Earth/solar position vs spiral arms

[ray b]

in a post on global warming linking to a PDF http://www.spacecenter.dk/research/...blications/svensmark_2007cosmoClimatology.pdf
there is a figure #9 showing the earth+ solar position over the last 200 million years
as our sun moves in and out of the spiral arms of the Milkyway

WHY does this happen
as if the other stars moved at a near speed
would not the solar position stay with in a given spiral arm
or the whole arm no long be in place as the stars move out
is our sun at a special orbit or speed
or what is going on?

 

[Wallace]

Spiral arms are 'density waves' rather than a permanent structure that rotates with the stars. This means that stars are sometimes in and sometimes out of the spiral arm.

 

[Janus]

The spiral arms are not due to star movement around the center of the galaxy. They are compression waves moving through the galaxy.

 

[ray b]

how much variation in speeds is there in our current local group of stars

I thought the whole need for "DARK MATTER" was to limit the speed of the outer
stars as they move at nearly the same speed in orbits around the galaxy

so how do compression waves move a star or group of stars
and how can someone say our sun was in a give arm at a given date 100 or 300 million years ago?