Algol: The Maliciously Winking Demon Star

Shimmering within the high reaches of the northern skies, Perseus shines like a beacon adorning the September heavens, each of its stars pulsating in a plight of endless light. The great warrior’s brightest star, Mirfak (Alpha Persei), is a yellow supergiant of stunning grandeur that leads a cluster across the aether. Additionally, Perseus possesses Menkib (Xi Persei), a shining sapphire that flanks the famed California Nebula. A large array of astronomers wouldn’t believe a constellation of such light could bear such a malicious member, yet conversely, they’d be wrong. Dangling upon the edge of the combatant’s outline lies his nictating swindler, a malefactor misleading astronomers for centuries, cackling as it coruscated above the heads of stumped scientists. The outlier of the skies, frowned upon by cultures across the globe for its unexpected behavior, may simply be misunderstood by the scientists who find it taboo. Perhaps our stellar scoundrel’s true goal is to guide us, so long as we play its cosmic game.

Algol (Beta Persei), also known as “The Demon Star”, is the second-brightest star in Perseus and the sixty-first brightest in our entire night sky. Whilst Algol is a binary system composed of three different stars (which I’ll cover later in this article), its primary and brightest component is of spectral class B8V, meaning it is a blue-white main sequence star with a temperature around 13,000 K (12,725 Celsius). It has 182 solar luminosities, meaning it emits 182 times as much light as our sun. Furthermore, if the Sun’s luminosity was equivalent to one foot in height, Algol would be just six feet shorter than the Leaning Tower of Pisa in Italy. Conversely, what makes Algol so special isn’t its own properties, but how it functions as a binary star system. In order to fully understand the demon star’s significance, let’s rewind to just what eclipsing binaries are, and how they tie into “Algol Variables”.

An eclipsing binary is a system of two stars that orbit each other around a center of mass, consequently blocking each other’s light from Earth. Eclipsing binary systems consist of two stars, one being larger than the other in solar radii, consequently making some eclipses shield more light than others. When the smaller star of the two goes in front of the brighter one, it is called the primary eclipse, blocking out the most light during the star’s variable cycle. When the larger star passes in front of the smaller one, completely shielding it from view, it is called a secondary eclipse. You can imagine this by placing your fists in front of your face, parallel to your ears, and having one pass in front of the other so only one fist is visible to you. That’s how stars in eclipsing binary systems block out each other’s light, as seen in the diagram below. Algol variables are a peculiar type of eclipsing binary star that have coined their own designation with a separate set of rules and regulations.

An Algol Variable is a special type of eclipsing binary in which the star with more mass is still on the main sequence, and the star with less mass is farther along in evolution (a subgiant or giant star). The existence of Algol Variables consequently contradicted the theories of stellar evolution, coining the “Algol Paradox”: when the masses of two stars oppose the theories of stellar evolution. Responsively, the puzzle was hastily pieced together by the theory of mass transfer between two stars, which I’ll explain from the beginning. When the less massive star in the Algol System originally swelled into a subgiant, it was larger than the primary blueish star but filled its Roche lobe and began to transfer its mass to the other star. If you don’t know what that means, allow me to explain.

Stars are spherically shaped thanks to the forces of gravity compacting their particles to be that way. Moreover, since stars have so much mass, they develop a gravitational force of their own, which has the ability to attract other stars. In binary systems, since both stars have their own gravitational pull, they end up tugging at each other’s mass whilst waltzing around each other. It’s like two spherical magnets orbiting each other in a funnel, and therefore tugging at each other, but not quite collapsing into each other. Conversely, if one star approaches too close to the other or accumulates too much mass, it will transfer its composition to the other member. The Roche lobe makes this process much easier to explain, which I’ll display now.

