How do you measure distances to astronomical objects?

Answer 1

For distant objects, astronomers use the units of "light years" and "parsecs".

Several different methods are used to measure the distances.

There is a series of yardsticks each one calibrated using the previous one.

Here are some of the main methods used:

The first measures the distance to Venus by using parallax on a transit. Venus is observed crossing the Sun simultaneously from two different places on Earth a known distance apart, and triangulation gives the distance. From that the diameter of the Earth's orbit is calculated exactly using Kepler's laws and the orbit model. Nowadays we can also measure the distance to Venus using radar signals.

Next is the parallax of nearby stars, which is done by measuring the change in position of a star against the background of distant stars as the Earth moves round the Sun. Bessel did this first in the 19th century using the nearby star 61 Cygni. It's a method that extends to about 1000 light years. That was when we discovered how unimaginably far away the stars are, and it confirmed the heliocentric theory that the Earth orbits the Sun.

Within 1000 light years there are several Cepheid variable stars like Delta Cephei that give away their distance because the period of variation depends on their absolute magnitude in a relationship discovered by Henrietta Swan Leavitt. This method can then be used to measure the distance to nearby galaxies if Cepheid variables can be found in them.

Another possible method involves estimating a star's luminosity. There is a relationship between a star's color (spectral type to be more precise) and its luminosity (real brightness). This is the Hertzsprung-Russell diagram. We can get an estimate of a star's distance by comparing its observed apparent brightness with its estimated luminosity. However this is not a good method because there are very many giant stars that are much brighter than their temperature would indicate, and these obviously give wrong answers for the distance.

At larger distances the distances to galaxies is measured by the Red Shift using the Hubble constant. Edwin Hubble showed that the shift in the spectrum of a galaxy towards the long wavelength end of the spectrum depends on its distance.

By these means we know how far it is to the objects in the Universe.