First look
The idea, in plain words
When a very big star runs out of fuel, its middle caves in. A core heavier than the Sun is squeezed into a ball about 20 km across, the size of a city. The rest of the star is blown away in a huge explosion called a supernova.
What is left is a neutron star. It is the densest object known. A piece the size of a sugar cube would weigh about a billion tonnes, as much as a mountain.
Many neutron stars spin, and fast. They also send out two narrow beams of light and radio waves. As the star turns, the beams sweep round like the lamp of a lighthouse. If one of them crosses the Earth, we see a flash on every turn. A neutron star seen this way is called a pulsar.
The best known is in the Crab Nebula. Astronomers in China saw its star explode in the year 1054. The neutron star left behind still flashes 30 times every second.
If the crushed core is heavier still, nothing can hold it up, and it becomes a black hole instead.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Neutron star
- The crushed core left by an exploded star: more than the Sun's mass in a ball about 20 km across.
- Supernova
- The explosion of a star. For a few weeks it can outshine a whole galaxy.
- Density
- Mass divided by volume: how tightly a thing is packed.
- Magnetic field
- The region round a magnet, or round moving electric charge, in which other magnets and charges feel a force.
- Black hole
- A place where so much mass is packed so tightly that nothing which goes in, not even light, can come out.
- White dwarf
- The hot, dense core left behind when a star like the Sun dies: about the size of the Earth, and no longer making energy.
- Gravitational wave
- A ripple in space itself, sent out when very heavy things move violently.
Where this comes from
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.