Time and the supernova.

I don't understand how scientists say that they have found stars that have gone supernova meer hours after they explode. In this article from a previous post last year, scientists state that they "Astronomers believe they caught the supernova within hours of its explosion, a rare feat." According to the Wikipedia article, the white dwarf which caused the supernova is 21 million ly away. Didn't this supernova occur 21 million years ago? So explain to me, as you would a child, please how it is that these scientists discovered this supernova hour after it occured. I really don't understand and would like to know.