Algol (Beta Persei) has another name, namely the Demon Star. The main reason for this is that it forms the eye of Medusa the Gorgon in the constellation of Perseus. The ancients thought that they could see an outline in the night sky of Perseus, the Greek hero, slaying the monster who could turn a man to stone with her stare. It is a somewhat fanciful notion, but then so are most of the assumed constellations! Perseus is close to the W-shaped constellation of Cassiopeia, and is pointed to by the left-hand V.
However, Algol had another claim to fame, in the eyes of medieval astrologers, as being a star that implied danger and misfortune. The reason for this is its odd behavior. Far from shining steadily in the night sky, its brightness dims to 44 per cent of its usual brightness, every two days, twenty hours, forty-eight minutes and fifty-six seconds. This is not what most stars do, so there must surely be some devilry at work here!
The explanation was given in 1782 by John Goodricke, a young amateur astronomer from York, who was profoundly deaf and who died at the age of only 21. He not only measured the periodicity of the star but also surmised that it was caused by Algol being not one star but two, orbiting around each other and with one star eclipsing the other.
However, the second star is invisible to observers because it is outshone by its brighter companion. It has only been "seen" by virtue of spectroscopic observations of the spectral lines from the two stars as they orbit each other. As one star comes closer towards us and the other recedes, the spectral lines of each are shifted due to the Doppler effect (the same effect that is noted when the pitch of an approaching emergency vehicle siren rises as it approaches and sinks as it leaves), and can therefore be distinguished from each other. This calculation was only made as recently as 1978, at the McDonald Observatory in Texas.
So Algol is what is usually termed a double star (or binary system), meaning that two stars have been caught by each other's gravitational pull and have never been able to escape since they were formed in a stellar nursery (the Orion Nebula is an example of such a nursery). One of the stars, which is regarded as the primary star, emits virtually all the light that we see, whereas the secondary star, which is dark by comparison, obstructs the light from the primary star as it passes between it and our line of sight. Actually, the two stars orbit around a common center of gravity, making their orbits elliptical.
We now know that the system is about 100 light years away, and that the primary star is a white, hydrogen-burning star that is about 2.6 million miles in diameter and with a mass approaching four times that of the Sun, but shining a hundred times as brightly. For comparison, the Sun is approximately 0.86 million miles in diameter. The companion star is larger than the primary star, at 3 million miles, but with a mass and luminosity similar to that of the Sun.
The two stars are 6.5 million miles apart. If we could imagine the primary star of Algol as being where our Sun is, both stars would fit easily within the orbit of Mercury, which at an average orbit of 36 million miles is usually thought of as being virtually on top of the Sun (Earth's orbit is 93 million miles). The two stars of Algol are, in astronomical terms, practically touching each other.
Given that the "dark" star is larger than the "bright" one, the question arises as to why it does not completely eclipse its companion as opposed to merely reducing its light. The reason for this is that they are not in exactly the same plane as seen from Earth, and part of the surface of the primary star is always going to be visible.
Algol presents something of a puzzle, in that the two stars, which must be assumed to have been born at roughly the same time, have features suggesting that they are of very different ages. The primary star is a massive supergiant that is still burning hydrogen (as opposed to helium when all the hydrogen has been used up). Its maximum age can therefore only be about 100 million years. However, the secondary star appears to be on its way to becoming a red giant, having reached the stage that our own Sun will reach when it is about twice its present age. This suggests that the secondary star of Algol must be about 10,000 million years old. So how can the two stars have been born as twins?
The explanation, as suggested in 1955 by John Crawford, is that the secondary star is not what it seems. Indeed, it was once far more massive than it is now, probably even more massive than its companion, and it soon reached the stage at which it had burned all its hydrogen and was ready to become a red supergiant. However, as it grew it was distorted by the gravity of the other, very close, star and started to lose material to it, having become pear-shaped rather then globular. This process has continued, so that it has now lost so much mass that it only has as much as our own, much smaller, Sun. The two stars have swapped roles, with the original primary star becoming the secondary, and vice versa.
Algol, a "close binary", has some very interesting features that are shared by some, but by no means all, similar systems that have been investigated. For example, some binary systems have been discovered where one partner has become a black hole that is rapidly consuming its other half. Perhaps there are therefore other systems that are far more deserving of the "demon star" tag!