• Neutron Stars
• Pulsars

• Neutron Star density ~ 3 x 10¹⁴ g/cm³
• Steel has a density of 7.7 g/cm³

The mass of a 1 cm cube of a Neutron Star is equivalent the mass in a 340 meter cube of steel!

• Neutron stars initially spin very rapidly.
• Conservation of angular momentum!
• mass x velocity x radius = constant
• Rotation period of Sun = 25 days
• Shrinking the Sun to 10 km would give
  a rotation period of much less than 1 second!

• Jocelyn Bell - 1967
• Graduate student at Cambridge, England
• Discovered a pulsating radio signal coming from the sky!!
• LGMs? (Little Green Men)
• The object is a pulsar (pulsating star).
• Antony Hewish (her advisor) won a Nobel Prize.

A short pulse is detected at regular intervals.

• Rotation Periods ~ 0.001 to 10 seconds.
• Pulsars cannot be normal stars!
• Even a white dwarf would fly apart!
• T. Gold (Cornell) and F. Pacini (Italy) made the connection between pulsars and neutron stars.
• Magnetic Field ~ 10¹² gauss.

• The intense magnetic field rips particles from the surface at the magnetic poles.
• As the particles are accelerated to relativistic speeds, the electrons emit synchotron radiation.
• The radiation is highly directional and photons are concentrated in narrow beam coming from the polar cap.
• The magnetic pole is not aligned with the rotation axis (like the earth).
• So the beam of radiation sweeps around the sky.
• We see a pulse when the beam points towards us.
• Pulsars slow down because they are radiating energy away!

• Pulsars are excellent clocks!
• Binary Pulsar (General Relativity test)
• Slow down of orbit confirms General Relativity.
• Won Nobel Prize for Taylor and Hulse
• Work done at Arecibo Observatory
• Pulsar in an Eclipsing Binary
• Planets around Pulsars!

• Pulsar period = 0.0016 seconds!
• Pulsar orbits companion every 9.7 hrs.
• Pulsar is eclipsed for 50 minutes each orbit.
• Masses: Pulsar ~ 1.4 M_sun
• Companion ~ 0.023 M_sun
• The pulsar may be evaporating the companion.
• In about a billion years it may be gone.

• Accurate timings of radio pulses from Pulsar PSR 1257+12 show evidence for small bodies orbiting it.
• This pulsar is ~300 pc (~980 lightyears) away.
• Three planet size objects have been detected!
  

• It is thought that a planet has been detected orbiting Pulsar PSR 1620-26, which is about 3.8 kpc (12400 lightyears) from us.

• Mass falling from companion on the NS emits x-rays
  • The falling material reaches such high speeds, when it hits the stars the temperature becomes very high
  • Hence, emission is in the x-ray part of the spectrum