This activity consists in measuring the mass of a spiral galaxy, viewed edge-on, using the same procedure employed by astronomers. It is surprising how just a few measurements and the knowledge of a few fundamental laws of physics make it possible to weigh the largest and most distant objects in the Universe, despite the impossibility of carrying out direct measurements and the fact that the only information available to us derive from a few photons which have travelled for tens and tens of millions of years. What is even more surprising is the fact that the simple measurement proposed here makes it possible to obtain experimental evidence for the well-known dark matter. Spiral galaxies contain a large amount of gas; the gas emits a spectrum of lines; if the galaxy is viewed edge-on (and not directly facing the observer, not perpendicular to the line of sight) and given that it rotates on its own axis, on one side the gas goes away from us and on the other, with respect to the centre, it draws closer to us (cf. figure). The lines emitted by a gas which moves with respect to the observer undergo the so-called Doppler effect, which shifts the observed frequency. |