In binary systems, a Roche lobe is the area around a star in which its mass is still gravitationally bound to it. Stars in binary systems are in a gravitational tug of war, with their own gravity fighting against the other member’s gravity. The vast majority of binary systems are known as detached binary systems (top image), where both stars are well within their Roche lobes and do not transfer any mass to each other. Algol variables, on the other hand, are systems in which one star is well within its Roche lobe, but the other has filled it. When a star fills its Roche lobe, the gravitational attraction of the other star in the binary system is much stronger than the gravitational force holding it together, causing it to lose its mass to the other star in the system. In the case of Algol Variables, the main-sequence star has taken so much mass from the subgiant/giant component that it has grown bigger, therefore making it an Algol Variable rather than a regular eclipsing binary. But how do we know that Algol is like this, and other stars behave the same way too? It takes a brief science history lesson to truly understand.
Algol was first historically documented 3,200 years ago in Ancient Egypt, however, its pulsations aren’t assumed to have been (debatably) observed until ancient Greece and Arabia, who depicted Algol as a gorgon/ghoul. Ordinarily, this is no direct proof of Algol’s variability being recorded, with the first verified documents of it being made by Italian astronomer Gemimiano Montanari, whose diary was unfortunately lost. Thankfully, in 1888, astronomer Francesco Porro found handwritten notes from one of Montanari’s students, proving he didn’t know Algol was a binary star and simply revealed that it was an ordinary pulsator. The discovery of Algol being a multi-star system was made by British astronomer John Goodricke, who first proposed it dimmed thanks to a dark object passing in front of it and was awarded the Copley Medal for his discovery, becoming its youngest ever recipient at nineteen years old. Unfortunately, just four days after Goodricke was elected a member of the Royal Society of Scientists, he tragically died of pneumonia at the heartbreaking age of twenty-one. A century after Goodricke’s untimely death, American astronomer Edward Charles Pickering deduced that Algol did not eclipse thanks to a dark object passing by, but because it was an eclipsing binary system. German astrophysicist Hermann Carl Vogel confirmed this just seven years later, making Algol one of the first spectroscopic binaries (a binary system that cannot be observed with the naked eye, but with astronomical equipment finding evidence of orbital activity) ever discovered. Observations by the American astronomer Joel Stebbins later led to the discovery of a third star in the system, finally rendering our astronomical image of Algol accurate after three centuries of observation (and the Algol Paradox being debunked in the 20th century).

With the kinematics of Algol Variables being established, we can finally cut to the star Algol’s own properties. The blueish-white primary star is known as Algol Aa1, and its subgiant companion (of spectral type K0IV, making it orange in a hue similar to that of Arcturus and Capella) is known as Aa2. The two stars orbit each other within a 2.8-day period, meaning the stars take 2.8 days to orbit around each other once. The system has an eccentricity of zero, meaning its orbit is nearly in a flawless circle, though the orbit’s semi-major axis (a satellite’s farthest point from what it is orbiting, whether another celestial object or a center of mass) is very slightly farther out than the semi-minor axis (a satellite’s closest point to what it is orbiting, whether another celestial object or a center of mass). Though the Algol system’s apparent magnitude (a star’s brightness as viewed from Earth) is usually 2.12, it can dip as low as 3.39 (a lower number means the object is brighter).

Algol Aa1 and Aa2 are orbited by a third companion, Algol Ab, a blueish-white main-sequence star (spectral class A5V; Wikipedia incorrectly cites it as F1V) circling the two primaries with an orbital period of just under two years. Algol Ab’s orbit has a slight eccentricity of 0.2 (lower means more circular), meaning it is perfectly circular notwithstanding a slight oblation. Algol Ab has been cited as an Am star in various articles (a peculiar type of white star that radiates heavy metals such as zinc and barium, although its composition is still relatively unknown to astronomers. Additionally, the impacts that Algol Ab has on the main system are also quite unknown, though it is highly suspected that it has little to no impact.

And so, while Algol was one slandered with a defaming nickname and terrible reputation, modern astronomers now believe it to be a gold mine for discovery and knowledge of the universe. With an increased understanding of Algol, astronomers have therefore picked up the pieces to other similar mysteries in their field, improved their stellar classification systems, and made many more groundbreaking discoveries in their field. All thanks to Algol, whose once-execrated charm has now revolutionized stellar astronomy for centuries.