How do we measure distances to space objects?

There are many methods to do this. Let me cite parallax, because we use it in our Gaia mission. For stars, we can exploit the fact that their position in the sky depends on our position as an observer. Much like nearer objects appear to move in front of background objects when you as an observer move, say, driving by a bunch of people in front of a landscape. In astrometry, we can use the fact move with the Earth around the Sun, so our observation position relative to the Sun changes by two astronomical units (AU) – about 300 million kilometres or

185 million miles – in six months. A star that moves by one second of arc due to the change in our position by 1 AU has a distance of one parsec (parallax second). One parsec is equal to 3.26156 light years, which is the distance light travels in 3.26156 years, and is equal to 30.9 billion kilometres (19.2 billion miles). With the Gaia spacecraft, for example, we can basically determine the distance of half of the Milky Way stars using this method.

For more distant objects, like other galaxies, we use ‘standard candles’. Those are variable stars, the absolute brightness of which is correlated with the period of the variation. Knowing the variation, we can derive the absolute brightness. Knowing the absolute brightness and the apparent, measured brightness we can determine the distance. Even further objects, like quasars, are measured by looking at the redshift due to the cosmic expansion. Dr Markus Landgraf, senior mission analyst at the European Space Agency (ESA)

“For more distant objects, like other galaxies, we use ‘standard candles’”

Markus Landgraf