If a spiral galaxy's mass consisted only of visible stars and gas, how would the orbital speed of material expectedly change as the distance from the core increases?

Answer

The orbital speed would decrease noticeably.

Based on Newtonian mechanics applied only to visible matter, the enclosed mass decreases beyond the luminous edge, leading to an expectation that orbital velocities should decrease, similar to how outer planets orbit the Sun more slowly.

If a spiral galaxy's mass consisted only of visible stars and gas, how would the orbital speed of material expectedly change as the distance from the core increases?
measurementgalaxyastronomymethodmass