We observes distant phenomena like supernovas using a variety of telescopes rather than visiting them using things like the space shuttle. In fact, all of our knowledge about the solar system minus the moon is from indirect observation using a variety of telescopes and other detectors of light.

A supernova shines about as brightly as all of the stars in a galaxy combined. Novas are not quite as bright, but they are much brighter than a normal star. We can see them because we see the light that they emit - just as we can see ordinary stars, even though none of our spacecraft have ever gone to them either. We can see (super)novas from farther away because they're just really bright.