Our Rotating Galaxy

How do we know the structure of our own galaxy?

Hydrogen, like all other elements, emits light with very definite wavelengths. This is because the electron around the Hydrogen atom can only absorb and emit definite amounts of energy.

One of these wavelengths is 21cm which falls in the radio range. Our galaxy, like all others, contains a huge amount of Hydrogen.

Waves with this wavelength are very useful to astronomers for several reasons. One is that they can penetrate through dust clouds, something that visible light struggles to do. Another is that there is an awful lot of hydrogen out there and by detecting it can we can make many deductions about the cosmos,

If we look in a certain direction and pick up a lot of 21cm radio waves that tells us that there is a lot of Hydrogen there. We are probably looking into the massive Hydrogen clouds within a spiral arm.

By detecting how much this radiation has red or blue shifted we know the speed of this hydrogen relative to us, i.e. is it moving towards or away from us and how fast. Knowing this we can map the spiral arms and calculate how fast they are rotating around the galactic centre.

Below is a graph showing how the speed of rotation of the milky way varies with distance from the centre. As we get further from the centre we would expect the velocity to get less as the gravitational pull from the centre gets weaker. What we notice is that it stays roughly the same. This is evidence for dark matter, i.e. lots of mass that we cant see.	Model of the Milky Way's spiral arms, by Taylor and Cordes 1993, created using data from 21 cm radio surveys. The circle with a dot in it is our Sun.