How to weigh a galaxy

Tuesday 21st June, 2016

Our speaker was David Morris FRAS who is not only an accomplished astrophotographer but has also successfully completed the Open University's Certificate in Astronomy and Planetary Science. In his previous visit to the Society he demonstrated the scale of the Solar System and on this occasion he talked about "How to weigh a galaxy".

He began by saying that, obviously, you cannot weigh a galaxy directly but it is possible to calculate a galaxy's mass by measuring the speed of stars orbiting at the galaxy's edge and knowing the galaxy's radius. This is because to stay in a stable orbit a star needs to have enough speed to counteract the pull of gravity towards the centre of the galaxy. In the same way the Earth does not fall into the Sun as its speed of roughly 70,000 mph is fast enough to resist the Sun's gravitational pull.

However, there are some problems with this method as we could use the speed of the Sun for the calculations but we know that the Sun is not at the edge of the Milky Way's disc. If we used our own star we would end up with a result for all the mass that was inside our Sun's orbit which would be too low an estimate. In fact, astronomers do not really know how much mass in the form of dark matter lies beyond all the stars, dust and gas that are relatively easy to detect. The visible disc appears to extend to about 60,000 light years from the centre (where a light year is the distance light travels in one year). The scientists think that the undetectable mass may continue out to over ten times that distance.

To overcome this problem scientists have looked beyond the Milky Way to all the galaxies that make up the galaxy cluster known as the Local Group and even a few beyond it. In a study led by scientists from the University of Edinburgh the relative motion of the Milky Way, the Andromeda galaxy and other nearby galaxies was measured along with the various distances between the galaxies. Using these data, they could calculate that our nearest neighbouring galaxy, Andromeda, is twice as heavy as the Milky Way and that 90 percent of the matter in both galaxies is invisible i.e. consists of dark matter.

Previous estimates of the mass of the Milky Way ranged from 100 billion to 1.6 trillion times the mass of our Sun. According to the latest research presented only last month at the Canadian Astronomical Society's Conference the Milky Way has a mass equivalent to about 700 billion times the mass of our Sun. The researchers obtained this result by studying the orbits of globular clusters. These are densely packed spherical collections of roughly a million stars orbiting the centre of our galaxy.