It has been suggested that the outer parts of the spiral nebulae are orbiting too fast to be explained by Newton’s laws, and are in fact rotating at the same velocity whatever their distance from the nebula centre of gravity. However it can be shown that if the mass density of the nebula falls off as 1/r then all parts will indeed orbit at the same velocity, and this is on the face of it quite feasible.

On the subject of the actual speed of rotation of the nebulae and the fact that they appear to be rotating slowly, this could be explained by extra mass due to cosmic dust between the stars, and there is plenty of space for that; and perhaps by the recently discovered brown dwarfs which are too cool to be seen at a distance.

Dark matter does not need to be involved at all.


John Fleming, 13 May 2010