Astronomer Tycho Brahe knew the position of nearly every star in the sky. But strolling home one November night in 1572, he saw a strange new light. The stella nova, modern astronomers now know, was actually a supernova. After 450 years, its remnant has faded like a firework and no longer glows visibly. Thanks to a secret energy source, though, it still shines in X-rays nearly as bright as it did in its youth.
When a massive star explodes, the spray of stardust collides with interstellar gas, forming an outgoing shock wave. The wave heats its surroundings billions of degrees, leaving a blazingly bright wake. Within months, the low-energy visible light dims as the plasma spreads and cools.
The high-energy X-rays should peter out, too. But, as what’s now called Tycho’s supernova remnant demonstrates, something still fuels the radiation fire hundreds of years after its energy should have been drained.
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