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Black holes and dead stars

What is left when a star dies, from an ember the size of the Earth to a place light cannot leave, and how we came to photograph one.

12 lessons in 3 parts. Every one can be read again at a deeper level.

A photograph. The black hole in the galaxy M87, published on 10 April 2019. A photograph, not a drawing. Credit: EHT Collaboration, CC BY 4.0.

Part 1

What you need first

Gravity, and why a star cannot last for ever.

1
What gravity is
Everything pulls on everything else. The pull grows with mass and fades with distance, and it never quite reaches zero.
With moving pictures and photographs
2
How stars live and die
Stars form in clouds of gas, shine steadily for most of their lives, and end as white dwarfs or in explosions, depending on their mass.
With moving pictures and photographs

Part 2

The explosion, and the three things a star can leave behind.

3
Supernovae
A supernova is a star blowing up. There are two ways it can happen, and for a few weeks one can outshine a whole galaxy.
With moving pictures and photographs
4
White dwarfs
A white dwarf is the core a star like the Sun leaves behind: the mass of a star in a ball the size of the Earth, with no fuel left, slowly cooling.
5
Neutron stars and pulsars
When a massive star dies, its core can be crushed into a ball the size of a city with more mass than the Sun. Some sweep beams past us like lighthouses.
With moving pictures and photographs
6
Magnetars and gamma-ray bursts
The strongest magnets and the brightest explosions there are both come from dying stars. One burst can give out more energy in seconds than the Sun will in its whole life.
With moving pictures and photographs
7
What a black hole is
A black hole is what is left when gravity wins completely. We cannot see one, but we can see what it does to everything near it.
With moving pictures and photographs

Part 3

The edge itself, the ripples when two collide, and the giants in the middles of galaxies.

8
The event horizon
The edge of a black hole is not a surface but a point of no return. What happens there depends on who is watching.
With moving pictures and photographs
9
Falling into a black hole
Nobody can go and look, but Einstein's equations say what a fall into a black hole would be like. In 2024 NASA ran them on a supercomputer.
With moving pictures and photographs
10
Do black holes last for ever?
Nothing gets out of a black hole, and yet theory says each one glows very faintly and will slowly shrink away to nothing. Nobody has seen it happen.
With moving pictures and photographs
11
Gravitational waves
When very heavy things swing round each other, space itself ripples. Since 2015 those ripples can be caught, a way of studying the universe that uses no light.
12
Active galaxies and quasars
In about one galaxy in ten, the centre outshines all the stars put together. The light comes from gas heating up as it falls towards a giant black hole.

Fly out to the Milky Way
Our own giant black hole sits at its centre, 27,000 light years from here.