John Goodricke

The Discovery of the Occultating Variable Stars

John Goodricke (1764-1786), Astronomer

John Goodricke was born September 17, 1764 in Groningen as a son of a British diplomat and a dutch merchant daughter. With the age of five he got scarlet fever and lost his hearing abilities completely because of it. But after a proper education he was able to read lips and to speak. For this, the rich parents had sent him to a specialized school in Edinburgh. 1778 with the age of thirteen he was able to go to the academy in Warrington near York which had no special treatment or equipment for handicapped persons.

The discovery of the variable stars started with David Fabricius (1564-1617) in the year 1596, when he discovered Mira (aka omicron Ceti) as a star with non equal brightness. At this time this was a sensation, because the invariability of the sphere of fixstars was a strongly believed theorem. Nearly three quarters of a century later the Italian astronomer Geminiano Montanari (1633-1687) wrote down in 1670 that the second brightest star in the constellation of Perseus is changing its brightness.
The star has the Arabic name "Algol", which stands for "Head of the Ghul". In fact at the place of Algol in the picture of the constellation there is the head of the Medusa, which the Greek hero Perseus had killed with his mirror shield by her own look. With this head Perseus was able to petrify the seamonster Cetus, whom the princess Andromeda, daughter of Cassiopeia and Cepheus should be offered. Another name for Algol is "Devils Star". This name may indicate that maybe before Montanari Arabian astronomer knew the special nature of Algol.
More than a hundred years after the discoveries by Montanari John Goodricke looked to the stars with a somewhat modest equipment. The Warrington tutor William Enfield brought John Goodricke to astronomy, and the young man observed the sky together with his cousin Edward Pigott. According to his observations he was the first one to calculate the period of Algol to 68 hours and 50 minutes, where the star was changing its brightness by more than a magnitude as seen from Earth.
Goodricke was reporting this observations in 1783 at the British Royal Society, and for explaning these observations he proposed two theories: that the distant sun is periodically occulted by a dark body, or that the star itself has a darker region which is directing to Earth periodically because of the star rotation. With his first theory Goodricke is noted as the discoverer of the occultating variable stars in the history of astronomy. For his report he got the Godfrey Copley medal from the Royal Society for important scientific discoveries.

Algol or beta Perseï is a multi star system 96 lightyears away with two main components, where the central star is a massive, bright, white blue main class star (B8) with 3.7 solar masses at 2.9 times solar diameter and a 100 times higher absolute brightness than our Sun. The orbiting secondary star is a yellow red undersize giant star (K2) with 0.8 solar masses at 3.5 times the solar diameter and a three times higher absolute brightness than our Sun. Both stars have a distance of around eight solar diameter. This double star system is orbited by a third main class star (F1) in around two astronomical units. The nature of the Algol system were discovered through the spectroscopic analization of Algol's light applying the Doppler effect, postulated by Christian Andreas Doppler (1803-1853).
The central star is draining off mass from the secondary star, which collects in an accretition disk around the main sun and which is spiraling downward to the center. At the point where the plasma of the secondary star hits the accretition disk a hot spot exists, where the temperature might be more than 100,000 Kelvin. The rotation axis of Algol is only angled by 8 degrees to the view plane of the Earth, therefor the "dark body" postulated by Goodricke is the lesser bright star, partly occultating the brighter central star.
The brightness of Algol drops from 2^m1 to ca. 3^m4 at such an occultation for more than 9 hours. If the secondary star gets behind the main star the total brightness has another minimum, but this is only a fraction of a magnitude. - The third star of Algol has no influence to the brightness of the Devils Star, but in a period of 1.862 years the spectrum of the star system changes, which is indicating the presence of another massive orbiting body.


Goodricke looked for other variable stars and he found in 1783/4 with Sheliak aka beta Lyrae another one. The period of Sheliak the young astronomer calculated to 12 days and 20 hours. In the picture of the Lyra the star is directly below the main star of the constellation Vega. - The Lyra was originally the instrument of the winged messenger Hermes, but the singer Orpheus could become the owner of this magic lyre and with it he could free his loved Euridike out of the underworld.
The light curve of Sheliak is to find between 3^m3 and 4^m1, the true period is 12 days and ca. 21.8 hours. The secondary minimum at 3^m7 is more significant than for Algol, but the curve is also much more smoother than the one of Algol.

Sheliak is a double star system with two giant suns close together, so that they are deforming each other and exchanging mass, which is ejected within a huge accretition disk around the stars. This disk is darkening both of the stars, but mostly the smaller, hotter, normally brighter star. The star system also might have at least three more companion stars and is 860 to 1000 lightyears away.
The brightness change of Sheliak is realized by the deformation of the starry atmosphere. If a deformed star is presenting its broad side to the viewer it looks brighter than if the top side is shown. Also because of the difference in visible brightness of the two stars, the light curve of Sheliak is a combination of occultation and deformation brightness change. The main minimum takes place when the darker red giant shows its top side and the secondary minimum takes place when the smaller brighter star is standing before the bigger star.

Goodricke discovered also the variable star Altais or better known as delta Cepheï. He calculated the period of this star to 128 hours and 45 minutes with an outstanding correctness. The visible brightness of the star is varying between 3^m6 and 4^m4 within this time span. But Goodricke's theories about the reason of the variability came to an end on this star.
The star delta Cepheï became name giving for an own class of massive suns - the delta-Cepheï stars or Cepheïds. Because of inner thermonuclear processes and an interaction between gravity and radiation pressure they blow up periodically and shrink in defined time spans. They not only change their size, but also their color, surface temperature and brightness in the same period. The brightness variation has a typical shark fin form.
John Goodricke was admitted to the Royal Society at April the 16th 1786 already when 21 years old. He didn't recognized this honor anymore, because he died in 1786 at April the 20th in York by pneumonia. One of the halls of the York university is named after him in remembrance.