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Imagine you had a spacecraft capable of travelling through interstellar space. You climb in, blast into orbit, fly out of the solar system and keep going. Where do you end up, and what do you see along the way? The answer is: mostly nothing. Space is astonishingly, mind-blowingly empty. As you travel through the void between galaxies your spaceship encounters nothing more exciting than the odd hydrogen molecule. But when it does come across something more exotic: wow! First and most obviously, stars and planets. Some are familiar from our own backyard: yellow suns, rocky planets like Mars, gas and ice giants like Jupiter and Neptune. But there are many more: giant stars, red and white dwarfs, super-earths and hot Jupiters. Elsewhere are swirling clouds of dust giving birth to stars, and infinitely dense regions of space-time called black holes. These clump together in the star clusters we call galaxies, and the