- How small is one, really?
- Twenty to twenty-five kilometres across, carrying more mass than the Sun — the best modern measurements, from X-ray timing of the hot spots on a few of them, put the radius near twelve kilometres. That is the density inside an atomic nucleus, scaled up to the size of a city: the star has been squeezed until its protons and electrons merged into neutrons and the neutrons are packed against each other.
- Why do they pulse?
- Because the magnetic axis is tilted away from the spin axis, so the beam of radio waves channelled along it sweeps the sky like a lighthouse. A pulsar is simply a neutron star whose beam crosses the Earth; only a minority of beams do, so most neutron stars point elsewhere and have to be found some other way.
- How fast do they turn?
- From over a minute per rotation for the slowest known down to under a millisecond and a half — the quickest known turns 716 times a second. At that rate a point on the equator is moving at a substantial fraction of the speed of light, and the rotation is steady enough to keep time against an atomic clock.
- Where do they come from?
- From the collapse of a star of roughly eight to twenty times the Sun's mass. The core falls inward in about a second, rebounds off its own stiffness, and drives the rest of the star off as a supernova. What is left behind is the core, and it is the neutron star.
- What holds one up?
- The neutrons themselves. Quantum mechanics forbids two of them from occupying the same state, and that refusal — stiffened by the nuclear force, which resists compression at close quarters — is what holds up the star's own weight; no fuel is being burned, and nothing is generating heat to push outward. It has a limit: the heaviest neutron star yet weighed carries about 2.35 times the Sun's mass, and not far above that the collapse continues to a black hole.
- What would a teaspoon of neutron star weigh?
- Roughly a billion tonnes — comparable to a small mountain. The number is a way of stating the density rather than a thing anybody could do: material at that density is held together by the star's own gravity, and a spoonful removed from it would expand violently rather than sit on a table.
- Can pulsars be used for navigation?
- In principle, and it has been demonstrated from orbit. A handful of millisecond pulsars keep time comparably to atomic clocks, and a spacecraft that measures when their pulses arrive can work out where it is in the same way a receiver on Earth uses satellite signals. It works anywhere in the Solar System, which is the appeal, and it needs an X-ray telescope, which is the cost.
- What is a magnetar?
- A neutron star with a magnetic field a thousand times stronger than the ordinary kind — strong enough that the field, rather than the rotation, powers what it emits. They flare in X-rays and gamma rays, they slow down quickly, and one of them briefly affected the Earth's upper atmosphere from about thirty thousand light-years away in 2004.
- Is there a neutron star close enough to be a danger?
- No. The nearest known are hundreds of light-years off, which is far enough that neither their gravity nor their radiation reaches here in any measurable amount. What arrives from the closest of them is a radio signal faint enough to need a large dish to detect at all.