How to measure a galaxy’s mass

How we work out the weight of galaxies in the universe

Image credit: X-ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL-Caltech

The idea of weighing a galaxy might seem a bit weird – it’s not like we can put it on a giant cosmic set of scales. But by using some clever observations and tricky equations, it is indeed possible to work out the masses of other galaxies in the universe.

There are actually a number of ways to do it, but one popular method is to look at the orbital motion of stars in a galaxy. Those in a more massive galaxy will move faster than those in a less massive one, so measuring their speed can help scientists work out the answer. Scientists also look at the overall rotation rate of galaxies to work out their mass. They do this by measuring the redshift or blueshift – the amount that a particular side of a galaxy is moving away from or towards us respectively – and seeing how much the light shifts to each end of the spectrum. Another method involves looking at the gravitational pull exerted on star clusters in space by nearby galaxies. The bigger the pull, the more massive the galaxy, and we can use this to estimate just how heavy it really is.

Yet another method involves gravitational lensing, which is the lensing effect caused when a galaxy passes in front of a distant object in our line of sight. Depending on the gravitational strength (and therefore mass) of the lensing galaxy, this can produce either a large or small lensing effect, something that was predicted by Einstein and that is known as an Einstein ring. However, these events are rare in the universe, so the chances of us measuring a galaxy in this way are slim.

 The Milky Way 

We can measure the mass of other galaxies, but how do we measure our own when we can’t see the big picture from afar? The best way to do it is to measure the speed and motion of stars in our galaxy, although to get an accurate reading you need to measure at least 100,000 stars, and maybe millions to be sure.

Another way to get a good estimate is to look at ‘tracers’ behind our galaxy. These are rogue stars and galaxy fragments that now trail behind us, and their velocity and angular momentum can tell us how much they have been pulled by our galaxy, thus indicating its mass. The most recent estimate suggests our galaxy is about 960 billion times the mass of our Sun.

Two measuring methods

Here are two ways used to estimate the mass of galaxy ESO 325-G004

Image credit: ESO, ESA/Hubble, NASA (Text: Future plc) Click to expand

This article was originally published in How It Works issue 118, written by Jonathan O’Callaghan

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