First look
The idea, in plain words
Throw a ball up and it falls back. Throw it faster and it goes higher. Throw it at 11 km every second and it never comes back at all: it has escaped the Earth.
Now imagine squeezing the Earth smaller while keeping all of its mass. The pull at the surface grows, and the speed you need to escape grows with it. Squeeze hard enough and the speed you need becomes the speed of light.
Nothing can travel faster than light. So from an object like that, nothing can leave. No light comes out, which is why it is black, and anything that falls in stays in, which is why it is a hole.
Black holes are real. Some are made when very heavy stars die. One with the mass of 4 million Suns sits at the centre of our own galaxy.
In 2019 the world saw one for the first time. This is a real photograph of the black hole in a galaxy called M87, which is 55 million light years away. The glowing ring is hot gas whirling round it. The dark middle is the black hole’s shadow. It has the mass of 6.5 billion Suns.
They do not suck things in from far away. If the Sun were swapped for a black hole of the same mass, the Earth would go cold and dark, but it would carry on round its orbit exactly as before.
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
- Black hole
- A place where so much mass is packed so tightly that nothing which goes in, not even light, can come out.
- Escape velocity
- The speed needed to leave a body for good without any further push.
- 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.
- Accretion disc
- A flat, spinning disc of gas spiralling in towards a star or a black hole.
- Gravitational wave
- A ripple in space itself, sent out when very heavy things move violently.
- Event horizon
- The edge of a black hole: the boundary from inside which nothing can get out.
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.