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Light, and how we know

Almost everything known about space was learnt from light alone. These lessons show how much a beam of it can tell you.

10 lessons in 4 parts. Every one can be read again at a deeper level.

Begin with lesson 1

A blurred orange disc with brighter and darker patches, on black.
A photograph. The face of another star: Antares, 550 light years away, built up from four telescopes working as one. Credit: ESO/K. Ohnaka, CC BY 4.0.

Part 1

What light carries

The only messenger we have from almost everywhere.

  1. 1
    Light is the only messenger

    Nearly everything known about space arrived as light, and light always brings old news.

  2. 2
    The whole spectrum: light we cannot see

    The colours we see are a thin slice of light. Radio waves, infrared, ultraviolet, X-rays and gamma rays are the same thing at other wavelengths.

Part 2

Reading it

Temperature, make-up and motion, all from colour.

  1. 3
    Colour and brightness

    A star's colour tells you how hot it is, and its true brightness tells you how big. Plot one against the other and the stars fall into order.

  2. 4
    Reading starlight

    Spread a star's light into a rainbow and it is crossed by dark lines. They tell you what the star is made of, how hot it is and how it is moving.

  3. 5
    Redshift and the Doppler effect

    A siren drops in pitch as it passes you. Light does the same thing, and that small shift is how the speed of a star and the growth of the universe are measured.

    With moving pictures and photographs

Part 3

Gathering it, and measuring with it

Telescopes, and the chain of methods that gives distances.

  1. 6
    Telescopes

    A telescope's first job is to collect light, not to magnify. Bigger mirrors see fainter and sharper, and that has driven four centuries of building.

  2. 7
    Telescopes in space

    The air blocks most kinds of light and blurs the rest. Lifting a telescope above it costs a fortune, and has changed what is known about almost everything.

    With moving pictures and photographs

  3. 8
    Measuring distance: kilometres, au, light years and parsecs

    Why astronomers keep four different rulers, and when each one is used.

  4. 9
    Variable stars and the distance ladder

    Some stars brighten and fade on a steady beat, and the beat gives away how bright they truly are. That is how the distances to other galaxies were first measured.

Part 4

Beyond light

Particles and ripples that bring news light cannot.

  1. 10
    Messengers other than light

    Three other things reach the Earth from deep space: neutrinos, cosmic rays and ripples in space itself. Each brings news that light cannot.

    With moving pictures and photographs

Go and look