So that will tell you
if the, the star in this region is sort of coming towards us.
Then we will expect the light should be bluer than what it actually should be.
If the stars are going inwards then the light
should look reder to us than what it should be.
That's exactly what you see in this picture.
So this line here is the center of the galaxy.
And what
is plotted in the vertical axis is the wavelength of light.
And what you see is that on the left and the
right of the center of the galaxy, you see different wavelength.
And that's because this part of the galaxy is now pushed, being pushed in.
So that the wavelength looks longer while this part of
the galaxy is coming towards us, the wavelength looks shorter.
But the most important thing here is that the wavelength looked
pretty much flat as you go to the outskirt of the galaxy.
Which means that rotation speed again is pretty much flat.
It doesn't seem to go down, like what it did in the case of solar system.
And that is true in all the galaxies that
people have studied, this is the case of Andromeda.
Again, the rotation speed is more less constant
as you go outwards in the Andromeda galaxy.
While if you think that the source of gravity is only stars and nothing else.
Rotation speed should be going down along this yellow curve.
But clearly something is missing here.
Again, we attribute that to the existence of dark matter.
Here's yet another example of a galaxy.
We again see a flat rotation curve while the
source of gravity coming from just the stars in the
galactic disk.
Should give you this folding curve so that
something else should be making up the rest.