Galaxies are held together by the force of gravity. Gravity is an attractive force that exists between all objects with mass. You will be most familiar with gravity as the force that keeps your feet on the ground. It is also the force that keeps the planets orbiting around the Sun and keeps stars orbiting in galaxies.

In spiral galaxies, the speed at which stars and gas orbit around the galactic centre depends on the distance from the centre.

In the inner parts of a spiral galaxy, we see enough stuff (stars and gas) that we would expect it to rotate like a solid (or rigid) body. In this case, the rotational speed should increase with distance from the centre (as is the case for a wheel).

(credit: Jen Gupta)

However, in the outer parts of the galaxy, we do not see as many stars, the mass is concentrated in the centre. We would therefore expect the rotational speed to fall off with distance from the centre, in a similar way to the motion of the planets in the solar system.

(credit: Jen Gupta)

By using these two relationships, we can draw a graph of how we would expect the rotational speed of a spiral galaxy to change with distance from the centre (see curve A below). This can then be compared to a graph of observed values.

Astronomers can measure the rotational speed using the Doppler shift of light from the galaxy - light from parts of the galaxy rotating away from us will be shifted towards the red part of the spectrum, while light from parts of the galaxy rotating towards us will be shifted towards the blue end of the spectrum.

When measured rotation curves are compared to the predicted rotation curve, something surprising is revealed. Instead of the rotational speed dropping off in the outer parts of the galaxy, the speed remains roughly constant, as shown by curve B below. Stars at these large distances are moving around the galaxy so fast that they should be flying off into space. This implies that there must be more stuff, or matter, in the outer parts of the galaxy providing the gravitational attraction keeping the stars bound to the galaxy; we just can’t see this extra stuff. Astronomers call this stuff dark matter and you'll learn more about this in a later section of the course.

Predicted (A) and observed (B) rotation curve of a galaxy. (credit: PhilHibbs (Wikimedia Commons) )