technique - How were these Astronomy Photographer of the Year images produced?

Looking at the winner of Astronomy Photographer of the Year, I'm wondering, how can one produce shots like these:

http://www.rmg.co.uk/whats-on/exhibitions/astronomy-photographer-of-the-year/winners-2011/special-prizes/

Specifically I'm referring to the "People and Space: winner" image. When the winner say that he used "525 separate exposures" to create the image, what does this mean? Any tutorials on this workflow?

I suppose this image uses the same method:

http://www.universetoday.com/86472/are-you-the-next-astronomy-photographer-of-the-year/

I mean, even if it is really dark, how could one possibly get that much detail from just one exposure without stars beginning to trail?