The Demon and the Wonder: Variable Stars

But if you’re in the Northern Hemisphere with a decently dark sky, you might be able to make out the constellations of Perseus and Cetus, which take up nice big chunks of the autumn evening sky. They sit more or less between the Square of Pegasus and the V-shape of Taurus’s head. 

They’re also the home constellations of a couple of special stars, Mira in Cetus and Algol in Perseus. These two stars are what are known as variable stars, stars whose brightnesses as viewed by someone on Earth appear to change over time. And that fact earned these two stars very different names and reputations. 

So let’s get to know Mira, the Wonderful, and Algol, the Demon, and see just what’s up with these variable stars.

Twinkle, Twinkle Little Star 

That classic song lyric is a bit misleading. What we see as stars “twinkling” is actually due to a star’s light passing through Earth’s ever-moving atmosphere on its way to your eye. That said, some stars do appear to change in brightness periodically, for one reason or another. These are variable stars. 

Sometimes this brightness change can happen because what looks like one star to the human eye is actually two or three (occasionally more). If the star system is far enough away or the stars are close enough together, an Earth-based observer will see the whole thing as a single star. 

Sometimes in these cases one star will move behind the other as seen from Earth and have its light blocked. When that happens, the system appears to our Earth-based observer to dim for the length of time one star is behind the other. This type of variable is called an extrinsic variable, indicating that what’s changing isn’t something inherent to the star itself, just how much of its light is reaching Earth. It’s also called an eclipsing variable, since the brightness change comes from one star eclipsing the other.

A second category of variable star is known as an intrinsic variable. In this case, the actual amount of light the star is putting out is changing due to something having to do with the star itself. This can happen for several reasons, ranging from the star changing size to the star blowing itself up. 

There are many different types of intrinsic variables. Many. Among the subtypes are the cataclysmic types, when something on the star—or the star itself—explodes, the eruptive type, caused by something making the star flare brightly and briefly, and the pulsating type. This last type is the most interesting to me. Cataclysmic and eruptive ones are cool because they’re explosions, but of course something blowing up will cause a change in brightness. A supernova may wipe out a star, but the brightness definitely changes. 

Pulsating variables, on the other hand, are caused by a star actually expanding and contracting on a regular basis. Imagine something as large and complex as a star growing and then shrinking! It’s like the star is breathing. The reasons for this depend on the star’s age and size, and there are a lot of sub-types of pulsating variables, but I feel like we’ve gotten far enough into the weeds with categories. 

The Wonder Star 

In the official record, it took humanity quite a while to figure out that variable stars were a thing. It wasn’t until the late 1500s, which is still before telescopes were invented, that an astronomer named David Fabricius, which is a fabulous name, took note of a previously anonymous star in the constellation Cetus.

In 1596 he was observing Mercury (or so he thought—it was later suggested that what he thought was Mercury may have actually been Jupiter, which is a hilariously rookie mistake to make). Fabricius wanted a comparison star to help with his observations. He chose that anonymous star in Cetus, then watched as his randomly selected star proceeded to brighten significantly over several weeks before fading so much over the next months that he couldn’t see it anymore. 

It wasn’t until 1638 (after telescopes had been invented) that an astronomer named Johannes Holwarda figured out the star changed brightness over a roughly 11 month period, and it isn’t until 1662 that another astronomer named Johannes, Johannes Hevelius, is credited with naming the star. He named it Mira, the Wonderful. 

Today we know that Mira is a pulsating variable. In this specific case, it’s an old star, approaching the end of its life. It has swelled up into a red giant and doesn’t have much fuel left in the tank. It’s still good and hot near its core though, and the heat from those innermost layers cause the outermost layers to heat up and expand, swelling the star. Of course, once the outer layers are far from the heat-generating inner layers, they cool off. When they cool off, they contract, and the star shrinks again. The whole cycle takes 332 days.

Since Mira is the first variable of this type ever studied, this sort of pulsating variable is now known as a Mira variable. It’s of particular interest to us because it’s a stage our Sun is likely to go through in the final era of its life. Since that is billions of years in the future, we’re not going to be around to see it, so watching it happen to stars like Mira is the best we can do. 

