I had the opportunity to go out and spend some time in the early morning hours with a couple of pro-wedding photographers who wanted to shoot some star trails. We had a good time visiting while our cameras snapped away shooting 30 second photos for a total of 90 minutes.
After doing a little searching, I figured out that it's steel wool. A few friends and I went out in the country to test it out. A pack of steel wool is around $3 at Lowe's. You can't get entertainment much cheaper than that!