Neutron stars and pulsars
A star's core squeezed into the size of a city, spinning hundreds of times a second. See what that does to density, spin and magnetism, and why some of them tick like clocks.
WHAT IS LEFT
A star made of neutrons
Ordinary matter is mostly empty: a tiny nucleus with electrons far out around it. In the collapse of a massive star's core, gravity crushes that empty space out. Electrons are pressed into protons and the two become neutrons. What remains is not a ball of atoms at all. It is closer to one enormous atomic nucleus, about 20 km across, and it holds itself up because neutrons packed that tightly refuse to be pushed any closer.
There is a limit to that refusal, just as there was for a white dwarf. Above roughly two to two and a half solar masses the neutrons lose, and nothing known can stop the collapse. That case is the next lesson.
a neutron star = about 1.4 Suns of mass
inside a ball about 20 km across
now squeeze the whole Earth to the same density.
how big would the Earth be?a ball about 250 metres across: a five-minute walk around itA sugar cube of this material would weigh a few hundred million tonnes. It is not a substance you could bring home. It only holds together while the star's own gravity is crushing it; taken out, it would blow apart instantly.
Check yourself
Ramin wonders what would happen if you could stand on a neutron star. Which is the best answer, given what you know about gravity and mass?
- Its gravity would crush you: a whole star's mass sits 10 km away
- It would feel much like standing on the Moon, since the star is small
- You would float, because a neutron star has no atmosphere
- Nothing at all, since gravity needs a large object to work
Show the answer
Its gravity would crush you: a whole star's mass sits 10 km away
Right. Gravity grows with mass and with closeness. A whole star's mass at 10 km is an extreme of both. Nothing could stand there; you would be spread across the surface in a layer thinner than a sheet of paper.
The skater who pulls her arms in
A spinning skater pulls her arms to her chest and speeds up without pushing off anything. Bring the mass closer to the axis and the spin quickens. A star's core does the same thing, on a scale that sounds absurd: a core the size of the Earth, turning lazily once a month, collapses to 20 km and comes out turning many times a second. Where the analogy breaks: the skater slows down again from friction in seconds, while a neutron star in empty space keeps spinning for millions of years, losing speed only very gradually.
THE BEAM
A lighthouse, not a blinking star
The collapse concentrates the star's magnetic field as well as its spin, leaving something like a trillion times the Earth's. Charged particles are funnelled out along the magnetic poles as two narrow beams of radio waves. The magnetic poles are usually tilted away from the spin axis, so the beams swing around like the lamp of a lighthouse. If one of them happens to cross the Earth, we receive a pulse on every turn. That is a pulsar.
The star is shining steadily. Nothing about it blinks. The flashing exists only for an observer the beam sweeps over, which means most pulsars in the galaxy are invisible to us: their beams simply never point our way.
Check yourself
A pulsar is a neutron star whose brightness genuinely switches on and off many times a second.
Show the answer
False
False. The star shines steadily; it is the beam that moves, because the star is spinning and its magnetic poles are tilted. You see a pulse each time the beam crosses you, exactly as a lighthouse appears to flash to a ship while its lamp burns without interruption.
From a collapsing core to a tick on a chart
- The core falls in
An iron core roughly the size of the Earth collapses to about 20 km in under a second, and becomes neutrons.
- Spin and magnetism multiply
Both were spread over a huge volume and are now concentrated in a tiny one. A slow turn becomes many turns a second; an ordinary stellar magnetic field becomes something no laboratory can approach.
- Two beams leave the magnetic poles
Charged particles stream out along the field and radiate, mostly as radio waves, in two narrow cones.
- The beams sweep the sky
Because the magnetic axis is tilted from the spin axis, the cones trace circles on the sky as the star turns.
- A radio telescope sees a pulse train
If a cone crosses the Earth, the telescope records a sharp blip every rotation: steady, evenly spaced, and repeating to an accuracy that can beat a wristwatch by a very long way.
Check yourself
What does the collapse do to each property of the core?
- Density
- Diameter
- Rotation speed
- Strength of the magnetic field
- Time to complete one turn
- Surface gravity
Show the answer
Goes up enormously: Density, Rotation speed, Strength of the magnetic field, Surface gravity
Goes down enormously: Diameter, Time to complete one turn
Two kinds of dead star, side by side
White dwarf
The core of a star like the Sun. About the size of the Earth, up to about 1.4 solar masses. Held up by electrons refusing to be packed closer. A sugar cube of it weighs a tonne or so. It just cools, for billions of years.
Neutron star
The core of a star of more than about 8 Suns. About the size of a city, typically 1.4 to 2 solar masses. Held up by neutrons refusing to be packed closer. A sugar cube weighs hundreds of millions of tonnes. Often spinning many times a second, with a magnetic field beyond anything on Earth.
Check yourself
- Object A sends a radio blip every 0.089 seconds. The blips keep coming at that spacing week after week, accurate to a tiny fraction of a second.
- Object B brightens and fades over about 5 days, and the timing wanders by hours from one cycle to the next.
Which one is a spinning neutron star, and what gives it away?
- B, because 5 days is a more realistic period for a star
- A, because a pulse that regular can only come from something rigid and small spinning
- Both, since anything that repeats must be spinning
- Neither; repeating signals always come from equipment faults
Show the answer
A, because a pulse that regular can only come from something rigid and small spinning
Right. Spin is the most reliable clock nature has. A puffed-up star that swells and shrinks keeps sloppy time, because it is a loose ball of gas. A 20 km lump of packed neutrons does not wander.
Check yourself
Maryam reads that astronomers know of roughly three thousand pulsars, but expect the galaxy to hold hundreds of thousands of neutron stars. What explains the gap best?
- Most neutron stars are too far away for their gravity to reach us
- Most neutron stars stopped spinning long ago
- Only the youngest neutron stars exist; the rest have evaporated
- We only see the ones whose beam happens to sweep across the Earth
Show the answer
We only see the ones whose beam happens to sweep across the Earth
Right. The beams are narrow cones, and a cone pointing anywhere else in the sky is a pulsar nobody on Earth can detect. What we count is not how many exist but how many are aimed at us.
Lesson recap
- A neutron star is a collapsed stellar core, roughly 1.4 to 2 solar masses in a ball about 20 km across, closer to a single giant nucleus than to ordinary matter.
- Collapse concentrates spin and magnetism: a core turning once a month comes out turning many times a second, with a field around a trillion times the Earth's.
- A pulsar is a neutron star whose tilted magnetic beams sweep past us. The star shines steadily; only the beam moves.
- We see only the pulsars aimed our way, so the galaxy holds far more neutron stars than we can count.
- Their timing is so precise that a pulsar in orbit around another star measured energy lost to gravitational waves, decades before waves were caught directly.