The outer most stars should be chosen to conduct this test. The outer most stars should be chosen as they would be the most accurate, due to them incorporating the majority of the matter and the gravitational effects imposed by the galaxy.
It is hypothesised that the orbital period of the stars will increase as the radius of the star from the galactic centre is increased. This is predicted as the star will have a larger orbital path than that of a star which is closer to the galactic centre, which means that it should orbit the galactic centre at a much slower rate.
The rotation curve of disc galaxies is the plot of the magnitude of visible stars or gas in galaxy orbital velocities against their radial distance from the centre of that galaxy.
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Dark Matter halos surround individual galaxies and groups or clusters of galaxies and its properties are only inferred to exist due to the gravitational effects it has on visible matter. Dark matter in the halo is inferred through the gravitational effect imposed on a spiral galaxy’s rotation curve. Evidence for dark matter is also found through observations of the motions of galaxies in groups and clusters. Within galaxies the velocities of galaxies in groups and clusters are so high that the groups or clusters would fly apart if the luminous matter were present. The large amount of mass throughout the halo means that the rotational velocity of the galaxy doesn’t decrease at large distances from the galactic centre. Observations of spiral galaxies from neutral atomic hydrogen reveal that rotation curve flattens out in most spiral galaxies, this means that the dark matter theory suggests that rotational velocities don’t decrease as the distance from the galactic centre increases. Due to the absence of any visible matter account for these observations that the unobserved dark matter exists or the theory of motion under gravity is incorrect. Dark matter theory makes it more challenging to see how the mass and acceleration of the rotation curve correlate so