Mira is officially the first ever known variable star. Unofficially, there’s plenty of evidence that ancient humans were aware of a different variable star millennia before Mira came to our attention. 

The Demon Star 

If you know the legend of Perseus (or if you’ve seen Clash of the Titans, in which case may I recommend the 1981 version featuring effects by the legendary Ray Harryhausen over the 2010 remake), then you know the story of how the hero Perseus cut off the head of the Gorgon Medusa, whose gaze could turn people to stone. He used this severed head to turn a great sea monster (often identified with Cetus, the constellation to which Mira belongs) to stone, thereby saving the princess Andromeda from being eaten and the city of Joppa from being wrecked.

These great feats earned him a place among the stars, where he is generally depicted as still holding Medusa’s severed head. And there’s one star in the constellation that specifically is meant to represent her evil, petrifying eye. Other cultures seem to have also associated this star with devils or with death. The Arabic name for this star, which is still in use, is Algol, the Demon’s Head or the Ghoul, often translated into English as the Demon Star. 

Given the number of peoples that apparently watched this star with suspicion, it seems likely that ancient folks knew something was different about this particular light in the sky. At both its brightest and its dimmest, Algol is easily visible to the eye, although its variation is not nearly as pronounced as Mira’s. 

Today we know that Algol is actually an eclipsing variable. While there are at least three stars in this system, and possibly as many as five, the variability come from the two brightest stars. Of these, the larger is actually the dimmer, and its orbit carries it directly between Earth and the smaller, brighter star, periodically blocking its light from Earth. 

When this happens, Algol appears to dim for about ten hours, before the smaller star emerges and Algol brightens again. The two stars are orbiting very tightly, so this happens roughly once every three days, giving ancient astronomers ample opportunity to notice the star’s light changing—or, you know, the Gorgon’s eye twinkling.We refer to this type of variable as (did you guess?) an Algol variable.

The lack of indisputable evidence confirming that ancient people knew about Algol’s variability means Mira still holds the title of first officially known variable star. But in my (thoroughly amateur and completely un-researched) opinion, it sure seems like this star earned itself a bad reputation amongst multiple ancient cultures somehow, and being a variable seems like a strong bet. 

Wonders and Demons 

The stories behind Mira and Algol have always fascinated me because they are so reflective of the eras in which they originated. Algol’s tale as something evil or demonic is an ancient one, a product of the long era of human history when the natural world was a terrifying mystery controlled by the whims of capricious deities, when the heavens were fixed and immutable and any change in them was an omen of malevolence. 

In contrast, Mira’s variability was discovered just on the cusp of the Enlightenment, when the secrets of the night sky were already being treated as mysteries with explainable solutions being teased out by the likes of Copernicus, Brahe, and Kepler. That a star could change its brightness was not a phenomenon to be feared but a new wonder to explore. 

We’ve solved a lot of those night sky mysteries in the centuries since Mira’s naming, including the ones behind Mira and Algol. But there are still wonders to explore. So bundle up to go take a look at the sky on the next clear night. Mira is near the dimmest point of its cycle, but Algol will be winking nice and bright. Go look for it. 

Or don’t. Just take the opportunity to stare at the stars. Bask in the spectacle that is our universe. It’s largely been stripped of demons but it’s still full of wonders and mysteries. Take a moment and enjoy it. You’ll thank yourself later. 

1. Artwork from Urania’s Mirror (1825) depicting the constellations Perseus (featuring the star Algol) and Cetus (featuring the star Mira). Credit: Wikimedia Commons

2. Telescope image showing a variable star at its brightest and dimmest. Credit: AAVSO

3. Diagram showing the brightness variation caused by an eclipsing binary star. Credit: NASA

4. The constellation Cetus with the star Mira marked. Image created by the author using Stellarium.

5. Chart showing the variations in the brightness of the star Mira over ten years. Credit: AAVSO International Database. 

6. The constellation Perseus with the star Algol marked. Image created by the author using Stellarium. 

7. Image showing artwork for the constellation Perseus, showing Algol’s position as the eye of Medusa. Image created by the author using Stellarium.