# Distances

Distance to the planets is measured by bouncing radar signals off them
and timing how long the signals take to get there and back.

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Parallax Shift

• nearby stars distance is calculated by measuring the slight shift in angle of each star in comparison to stars far away, as the Earth orbits the Sun; this is called “Parallax Shift”.
• parallax shift, it 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.

Parallax Shift

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Hertzsprung-Russell (H-R) diagram

• middle distance stars Â – Â astronomers compare colour with brightness using the Hertzsprung-Russell (H-R) diagram.
• beyond 30,000 light-years, stars are too faint for main sequence fitting to work.

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Standard Candles

• distances to nearby galaxies can be estimated using “standard candles” – stars that astronomers know the brightness of, such as supergiants and supernovae.

Standard Candles
(For distances which are too large to measure using parallax, astronomers useÂ ‘standard candles’. Light sources which are further away appear fainter because the light is spread out over a greater area.)

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Tully-Fisher Technique

• expected brightness of a galaxy that is too far away to pick out its stars may be measured using the Tully-Fisher technique (wikipedia) – based on how fast galaxies spin.
• counting planetary nebulae (the rings of gas left behind by supernovae explosionsÂ  is another way of measuring how bright a distant galaxy should be.

Example of supernova explosion

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Distant galaxy

• third method of calculating the brightness of a distant galaxy is to gauge how mottled it looks by marked with spots of stars.

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