Historical Observations of Algol
1660s – 1920s

Seventeenth century

In 1669, Geminano Montanari, Professor of Mathematics at the Italian University of Bologna, found that the apparent brightness of Algol occasionally dropped appreciably below normal, and became so impressed by this curious phenomenon that he wrote a special pamphlet about it: “Sopra la sparizione d’alcune stelle e altre novit-celesti”, in Prose di Signori Academici Gelati di Bologna, 1671. The relevant passage is tantalizing by its incompleteness ―

The brightest star that shines in [the head of Medusa], affected by frequent mutations, attains but occasionally its greatest magnitude. I had observed it for several years to be of the third magnitude. It faded in 1667 to the fourth magnitude; in 1669 it recovered again its previous lustre up to the second magnitude, but in 1670 it exceeded only by little the fourth magnitude…

This was all that Montanari had to say in his report, mentioning neither the month nor the day of his observations. Twenty-five years later the variability of Algol was confirmed by G. F. Maraldi. Neither he nor Montanari seem to have had any inkling that the phenomenon they happened to discover was periodic in nature: this was not to be established until nearly a century later by a most unusual astronomer – John Goodricke, Junior, of York.

Eighteenth century

Goodricke was born at Groningen, The Netherlands, in 1764, as the grandson and heir of Sir John Goodricke of Ribston Hall, Yorkshire. He was deaf and dumb, and remained so throughout his unfortunately short life; but there was nothing the matter with either his eyesight or his brain, and he became an expert observer as well as a theorist. In spite of his handicap, he had shown every sign of becoming one of the leading astronomers of his time.

Goodricke made the first observations of a minimum in the brightness of Algol on 12th November, 1782, and with his friend Edward Pigott (the future discoverer of the variability of η Aquilae) assured himself shortly there after of the periodicity of the phenomenon. The two young men continued their observations through the rest of the season, and it was not until May 12th, 1783, that Goodricke communicated (through the offices of Reverend Anthony Sheperd, then Plumian Professor of Astronomy at Cambridge) the results to the Royal Society, in the form of a letter containing also some speculations as to the nature of the phenomenon.

This communication was read before the Society on 15th May, and created at once a considerable interest and excitement in astronomical circles- which prompted the Council of the Society to award to the youthful astronomer the Copley medal for 1783. Goodricke had discovered the first short-period variable ever known (all variables known in his day were long-periodic or irregular in nature) and established an estimate of its period differing from the true value by only four minutes. (A year later he revised his value to one of two days, twenty hours, forty-nine minutes and nine seconds – a result on which modern observations have had little to improve.)

Of especial interest, however, is the following from Goodricke’s letter to the Royal Society:

If it were not perhaps too early to hazard even a conjecture on the cause of this variation, I should imagine it could hardly be accounted for otherwise than… by the interposition of a large body revolving around Algol… But the intention of this paper is to communicate facts, not conjectures; and I flatter myself that the former are remarkable enough to deserve the attention and farther investigation of astronomers.

Goodricke’s bold suggestion that Algol was an eclipsing binary was, perhaps, made too early for it to gain ready acceptance. At least William Herschel was plainly non-committal: “The idea of a small Sun revolving around a large opake body has also been mentioned in the list of such conjectures”. However, he believed that he had sufficient reason for doubt, for, prior to his report to the Royal Society concerning “Observations on Algol”, he had repeatedly observed Algol with his 84" telescope and found it to be “distinctly single”. It was not until 1889 when Hermann Vogel, then Director of Potsdam Observatory, recognised Algol as a spectroscopic binary whose conjunctions coincide with the minima of light that the binary nature of the star was at last established beyond any reasonable doubt.

Thus Algol, the “demon star” of the Arabic astronomers, became the first known eclipsing binary system.

Nineteenth century

It had long been known that the interval between successive light minima of Algol varied minutely and apparently irregularly, but it was not until 1888, the year before Vogel made his spectroscopic observations, that the character of this variation was recognised: Chandler showed that the inequalities could be closely represented by an empirical periodic expansion involving three sine terms with periods of 130, 35 and 16 years.

To account for the long-periodic term, Chandler postulated that Algol was, in fact, a three-body system, about whose centre of mass the eclipsing pair revolved with a period of 130 years, in an orbit similar to that of Uranus about the Sun. However, in 1894 Bauschinger showed that Chandler’s corroborative evidence far the existence of the third mass, based on variations in the proper motion of the star, was apparently illusory.

In the following year Tisserand proposed the alternative hypothesis that the long-period variation was a result of apsidal motion (see Appendix), this revolution being due to the oblateness of one star of the eclipsing pair.

