Light is the only messenger

Nobody has touched a star. Apart from a few rocks and some dust brought back from very nearby, everything we know about space came to us as light, and had to be read from it.

Light is fast, but space is big enough that even light takes its time.

• From the Moon: a little over 1 second
• From the Sun: 8 minutes
• From the nearest other star: more than 4 years
• From the Andromeda Galaxy: about 2.5 million years

So you never see the sky as it is. You see the Sun as it was 8 minutes ago. On a dark night you can see the Andromeda Galaxy as a faint smudge, and that light set out before there were any people.

Looking further away always means looking further back. It is the nearest thing to a time machine that anyone will ever have.

Light crosses empty space at 299,792 km every second, and nothing that carries news goes faster. Because that speed is fixed, a distance and a delay are the same thing: the Sun is 8.3 light minutes away, and what we see of it is 8.3 minutes old.

A beam of light carries far more than a picture. Spread it out by wavelength into a spectrum and four separate things can be read from it.

How hot. A glowing body shines brightest at a wavelength set by its temperature. Cool stars at about 3,000 K look red. The Sun, at 5,772 K, peaks in green and looks white. The hottest stars, above 30,000 K, look blue.

What it is made of. Each kind of atom takes in and gives out light at its own exact wavelengths, which show up as narrow lines across the spectrum. The pattern is as particular as a barcode. Helium was found as a set of unexplained lines in sunlight in 1868, nearly thirty years before it was found on Earth.

How it is moving. If the source is coming towards us every line shifts a little towards blue, and if it is going away, towards red. The size of the shift gives the speed. A star being tugged back and forth by an unseen planet gives itself away like this.

How far and how powerful. Light spreads out as it travels, so the same star twice as far away looks a quarter as bright. Know either the distance or the true power and you can work out the other.

More than the eye sees. Visible light is one narrow band of a far wider range, running from radio waves through infrared, visible and ultraviolet to X-rays and gamma rays. Different things shine in different bands: cold gas in radio, dust and young stars in infrared, gas at millions of degrees in X-rays. The Earth’s air blocks most of them, which is the main reason telescopes are put in space.

Light is no longer quite the only messenger. Ripples in space itself, called gravitational waves, were first picked up in 2015, and ghostly particles called neutrinos reach us from the Sun’s core. But they are whispers beside the flood of light.