1. Extremely High Density and Small Size:
* Observed Mass and Radius: Pulsars have extremely high densities, comparable to atomic nuclei. This is inferred from their measured masses (typically 1.4 solar masses) and the fact that they are incredibly compact, with radii estimated to be only about 10-20 kilometers.
* Theoretical Model: Neutron stars are predicted by theoretical models of stellar evolution. When massive stars exhaust their nuclear fuel, they undergo a supernova explosion. The core, collapsing under its own gravity, reaches incredibly high pressures and densities, forcing protons and electrons to combine and form neutrons. This creates a super-dense object, consistent with what we observe in pulsars.
2. Rapid, Regular Pulsations:
* Precise Timing: Pulsars emit extremely regular pulses of electromagnetic radiation (radio waves, X-rays, etc.) with periods ranging from milliseconds to seconds. This precise timing is a defining characteristic of pulsars.
* Rotating Neutron Star Model: The most accepted explanation for these pulses is that the neutron star is rapidly rotating, emitting radiation from its magnetic poles. As the star spins, these beams sweep across space, like a lighthouse beam, causing the observed pulsations.
3. Strong Magnetic Fields:
* Polarized Radiation: The radiation from pulsars is highly polarized, indicating the presence of extremely strong magnetic fields.
* Synchrotron Radiation: The observed radio emission is likely caused by synchrotron radiation, a process that occurs when charged particles spiral around magnetic field lines. The strength of the magnetic field needed to produce synchrotron emission at the observed frequencies is consistent with the theoretical magnetic fields of neutron stars.
4. Observed Properties Consistent with Neutron Star Models:
* Cooling Rates: The observed cooling rates of pulsars match theoretical predictions for neutron stars. The initial high temperatures of the newly formed neutron star gradually decrease over time, as heat is radiated away.
* Glitches: Pulsars occasionally exhibit sudden, brief changes in their rotation rate, known as glitches. These glitches are consistent with the idea that the neutron star's superfluid interior interacts with its solid crust, causing these disruptions.
5. Direct Observation of Neutron Star in a Pulsar:
* Crab Nebula Pulsar: The pulsar in the Crab Nebula, a supernova remnant, has been directly observed. Its properties, including its mass, radius, and magnetic field strength, are consistent with predictions for neutron stars.
Conclusion:
The combination of observational evidence, theoretical models, and the consistency of properties with neutron star predictions make a compelling case that pulsars are indeed neutron stars. While some details about their internal structure and magnetic field behavior are still being studied, the overwhelming evidence supports this conclusion.
