulting in a gamma-ray burst.

Why are pulsars so hot?

Gravitational energy was converted into thermal energy during formation.

In what sense is a black hole actually black?

It absorbs all light within the Schwarzschild radius and emits no light itself.

How can a neutron star not be a pulsar?

Its magnetic field may be too weak to generate beams of radiation. A pulsar may be too old and rotate too slowly to pulse. A pulsar's magnetic field may not sweep past Earth. Its magnetic field may be too weak to generate beams of radiation and a pulsar may be too old and rotate too slowly to pulse.

Why do you expect neutron stars to spin rapidly?

Neutron stars are formed by the collapse of massive stars. Since all stars rotate, the principle of conservation of angular momentum predicts that as a massive star collapses it must rotate faster to conserve angular momentum.

How did astronomers conclude that pulsars actually could not be pulsating stars?

Normal stars are much too large to pulse with a period as small as the period of a pulsar.

If the Sun has a Schwarzschild radius, why isn't it a black hole?

Not all of the Sun's mass is inside its Schwarzschild radius.

What evidence can you cite that pulsars are neutron stars?

Only neutron stars are small enough to produces such short pulse lengths.

Why are all pulsars not located in supernova remnants? (Choose the best answer.)

Pulsars persist longer than supernova remnants and some pulsars given high velocities upon formation can flee the scene of destruction.

White dwarfs and neutron stars are both end products of stellar evolution. White dwarfs are composed of mostly carbon, oxygen, and electrons, whereas neutron stars are composed of mostly neutrons. What happens to the protons in the atomic nuclei and the degenerate electrons that were inside the star that creates a neutron star?

The electrons and protons combine to form neutrons and neutrinos.

Shown here is an artist's rendering of the suspected black hole and its O9 supergiant companion in the Cygnus X-1 binary system. If mass transfer is a relatively recent occurrence and the binary pair was originally two main-sequence stars, what would we suspect is true about the original mass of the star that has become the black hole?

The original mass was greater than that of the O9 supergiant.

Why would astronomers naturally assume that the first discovered millisecond pulsar was relatively young?

When a pulsar first forms, it is spinning fast, and its rotation begins to slow as it radiates energy into space.

Why do you expect neutron stars to have a powerful magnetic field?

When a star collapses into a neutron star, its magnetic field is squeezed into a small volume, which can make the field as much as a billion times stronger.

How can a black hole emit X-rays?

X-rays emitted just before material crosses the event horizon can escape the gravitational pull

What is the lower limit for the mass of neutron stars?

about 1.4 solar masses

What is the origin of the X-ray bursters?

matter accreting around a binary system in which there is a neutron star