In 1835 the French philosopher Auguste Comte offered an example of knowledge humans could never have: we would never learn what the stars are made of. We cannot visit them, scoop up a sample or run a test. Within about 25 years he was proven wrong. The key was light. Almost everything we know about the universe beyond our solar system comes from light and its invisible relatives, and learning to read it was one of the great leaps in the history of science.

The speed of light and looking back in time

Light is fast, but not infinitely fast. In 1676 the Danish astronomer Ole Rømer noticed that eclipses of Jupiter's moon Io came earlier than predicted when Earth was near Jupiter and later when it was far away. He realized that light needs time to cross the extra distance. Today we know light travels about 300,000 kilometers per second. Even so, space is so large that astronomers measure it in light-years, the distance light travels in a year, about 9.5 trillion kilometers.

This has a strange and wonderful consequence: to look out into space is to look back in time. You see the Moon as it was just over a second ago, the Sun as it was about eight minutes ago, and the nearest star beyond the Sun, Proxima Centauri, as it was about four years ago. The faint smudge of the Andromeda Galaxy that you can see from a dark site left its source about 2.5 million years ago, when our ancestors were first making stone tools.

Splitting light into a rainbow

Around 1666 Isaac Newton passed sunlight through a glass prism and showed that white light is a mixture of colors, a spectrum. In 1814 the German lens maker Joseph von Fraunhofer looked at the solar spectrum through a finer instrument and found it crossed by hundreds of dark lines, as though some colors were missing. He mapped them carefully but did not know what they meant.

The answer came around 1860 from Gustav Kirchhoff and Robert Bunsen in Heidelberg. Heating different chemical elements in a flame, they found that each one gives off light only at its own particular set of colors, a pattern as unique as a fingerprint. Cool gas absorbs exactly those same colors, which is what produces dark lines. Comparing the Sun's dark lines with their laboratory flames, they found sodium, iron and other familiar elements in the Sun. The science of spectroscopy had been born, and Comte's impossible question had an answer.

In 1868, observers studying a solar eclipse found a yellow line that matched no known element. The English astronomer Norman Lockyer named the mystery element helium, after the Greek word for the Sun. It was not found on Earth until 1895. An element was discovered in the Sun before it was discovered at home.

Light that tells us motion

Spectra reveal motion as well as chemistry. You have heard the Doppler effect when an ambulance siren drops in pitch as it passes: sound waves bunch up ahead of a moving source and stretch out behind it. Light behaves the same way. An object moving toward us has its spectral lines shifted toward the blue end of the spectrum, a blueshift, and an object moving away has them shifted toward the red, a redshift. By measuring how far the familiar fingerprint lines have moved, astronomers can tell how fast a star or galaxy is moving toward or away from us. This simple tool would later reveal the expansion of the universe.

The invisible universe

Visible light is only a thin slice of the full range of light, the electromagnetic spectrum. Beyond red lies infrared and then radio; beyond violet lie ultraviolet, X-rays and gamma rays. Different objects shine in different bands. Cool dust glows in infrared, the hottest gas in X-rays, and cold hydrogen gas gives off radio waves. Karl Jansky detected radio waves from the center of our galaxy in the early 1930s, opening a new window on the sky. Because our atmosphere blocks most X-rays, ultraviolet and much of the infrared, many telescopes must work in space. The Hubble Space Telescope, launched in 1990, and the infrared James Webb Space Telescope, launched on Christmas Day 2021, have shown us views no ground telescope could.

Worked example: how old is the light you see?

Divide distance by the speed of light and you get the light's age. Try a few objects.

• The Sun: about 150 million kilometers away. 150,000,000 ÷ 300,000 = 500 seconds, a little over eight minutes. If the Sun vanished, you would not know for eight minutes.
• Proxima Centauri: about 4.2 light-years away, so its light is about 4.2 years old.
• The Andromeda Galaxy: about 2.5 million light-years away, so its light set off long before modern humans existed.

Every telescope is a time machine. When astronomers study very distant galaxies, they are not guessing about the early universe. They are watching it, in light that has been traveling for billions of years.