- What is a black hole? How do we know that stars and galaxies are billions of years old? What is the difference between stars and planets? Glenn Murphy, author of Why is Snot Green?, answers these and a lot of other brilliant questions in this funny and informative book. Packed with doodles and information about all sorts of incredible things, like supermassive black holes, galaxies, telescopes, planets, solar flares, constellations, eclipses and red dwarfs, this book contains absolutely no boring bits!