Astrophysics for People in a Hurry

Overview

Neil deGrasse Tyson explains the main ideas of modern astrophysics in twelve short chapters, most of them adapted from essays he wrote for Natural History magazine. He begins with the first moments after the big bang and goes on to galaxies, dark matter, dark energy, the elements and the planets. There is almost no mathematics. The book is brief and moves fast, so some topics get only a few pages.

In brief

How the universe began

Tyson opens nearly fourteen billion years ago, when all the matter, energy and space we know of was packed into a volume far smaller than the dot at the end of this sentence. It was unimaginably hot. As it expanded it cooled, and the cooling allowed structure to appear.

In the first fraction of a second the forces of nature, which had been unified, separated from one another. Tiny particles called quarks joined to form protons and neutrons. There was slightly more matter than antimatter, and everything that exists today is made from that small surplus. Within a few minutes the simplest atomic nuclei had formed, mostly hydrogen and helium.

He is frank about the limits. Physics can describe the universe from a tiny fraction of a second after the beginning, but nobody knows what came before. Tyson leaves this as an open problem for science.

The same laws everywhere

One of the quieter ideas in the book is that the laws of physics worked out on Earth apply across the whole universe. For most of history people assumed the heavens followed their own rules. Newton changed that by showing that the force pulling an apple to the ground also holds the Moon in orbit.

Later, scientists learned to split light into its colours and read the pattern like a fingerprint. Each chemical element leaves its own marks. When astronomers studied sunlight this way, they found the same elements that exist on Earth. Helium was detected in the Sun before it was found here, and its name comes from the Greek word for the Sun.

This sameness is what makes astronomy possible. We cannot visit a distant galaxy, but we can trust that hydrogen there behaves like hydrogen in a laboratory.

The oldest light

For about 380,000 years after the beginning, the universe was a hot fog. Light could not travel far before hitting a loose electron. Once things cooled enough for electrons to settle into atoms, light was set free, and it has been travelling ever since. Stretched by the expansion of space, it now reaches us as faint microwaves coming from every direction. This is the cosmic microwave background.

Its discovery was an accident. In the 1960s two researchers at Bell Laboratories, Arno Penzias and Robert Wilson, could not remove a steady hiss from their radio antenna. They checked everything. The noise stayed. It turned out to be the glow that theorists had predicted, and it earned them a Nobel Prize.

Tyson uses the story to show that the big bang is supported by evidence you can measure.

What we cannot see

The most humbling chapters deal with dark matter and dark energy. In the 1930s the astronomer Fritz Zwicky measured galaxies in a large cluster and found them moving too fast. The visible matter did not have enough gravity to hold the cluster together. Decades later Vera Rubin found a similar problem inside single galaxies, where the outer stars orbit faster than they should. Something invisible is supplying extra gravity. It gives off no light, and nobody knows what it is.

Dark energy is stranger. Einstein once added a term to his equations to keep the universe from collapsing, then dropped it with regret. In 1998 two teams studying distant exploding stars found that the expansion of the universe is speeding up, and a version of his term came back. By the estimates Tyson gives, ordinary matter makes up only about five percent of the universe.

Where our atoms come from

The big bang produced hydrogen, helium and a trace of lithium. Heavier elements were made later, inside stars. Deep in a star, pressure and heat fuse light nuclei into heavier ones, building carbon, oxygen and others. When large stars die they explode, and the explosion scatters these elements into space, where they mix into the gas clouds that form new stars and planets.

So the calcium in your bones and the iron in your blood were once inside a star. Tyson walks through the periodic table with this in mind, linking elements to their cosmic history.

He also explains why large objects in space are round. Gravity pulls equally toward the centre, and once a body is massive enough, it crushes itself into a ball. Spinning flattens things, which is why galaxies and the solar system are shaped like discs.

The cosmic perspective

The final chapter is the most personal. Tyson argues that knowing the scale of the universe should change how we behave. Earth is a small planet around an ordinary star in a galaxy of billions of stars, among billions of galaxies. He does not find this depressing. We are made of the same material as the stars, and we are a part of the universe that has learned to study itself.

From that view, he says, many human quarrels look small.

An earlier chapter imagines how Earth would look to distant observers. They might detect signs of life from the gases in our air, which is the method our own astronomers use on planets around other stars.

Try this week

1. Go outside on a clear night this week, away from street lights if you can, and find one planet using a free sky map app.
2. Read one chapter a day, since each is short and stands alone.
3. Explain to a friend or younger sibling, in your own words, why dark matter is thought to exist.
4. Pick one topic from the book that confused you and watch a university lecture on it.

"The universe is under no obligation to make sense to you."

— Neil deGrasse Tyson, Astrophysics for People in a Hurry

If you liked this

A Brief History of Time by Hawking for the heavier version. The Order of Time by Carlo Rovelli for a more philosophical take.