Early twentieth century

In 1908 Curtiss published the results of an investigation of data obtained from observations of Algol made by Belopolsky, Schlesinger and himself at Allegheny Observatory, Pennsylvania, between 1905 and 1907, and from earlier observations made by Belopolsky, at Pulkowa, and by Vogel and Scheiner, at Potsdam.

He determined a value for the eccentricity of the orbit of Algol A (the bright component) about the centre of mass of the eclipsing system consistent with the value e = 0.12 required by Tisserand’s theory of apsidal motion. His investigation also led him to propose the existance of a third member of the system separated from the eclipsing pair by a distance similar to the Earth’s distance from the Sun, which orbited the centre of mass of the system with a period of 1,899 years.

Curtiss suggested that the perturbations of the system due to the presence of this third mass would account for the observed variations in the period of the eclipsing pair. Such a 1.9 year variation was not given by Chandler since, as Curtiss noted, the nature of his observational data would have concealed any short period term of small amplitude.

Stebbins (1910) obtained a light curve for Algol from observations made with a selenium photometer. He noted the existence of a secondary minimum, with a depth of 0.06 magnitudes, which had not been seen by visual observers ; and the continuous variation of light between minima, with the maximum brilliancy occuring just before and after the secondary minimum, which he explained by supposing that the companion – Algol B was brighter on the face towards Algol A due to surface heating. In any case, the presence of the secondary minimum would, by itself, indicate that Algol B was not wholly dark, as had previously been supposed.

From the light curve, Stebbins obtained a value for the inclination of the orbit of the eclipsing pair of 82.3°. Making the assumption that Algol A was twice as massive as Algol B, he also computed the absolute dimensions [relative to those of the Sun, ☉], viz.:

rA, the radius of Algol A	1.45 r☉
rB,the radius of Algol B	1.66 r☉
mA, the mass of Algol A	0.37 m☉
mB, the mass of Algol B	0.18 m☉
ρA, the density of Algol A	0.12 ρ☉
ρB, the density of Algol B	0.04 ρ☉
ρAB, the mean density of Algol AB	0.07 ρ☉

In 1921 the same author published the results of a new series of observations made in 1919 and 1920 with a photoelectric instrument. The light curve obtained differed from that of 1910 only in the regions between the minima (see Fig. 2). Stebbins revised his values for the inclination of the orbit and the radii of the stars to i = 81.8°, r_A = 1.43 r_☉ and r_B = 1.69 r_☉.

Figure 2. Light curve of Algol, based on photometric (1910) and photoelectric (1921) observations. (After J. Stebbins [1910, 1921].)

Using instruments of different colour sensitivity, he obtained a colour index which indicated that Algol B was somewhat yellower than Algol A, whose spectral type had previously been determined to be B8. A study of the variation of the period (Stebbins, 1922) supported Curtiss’s suggestion of the presence of a third body. (Sahade and Wood [1978] erroneously credit Stebbins with being the first to suggest this.)

McLaughlin (1924) presented some of the results of a study of the Algol system based on 156 plates taken with a single-prism spectrograph of the 37-1/2" reflecting telescope at Detroit Observatory, Ann Arbor, Michigan, in 1913, 1920 and 1923. From 39 plates taken outside of eclipse (epoch 1923.66), he obtained the following preliminary elements of the orbit of Algol A:

e, the eccentricity of the orbit	0.038
T, the time of passage through the periapsis point (see Appendix)	1.506 days
ω, the arguement of the periastron (see Appendix)	277.5°
K, the semi-amplitude of the velocity curve	44.1 km s-1
γ, the systematic velocity	∼ 16.9 km s-1
a sini	1.736 x 106 km

(Notice that McLaughlin’s value for the eccentricity is incompatible with Tisserand’s theory of apsidal motion.) He also computed the preliminary elements of the long-period orbit of the centre of mass of Algol AB, viz.:

E, the epoch of minimum velocity	1901.85
P, the period	1.885 a
e	0.13
T	0.943 a
ω	0°
K	10.0 km s-1
γ	∼ 5.7 km s-1
a sini	9.3 x 107 km

Working from the assumption that Algol A was five times as massive as Algol B (which he made for spectroscopic reasons), McLaughlin determined the following dimensions of the eclipsing pair:

rA = 3.12 r☉	rB = 3.68 r☉
mA = 4.72 m☉	mB = 0.95 m☉
ρA = 0.16 ρ☉	ρB = 0.02 ρ☉
RAB, the mean separation of Algol A and B = 1.0522 x 107 km.

He considered these dimensions to be more probable than those computed by Stebbins (1910, 1921). From these dimensions and other data, McLaughlin was able to calculate a value for the parallax of Algol of 0.031", indicating the system to be 32 pc distant from the Sun.