- The distance to the Moon is measured with a laser beam.
- The distance to the planets is measured by bouncing radar signals off them and timing how long the signals take to get there and back.
- The distance to nearby stars is worked out by measuring the slight shift in the angle of each star in comparison to stars far away, as the Earth orbits the Sun. This is called parallax shift.
- Parallax shift can only be used to measure nearby stars, so astronomers work out the distance to faraway stars and galaxies by comparing how bright they look with how bright they actually are.
- For middle distance stars, astronomers compare color with brightness using the Hertzsprung-Russell (H-R) diagram. This is called main sequence fitting.
- Beyond 30,000 light-years, stars are too faint for main sequence fitting to work.
- Distances to nearby galaxies can be estimated using ‘standard candles’ – stars whose brightness astronomers know, such as Cepheid variables (see variable stars), supergiants and supernovae.
- The expected brightness of a galaxy too far away to pick out its stars may be worked out using the Tully-Fisher technique, based on how fast galaxies spin.
- Counting planetary nebulae (the rings of gas left behind by supernova explosions) is another way of working out how bright a distant galaxy Distant should be.
- A third method of calculating the brightness of a distant galaxy is to gauge how mottled it looks.
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