Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: EvaH on 01/04/2021 13:59:27
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David wants to know:
How do you measure the distance of a star or galaxy, and in what units does this measurement take (apart from light years - one would presume it is converted to light years)?
Can you help?
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There are a number of methods used, depending the distance range being measured:
https://en.wikipedia.org/wiki/Cosmic_distance_ladder
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It is true that astronomers don't measure in light-years.
- It would take someone to go the far star or galaxy with a clock, and then measure the time for a pulse of light to get from the star back to Earth
- Interstellar travel is not possible with our current technologies.
- As Einstein showed, keeping two clocks synchronized when one party is traveling at high speed to another star is tricky (but you can calculate corrections that would adjust for the accelerations)
- At best, travelling at 25% of the speed of light, you could measure the distance to the nearest star in about 20 years
- But with more realistic technologies we can use today, it might take a human 30,000 years to get to the nearest star...
Fortunately, the parallax method can measure the distances to many stars by using the orbit of the Earth around the Sun. This does not require a hypothetical interstellar spaceship, and was originally done from the surface of the Earth.
- This measures the distance to stars in parsecs (which can then be converted to light-years)
- The Gaia space probe is measuring the distances and velocities of something like a billion objects in space
- The most accurate results will be for stars in our arm of the Milky Way galaxy
The Gaia team are hoping to get some measurements on the movement of the Andromeda galaxy
- Andromeda is known to be moving towards our galaxy, but it has been hard to determine how much "sideways" motion it has; this will determine whether the first pass is a "direct hit", or a "glancing blow".
- But it is expected that Andromeda and the Milky Way will eventually merge, after a number of passes.
See: https://en.wikipedia.org/wiki/Gaia_(spacecraft) (https://en.wikipedia.org/wiki/Gaia_(spacecraft))
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Two methods include (I) the brightness of Type 1a supernovae, which is a standard candle because these objects have a known brightness. You then can apply the inverse square law. I.e. if the object is twice as far then the brightness is reduced to a quarter; three times further means the brightness is then a ninth. What is advantageous here is these supernovae are very bright and can be observed from distant galaxies.
(Ii) Using the Doppler effect with Hubble's law equation allows distances of objects to be determined by measuring the redshift.
What is interesting is that for distant objects the two methods do not agree in distance, as the objects appear dimmer than they should be. This proved the Universe is expanding at an accelerating rate.