Calculating Acceleration Due to Gravity in a Galaxy

[Bjarne]

Galaxy gravity ?

Which equation must be used to calculate Acceleration due to gravity inside a galaxy

I mean it is not the same as to calculate the acceleration towards the Earth GM/r², - right ?

 

[Jonathan Scott]

Which equation must be used to calculate Acceleration due to gravity inside a galaxy

I mean it is not the same as to calculate the acceleration towards the Earth GM/r², - right ?

Yes, apart from very small corrections for General Relativity, it's essentially the vector sum of that expression for each mass within the galaxy, where M is the mass and r is the distance to it, and the vector is in the direction of the mass.

That's obviously not very useful for practical purposes, so astronomers attempt to estimate the mass distribution at different positions within a galaxy and add up the effect, using numeric simulations if necessary.

When they do that, the result in most cases seems to be insufficient to account for the acceleration apparently being experienced by stars in the outer parts of galaxies. One theory to account for this discrepancy is the existence of "dark matter". Another class of theories (such as MOND) involves modifying gravity theory.