h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Can anyone help? As I am really struggling with this one.
Stars orbit the galaxy faster in the outer reaches because there is a dark matter halo around it.
This seems backwards to me, if there is mass outside the galaxy with a gavitational effect would this not make the stars go slower to balance out the pull from outside the galaxy with the pull from the core.
If the dark matter halo has a repulsive effect then the stars would need to orbit faster to stop being pushed towards the core.
In which case there would be no need for dark energy, or have I got this completely wrong?

Okay..

Any star in the galaxy is influenced by the gravitational field of every other star, so it is pulled in all directions, but the net effect is towards the centre of the galaxy.

If you draw a circle around the centre of the galaxy at the radius of the star you're looking at. Then when calculating the speed of the star, you can ignore everything outside the circle because the pull from one side of the galaxy cancels the pull from the other side. (Not entirely true as the galaxy is not perfectly symmetric, but a good approximation).

The speed of the star depends on two things:

A - The mass it is orbitting (what's inside the circle). The more mass, the faster it must move

B - The distance from the centre of this mass. The further away, the slower it moves

If the mass of the galaxy is concentrated in the centre, then as we move out into the disk, A is constant, but B is increasing. So the stars move slower and slower the further out we look.

If the mass is spread out in a dark halo extending far beyond the disk, then as we move out into the disk, both A and B are increasing, so the speed of the stars stays approximately constant.

Hope this helps. It takes a bit of maths to prove it.

Thanks for the reply

I think I am just about coping with it now

It must all depend on the percentage of mass between the outer reaches
and the galactic centre.

I shall think on it further

Inadequacy of perception adds to Confused logic.
Need to search beyond Dark mode Concepts in order to understanct Cosmic Function of the Universe. Space Time Energy concepts are distinct.Lead Kindly Light.
At the Milky way Galactic plane, one can introduce Plasma Regulated Electromagnetic phenomena in Magnetic field Environment.
Many of these concepts are missing in this discussion or Forum
How do ene interface through Origin of the Universe-from vedas to interlink present Space-astronomical data. obvious that Cosmological Index is missing.

Dark matter should not become Dogma- undefined and does not inspire logic.
Science need to establish credibility index.
Even the Red rectangle at 2300 LY shows the Enlightened Spirit.
integrate Nature and Philosophy -the Cosmos Quest has a larger frame.
Origins vedas provide interlinks. create East West interaction.
COSMOS QUEST-VI-JNANAM-DHIYO-YONAH-VIDYARDHI NANDURI
http://www.youtube.com/watch?v=9RzLEOHk68I&feature=plcp