# The Hubble Tuning Fork below is a diagram that describes the galaxies in our universe. ellipticals are not its separate branch as they are precursor/merged galaxies. ![[Pasted image 20230712094842.png]] SB --> barred spiral galaxies (with a galactic bar in the middle) S0 --> lenticular galaxies E --> elliptical S --> spiral galaxy, no bar # Our Milky Way -->the basic structure of our galaxy --> we live n a spiral type galaxy ![[Pasted image 20230712090530.png]] \- the milky way face-on (left), the milky way edge on (right) ## Galaxial Rotational Velocities F = ma so therefore: $\frac{GM(r)}{r^2} =- \frac{v^2}{r}$ where the left side is the gravitational acceleration and the right side is th centripetal acceleration --> r is the radius of the star around the galaxy. rearrange to get $v$: $v = \sqrt{ \frac{GM(r)}{r^2} } = \sqrt{ \frac{GM}{r} }$ therefore $v \propto r^{-\frac{1}{2}}$ we would expect the rotation rate of stars around a galaxy to decrease! However, this is not the case, as referenced by the graph: ![[Pasted image 20230712092814.png]] We can use gthe above equation to solve for M $M = \frac{v^2r}{G}$ This gives us a bigger mass value for the mass of the galaxy, more than we can account for in visible matter. This matter does not interact with the electromagnetic force and is therefore known as **dark matter!** //centre of the milky way ![[Pasted image 20230712093349.png]]