A neutron star is the ultra-dense leftover core of a massive star after it explodes as a supernova. It can pack more mass than the Sun into a sphere only about the size of a city. Neutron stars matter because they show what matter becomes under some of the strongest gravity, pressure, and magnetic fields in the universe.
They also help astronomers test ideas about relativity, nuclear matter, and stellar life cycles.
Inside a neutron star, gravity is so intense that protons and electrons are squeezed together, forming mostly neutrons. A typical neutron star has a thin crust, a dense neutron-rich interior, and possibly an exotic core where matter behaves in ways not found on Earth. Many neutron stars spin rapidly and produce beams of radiation along their magnetic axes, which can appear as pulses when the beams sweep past Earth.
These objects are observed as pulsars, magnetars, X-ray sources, and merger remnants that can create gravitational waves.
Key Facts
- Typical neutron star mass: about 1.4 to 2.3 solar masses.
- Typical neutron star radius: about 10 to 15 km.
- Density can exceed nuclear density: about 3 x 10^17 kg/m^3.
- Surface gravity is enormous: g = GM/R^2.
- Escape speed can be more than half the speed of light: vesc = sqrt(2GM/R).
- Pulsar rotation periods range from milliseconds to several seconds.
Vocabulary
- Neutron star
- A neutron star is the collapsed core of a massive star made mostly of neutrons and packed into an extremely small volume.
- Supernova
- A supernova is a powerful stellar explosion that can occur when a massive star's core collapses at the end of its life.
- Pulsar
- A pulsar is a rotating neutron star whose radiation beams sweep across Earth like a lighthouse signal.
- Magnetar
- A magnetar is a neutron star with an extremely strong magnetic field that can produce intense X-ray and gamma-ray bursts.
- Degenerate matter
- Degenerate matter is matter supported by quantum pressure when particles are squeezed so tightly that ordinary gas behavior no longer applies.
Common Mistakes to Avoid
- Calling a neutron star a small normal star is wrong because it is a collapsed stellar core, not a star producing energy by ordinary hydrogen fusion.
- Assuming neutron stars are made only of neutrons is wrong because they also have a crust, charged particles, magnetic fields, and possibly exotic matter in the core.
- Confusing pulsars with all neutron stars is wrong because a pulsar is a neutron star whose radiation beam happens to be detectable from Earth.
- Using everyday density intuition is wrong because neutron star matter is far denser than ordinary solids, so a tiny amount could have the mass of a mountain.
Practice Questions
- 1 A neutron star has a mass of 1.4 solar masses. If 1 solar mass is 2.0 x 10^30 kg, what is the neutron star's mass in kilograms?
- 2 Estimate the average density of a neutron star with mass 2.8 x 10^30 kg and radius 12 km. Use density = mass/volume and volume = 4/3 pi r^3.
- 3 A pulsar sends radio pulses to Earth every 0.05 s. Explain what this period tells astronomers about the neutron star's rotation and why not every neutron star is observed as a pulsar.