Wupadoo

I understand that light takes 8 minutes, to travel from the Sun, to Earth. I can see how the light from a Galaxy, thousands of light years away, would take years,and years to reach Earth. By looking through a Telescope, the image is closer, and therefore, the time the light needed to reach us is less. By how much is it less? If we are looking at a Star, through a powerful Telescope, and we see it explode, then take our eye away from the Telescope, and view it with our naked eye, would we see it explode again, a fraction of a second later?

If Jupiter, and Saturn are made up of gases (methane, hydrogen, etc,). Why didn't Jupiter catch fire, and ignite, when the Shoemaker-Levy 9 Comet smashed into it? Lack of Oxygen?