There is a demon's head winking at us every few nights. It's brightest during the winter months, so you might be able to see it on clear nights in the northern hemisphere. Some considered it bad luck — and some considered it the cause of a paradox of astronomy.

Algol the Demon

In the constellation of Perseus there is a certain star that flickers at us. Every three days it dims down, and brightens back up over the course of 10 hours. This flicker was not greeted with much enthusiasm by the ancient Greeks, due to the star's name, Algol.


For those who didn't spend much of their youth studying Greek mythology, Perseus was the hero who went up against the demon Medusa, whose face turned people to stone. The star is meant to represent Medusa's head, specifically her eye. "Algol" is taken from the Arabic "al Ghul," and contextually translates to "the demon's head." It's not something anyone would want winking at them.

The cause of the wink is a smaller star that orbits around Algol every few days, blocking out its light. This second star is dimmer and so astronomers didn't notice it until they started working with telescopes that could get a good look at how Algol moved and what was near it. The smallness of this second star caused nearly as much consternation to astronomers as Algol's wink did to the ancient Greeks. It set up what became known as the Algol Paradox.

Algol the Astronomical Paradox

Stars go through a predictable sequence. They begin as clouds of hydrogen gas that condense down until they are so dense, and the pressure on their center is so great, that hydrogen in their core starts fusing into helium. Once the hydrogen in their core is used up, the helium begins combining. The fusing elements get progressively heavier. There is a bottom limit on the mass of stars below which there won't be enough pressure to get hydrogen to fuse. Pretty much everything above that is fair game, and stars can get much bigger than our own sun. With mass comes a trade-off — bigger stars have more fuel, but they put so much pressure on their centers that they burn through their fuel faster. They go through all the stages of smaller stars, but they do so more quickly.


Algol and its companion were created at the same time and are, chronologically, the same age. Algol is larger, but its smaller companion has "aged," or developed faster. It's a wizened old star with the body mass of an infant. How is this possible?


The idea puzzled people long enough for it to be called a "paradox," but there is a solution. Algol and its companion star are very close to each other — about five million miles away from each other. As a star ages, its core fuses heavier stuff and puts out more energy, puffing out its outer layers. Those outer layers must have come in range of Algol's gravity and, like a good demon, Algol sank its claws into them. Having stolen its companion's extra mass, Algol itself continued on its way, preternaturally young, beautiful, and ready to baffle scientists.

So if you get a chance to look up into the sky during these months, steal a glance at the winking head of the demon. There's nothing to fear, now. Go ahead. Look.


Image: ESO/M. Kornmesser/S.E. de Mink. Perseus Image: Till Credner.