The first known was Algol, the Ghoul or Demon Star in Perseus, and the brightest southern star of the Algol type is δ Librae. It shines steadily for a little more than two days as a fifth-magnitude star, then in a few hours drops suddenly to below sixth magnitude, becoming invisible to the naked eye, and as quickly recovers its usual brightness. The entire change takes place regularly in less than two and a half days. A few bright Algol stars in the south are: SOUTHERN ALGOL VARIABLES +---------------+----------+----------+------------+ | Name. | Maximum | Minimum | Period. | | |Magnitude.|Magnitude.| | +---------------+----------+----------+------------+ | | | |Days. Hours.| |RS Sagittarii | 5.9 | 6.3 | 2 9 | |R Canis majoris| 5.8 | 6.4 | 1 2 | |δ Librae | 4.8 | 6.2 | 2 7 | |R Arae | 6.8 | 7.9 | 4 9 | +---------------+----------+----------+------------+ Altogether nearly a hundred Algol stars are now known, and seventy-four of these lie in or near the Milky Way. Unlike the mysterious Mira stars, the variation of Algol stars has been explained. The sudden drop in the light is a partial eclipse, caused by a dark or partly dark companion which for a time hides the bright star from us. When a source of light is coming towards us, the lines in its spectrum are shifted towards the violet, when going away they are shifted towards the red, exactly as the whistle of an engine becomes more shrill when approaching us, and falls to a lower pitch when going away. In this way it has been discovered that an Algol star is revolving round an invisible companion, for it alternately approaches and recedes, and these movements correspond with its light-changes. It is, in fact, a spectroscopic binary which happens to have an orbit whose plane lies just in our line of sight, so that at every revolution one star passes behind the other. The speed of the star in its orbit can be accurately determined (by the amount of shift in the spectrum lines) in miles per second, even when we do not know its distance from us; hence the size of its orbit can be calculated, since we know the period in which it is completed; and, further, the size of the orbit gives us the mass of the stars, for their movements depend upon the attraction they exercise over one another, and this is proportional to their mass; and so we are able to picture the system, although the eclipsing star is never seen and the distance from us may never be known. Here is indeed a triumph of modern astronomy.