MEPX
Chapter 8 of 12All chapters
  1. 1The scale of things
  2. 2The Sun
  3. 3The rocky planets
  4. 4The giant planets
  5. 5Moons
  6. 6Asteroids and comets
  7. 7Gravity and orbits
  8. 8Light and distance
  9. 9Stars and their lives
  10. 10Galaxies
  11. 11The expanding universe
  12. 12Observing for yourself

Chapter 8 of 12

Light and distance

Looking out is looking back.

The light year

A light year is a distance, not a time: how far light travels in a year, about 9.5 trillion kilometres. Saying a star is 4 light years away also says its light left 4 years ago.

  • The nearest star system, Alpha Centauri, is about 4.2 light years off.
  • Every telescope is a time machine pointed at the past.

Beyond visible light

Radio, infrared, ultraviolet, X-ray and gamma telescopes see what the eye cannot. Infrared cuts through dust clouds, which is why the newest space telescopes work in it.