How SDSS Uses Light to Measure the Distances to Galaxies

Here at the Sloan Digital Sky Surveys our mission is to explore and map the Universe, from planets to the edges of the observable Universe. The way we do this is to collect light from specially selected objects we see in the night sky – but we can’t visit them in order to measure how far away they are. So how do we actually know how far away they are in order to make a map of the Universe?

Measuring the distance to objects in the Universe has always been one of the biggest challenges for astronomers. Until we know the distance to something we cannot really understand its physical properties, and the history of astronomy is full of examples where new techniques for measuring distances opened up entirely new areas of study. For example when the “spiral nebulae” were first discovered there was a long debate over if they were small clouds of gas in our own Galaxy, or external galaxies in their own right each made up of millions or billions of stars. Only by measuring their distances was this finally settled, and our understanding of the size of the Universe suddenly jumped many orders of magnitude.

A collection of "spiral nebulae". But how can we tell that they are distant galaxies rather than nearby gas clouds? Credit: SDSS

A collection of “spiral nebulae”. But how can we tell that they are distant galaxies rather than nearby gas clouds? Credit: SDSS

There’s some really useful bits of physics we can use to help measure distances to the galaxies from their light. To do this we need to understand spectroscopy. Once SDSS had finished imaging more than a quarter of the sky with its camera, it became entirely focused on “spectroscopic” surveys. Our telescope in New Mexico collects the light from stars and galaxies and uses instruments called spectroscopes to split it up into its different colours (we actually have two different spectroscopes working right now – the APOGEE spectroscope and the BOSS spectroscope). These measurements split the light into a rainbow (or a spectrum), and we look for the precise colours of series of emission and/or absorption lines to tell us all sorts of things about the light source we’re looking at.


A hot bright light source (like a star) will have a “continuous spectrum” (with the peak colour depending on its temperature – hot things glow red, even hotter things glow white or blue hot). If the light from that passes through a cool cloud of gas before we measure it, that will create “absorption lines” where very specific colours (or “wavelengths” in proper scientific terms) are absorbed by atoms in the gas cloud. The exact pattern of colours/wavelengths which are absorbed tell you which atoms are in the gas cloud. If the gas cloud gets heated up enough we might instead see emission lines – at the same specific colours, where the atoms are now re-emitting these very specific colours/wavelengths. Each atom has a very distinctive pattern of lines it emits – for example hydrogen (the most abundant element in the Universe) has a very distinctive and bright emission/absorption line in the red part of the spectrum (at a wavelength of 656.3nm).


Emission spectrum of hydrogen in visible light (wikimedia commons)

Astronomers have been using this technique to work out the materials which make up the Sun and other stars for decades. It’s not always easy (it has been compared to trying to reconstruct a piano from the noise it makes falling down the stairs), but it works. When astronomers first used the technique to look at galaxies however they were very surprised by what they found. The patterns of lines seemed to be in completely the wrong places – for example the famous hydrogen lines weren’t even visible in some cases – they had moved right into the infra-red part of the spectrum.

In order to understand why this could happen we need to learn about another part of physics – the Doppler effect. First proposed in 1842, by a Physicist named Christian Doppler this is the observation that when a source emitting a wave is moving, the waves are shortened if the source is moving towards the observer, and lengthened if it is moving away. Most people are familiar with this effect when they have listened to ambulance sirens passing them on the street; the siren is higher in pitch when the ambulance is moving towards you and lower when it’s moving away (when sound waves are lengthened the pitch drops, and when they are shortened the pitch rises).

Wikimedia commons illustration of the Doppler effect.

Since light is a wave, the same effect happens when light is emitted from a moving source. When the waves of light are shortened the light becomes bluer, and when they are lengthened the light becomes redder.

An astronomer named Vesto Slipher, was the first person to try this out on galaxies, and he found that almost all galaxies he looked at showed enormous “redshifts”, implying that almost all the galaxies were moving away from the Earth at very high speeds.

Edwin Hubble is given the credit for explaining this observation by realising that we live in a Universe which is constantly expanding. In such a Universe any observer will observe almost all other galaxies moving away from them. Hubble published the first description of a relationship between how fast galaxies appear to be moving away from us (their “redshifts”) and their distances – this relationship is now called Hubble’s Law.

It is this relationship that we use to measure the distances to the galaxies from detailed observations of the light they emit, and astronomers are now used to describing the distances to galaxies as simply their “redshift”.


A map of the Universe from SDSS where the distance to galaxies is given in terms of their redshift. Credit: SDSS

This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe.