Astronomy

Astronomy is the science of everything beyond Earth: the Sun and the planets, the stars and galaxies, the black holes that swallow light, and the distant edges of the universe itself. It is the oldest science in the world, but it is also one of the fastest-changing, because every year we build bigger telescopes, send out new spacecraft and discover things nobody has ever seen before.

• Age of the universe13.8 billion yearsWorked out from the cosmic microwave background
• Galaxies in the universeapprox. 2 trillionBest estimate from the Hubble and James Webb telescopes
• Stars in the Milky Wayapprox. 100 to 400 billionOur galaxy alone
• Planets in our solar system8Plus 5 dwarf planets like Pluto
• Speed of light299,792,458 m/sThe fastest anything can travel
• Distance to the nearest star4.24 light yearsProxima Centauri, just past Alpha Centauri

What does an astronomer do?

An astronomer is a scientist who studies things in space. They might point a telescope at a planet and watch it for years to learn how its weather works. They might catch the faint light from a galaxy so far away that it has taken billions of years to reach us. They might write computer programs that simulate the birth of stars, or build robots that drive across other planets. Some astronomers work at huge observatories on top of mountains; others fly telescopes on aeroplanes or launch them into space.

Astronomers do not actually go to space. The people who do that are called astronauts (or, in Russia, cosmonauts). Astronomers study space from here on Earth (or from spacecraft we send out) and figure out the universe by carefully looking at the light, radio waves and particles that come from it.

A quick tour of the cosmos

Start small. You and everything you can see live on Earth, the third planet from the Sun. Earth and the other 7 planets, plus dozens of moons and millions of asteroids, all orbit our local star: the Sun. That whole family is the solar system.

Now zoom out. The Sun is one of hundreds of billions of stars in our home galaxy, the Milky Way. The Milky Way is one of about 2 trillion galaxies in the observable universe. And the observable universe is just the part we can see. Beyond it, the rest of the universe stretches on, and may even be infinite.

Fact The light from the Sun takes 8 minutes 20 seconds to reach Earth. So when you look at the Sun (never directly!) you are seeing it as it was over 8 minutes ago. Look at the next nearest star and the light is over 4 years old. Look at the most distant galaxies and the light has been travelling for over 13 billion years, almost as long as the universe has existed.

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