Question: how can you measure the distance to a star?

Jack Miller answered on 20 Jun 2013:

Hi Jeromeslurpslurp,

Great question! Historically, people measured distances to nearby stars by simple trigonometry: as the earth moves throughout its orbit, nearby stars change position very slightly. You can draw two right-angled triangles and, as we know the distance to the sun and can measure the angle the star appears at, work out its distance. There’s a great picture of this principle in action on the wikipedia article (http://en.wikipedia.org/wiki/Parallax).

This method only works if the star you’re looking at is *relatively* nearby — within 1600 light years or so (around 1% of the diameter of our galaxy).

If you’re looking at stars farther away, this method doesn’t work — you just can’t resolve the difference in angles. Instead, another method is used, which I’ll just explain a bit about now.

You’ve probably often heard that the universe is expanding. One of the effects of this expansion is that everywhere, everything seems to be moving away from you. A good analogy if that sentence made your head hurt is to imagine coins taped on the surface of a balloon — if you could stand on a coin as the balloon was being blown up, you’d think that all the other coins were moving away from you, as you stayed still. That’s the same for every coin, and for every observer in our universe. Now, imagine a distant star. It appears to be moving away from you as the universe expands. This has an effect: let’s consider a ray of light emitted by that star. The distance between successive peaks or troughs in that ray of light appear increased to you, relative to the star, as it’s essentially moving away from you — this is the origin of the dopper effect, which you’re probably familiar (with wah-wah noises coming from sirens). As it’s the star’s light that’s being shifted, and it’s moving away from you, it gets lower in frequency, and consequently appears in the redder end of this spectrum. This is called, funnily enough, ‘redshift’.

We can measure the distribution of light coming from a star easily, and stars which are redshifted a lot come from somewhere a lot farther away. It’s possible, by using a reference of types of stars that have the same or very similar properties (collectively called ‘standard candles’) to turn this redshift into a distance.

Hope this helps!

— Jack