PHYS 182 Lecture Notes - Lecture 16: Electromagnetic Spectrum, Escape Velocity, Cosmic Distance Ladder
PHYS182: Our Evolving Universe
2017-11-02 LEC 16
Neutron stars & pulsars
- We see white dwarfs because they start out very hot and very slowly radiate their heat, and
therefore appear on the H-R diagram
- But what about neutron stars?
o They were first discovered via pulsars
o If you compress something with angular momentum, the momentum is conserved, so the
neutron star will be spinning incredibly fast
o When material collapses to form a neutron star you expect huge magnetic fields at the
surface
- For the Earth, the spin axis is not exactly the same as the magnetic axis
o I.e. the north magnetic pole is much closer to us than the actual north pole
- So for neutron stars, we should expect that the magnetic axis is displaced from the spinning axis
- The magnetic field lines will beam the things that are ejected in the direction of the magnetic
axis
o I.e. out the top and the bottom
- We observe periodic flashes of light that we have identified as pulsars
o Pulsars were discovered before the concept of a neutron star was established
- Around the neutron star is the remnant of the outer shells
- All pulsars are neutron stars but not all neutron stars are pulsars
o If we don’t see the neutron stars as pulsars, it might just be because the light house pulses
are not radiated in our direction
Pulsar periods
- Time vs. intensity was plotted and you get periodic peaks in intensity
Pulsar binary
- Binary can be between a pulsar & neutron star, 2 neutron stars,
or 2 pulsars
- Orbiting mass = mass that is being accelerated
- If you have such a system you should expect that energy is
radiating as gravitational waves
- Period of rotation will decrease, the system will lose energy, the
two objects will approach each other
o The pulses should become closer together
à
period
decay
- This prediction confirms Einstein’s general relativity
X-ray Burst
- This is another feature associated with neutron stars
- There is a flux of material from the outer region toward the neutron star
Document Summary
We see white dwarfs because they start out very hot and very slowly radiate their heat, and therefore appear on the h-r diagram. But what about neutron stars: they were first discovered via pulsars. If you compress something with angular momentum, the momentum is conserved, so the neutron star will be spinning incredibly fast: when material collapses to form a neutron star you expect huge magnetic fields at the surface. For the earth, the spin axis is not exactly the same as the magnetic axis. I. e. the north magnetic pole is much closer to us than the actual north pole. So for neutron stars, we should expect that the magnetic axis is displaced from the spinning axis. The magnetic field lines will beam the things that are ejected in the direction of the magnetic axis. We observe periodic flashes of light that we have identified as pulsars: pulsars were discovered before the concept of a neutron star was established.
