The lights in the night sky are a bit deceptive. Just because one of them looks brighter than another doesn’t mean that it really is brighter — it might simply be closer. As an example, consider two pinpoints of light that sweep past each other in the dawn sky the next few mornings — the planet Mercury and the star Spica. Mercury is by far the brighter of the two. But that’s only because of the difference in distance. Mercury is about a hundred million miles away — a yawning gulf by human standards, but a mere hop on the astronomical scale. But Spica is roughly 15 million times farther. If you moved Mercury out to that great distance, it would be too faint to see with even the most powerful telescopes. Mercury is a ball of rock that shines only by reflecting sunlight. Spica, on the other hand, consists of two bright stars. They shine by radiating the energy produced by the nuclear reactions in their cores. Both stars are far more impressive than the Sun. At the wavelengths that are vis
