The night sky is full of stars, and those stars are incredibly distant. The distances are measured in light years , which corresponds to the distance travelled by light in one year. That’s about 9 trillion km. Alpha Centauri , the nearest star visible to the unaided eye, is at a distance 270,000 times the distance between Earth and the Sun. That’s 4 light years, so we see Alpha Centauri as it was 4 years ago.