AfterWorld Survival Forum

id like to know one thing concerning dark matter:
its dark matter that allows stars in a galaxy to spin around the center of a galaxy at approx same radial speed.
means, a star close to the center is spinning at approx. same radial speed as one on the edge of the disc.
its said, that without dark matter the edge stars would be ejected after 2-3 spins.

so if this is an indicator for the existance of dark matter (because its gravity keeps the system together) and if dark matter is everywhere (a valid physical rule must be generally applicable), why mercury and pluto are spinning at si different radial speeds around our sun? 
why are the planets in our solar system slower the bigger the distance from the sun?

is there any physicist who can explain me this pls?

I've read/listened to a fair number of physics audio books and don't recall any mention of this effect and its connection to dark matter so I had to look it up (Google to the rescue!) and found this Wikipedia page. It basically states that the reason dark matter is believed to be the cause of the relatively uniform rotational velocity of stars in a spiral galaxy is because its gravitational effects would/could be equivalent to the gravitational effects of the central bulge. This "extra" gravity causes all of the large-scale space in the galaxy to act like a sticky sheet that all of the stars are attached to and the whole thing is rotating. Without dark matter Kepler's Law says that the stars should rotate about the center of the galaxy in the same way that the planets rotate about the sun, with objects further from the center having lower rotational velocity than those closer in.

What I do remember hearing about in the physics audio books is the MOND theory. It stands for Modified Newtonian Dynamics and essentially says that there are small and subtle changes to Newton's Laws that could also account for the discrepancy in rotational velocities, among other things. There are apparently a number of alternative theories for why the stars in a spiral galaxy nearly all have the same rotational velocity but dark matter is the favored theory because it also explains a number of other galactic features, including galaxy formation and galaxy grouping and clustering.

This is all in the Wikipedia page linked above but before reading it I was of the belief that the stars in spiral galaxies such as our own rotated similarly to the planets in our solar system, so thanks! The short video about half way down the page is especially helpful in seeing the difference, especially when played in full screen mode. I think what makes me chuckle the most is the fact that Monty Python got the rotational velocity of our solar system rather close to its currently-accepted rate back in 1983!