seriousnotshirley t1_j4kxbsk wrote on January 16, 2023 at 1:11 PM

We look for radiation from hydrogen that occurs when the electron’s spin flips. This releases a photon with a very precise frequency that is seen with a radio telescope. It’s very rare for any single hydrogen atom to exhibit this spin flip but there’s a lot of hydrogen in space so it happens regularly.

While the frequency of the photon that’s emitted is precise the frequency we observe is not. The frequency we observe can be blue shifted or red shifted by the hydrogen moving towards or away from us as we and the hydrogen move around the galaxy.

With an estimate of the gravity of the center of the milky way we can estimate how from out the hydrogen we observe must be for it to be moving at the velocity that it does. That provides a basic model of the structure of the Galaxy.

BrooklynVariety t1_j4lvkib wrote on January 16, 2023 at 5:18 PM

> With an estimate of the gravity of the center of the milky way we can estimate how from out the hydrogen we observe must be for it to be moving at the velocity that it does.

This would be a terrible way of doing this since you have know the geometry and mass distribution of the galaxy to have a model of how stars should be rotating. On top of that, even if you had a good model, you only measure line-of-sight velocities, making this pretty useless.