The Astronomy of Milton's 'Paradise Lost'

by Thomas Nathaniel Orchard

The most rapid of known revolving pairs is δ Equulei. The components are so close that only the finest instruments can separate them, and this they cannot do at all times. They accomplish a revolution in eleven and a half years. The slowest revolving pair is ζ Aquarii. The motion of the components is so tardy that to complete a circuit of their orbits they require a period of about sixteen centuries. Other binary stars have had different periods assigned to them; eleven pairs have been computed to revolve round each other in less than fifty years, and fifteen in less than 100 but more than fifty. There are other compound stars whose motions appear to be much more leisurely than those just mentioned, and although no orbital movement has, so far, been detected among them, yet, so vast is the scale upon which the sidereal system is constructed, that thousands of years must elapse before they can have accomplished a revolution of their orbits.

The Pole Star is an optical double, but the components are of very unequal magnitude. The Pole Star itself is of the second magnitude, but its companion is only of the ninth, and on account of its minuteness is regarded as a good test for telescopes of small aperture. Mizar, in the constellation Ursa Major, is a beautiful double star. The components are wide apart, and can be easily observed with a small instrument.

There is a remarkable star in the constellation of the Lyre (ε Lyræ), described as a double double. This object can just be distinguished by a person with keen eyesight as consisting of two stars; when observed with a telescope they appear widely separated, and each star is seen to have a companion, the entire system forming two binary pairs in active revolution. The pair which first cross the meridian complete a revolution in about 2,000 years; the second pair have a more rapid motion, and accomplish it in half that time. The two pairs are believed to be physically connected, and revolve round their common centre of gravity in a period of time not much under one million years.

Cor Caroli, in Canes Venatici, is a pleasing double star, the components being of a pale white and lilac colour.

Albireo, in the constellation of the Swan, is one of the loveliest of double stars. The larger component is of the third magnitude, and of a golden yellow colour; the smaller of the sixth magnitude, and of a sapphire blue.

ε Boötis, known also as Mirac, and called by Admiral Smyth ‘Pulcherrima,’ on account of its surpassing beauty, is a delicate object of charming appearance. The components of this lovely star are of the third and seventh magnitudes: the primary orange, the secondary sea-green.

The late Mr. R. A. Proctor, in describing a binary star system, writes as follows: ‘If we regard a pair of stars as forming a double sun, round which—or, rather, round the common centre of which—other orbs revolve as planets, we are struck by the difference between such a scheme and our own solar system; but we find the difference yet more surprising when we consider the possibility that in some such schemes each component sun may have its own distinct system of dependent worlds. In the former case the ordinary state of things would probably be such that both suns would be above the horizon at the same time, and then, probably, their distinctive peculiarities would only be recognisable when one chanced to pass over the disc of the other, as our Moon passes over the Sun’s disc in eclipses. For short intervals of time, however, at rising or setting, one or other would be visible alone; and the phenomena of sunset and sunrise must therefore be very varied, and also exquisitely beautiful, in worlds circling round such double suns. But when each sun has a separate system, even more remarkable relations must be presented. For each system of dependent worlds, besides its own proper sun, must have another sun—less splendid, perhaps (because farther off), but still brighter beyond comparison than our moon at the full. And, according to the position of any planet of either system, there will result for the time being either an interchange of suns, instead of the change from night to day, or else double sunlight during the day, and a corresponding intensified contrast between night and day. Where the two suns are very unequal or very differently coloured, or where the orbital path of each is very eccentric, so that they are sometimes close together and at others far apart, the varieties in the worlds circling round either, or around the common centre of both, must be yet more remarkable. “It must be confessed,” we may well say with Sir John Herschel, “that we have here a strangely wide and novel field for speculative excursions, and one which it is not easy to avoid luxuriating in.”’

Anyone who takes a cursory glance at the heavens on a clear night can readily perceive that there exists considerable diversity of colour among the stars. The contrast between some is pronounced and well marked, whilst others exhibit refined gradations of hue.

The most numerous class of stars are those which are described as white or colourless. They comprise about one-half of the stars visible to the naked eye. Among the most conspicuous examples of this type are Sirius—whose diamond blaze is sometimes mingled with an occasional flash of blue and red—Altair, Spica, Castor, Regulus, Rigel, all the stars of Ursa Major with the exception of one, and Vega—a glittering gem of pale sapphire, almost colourless. The light emitted by stars of this class gives a continuous spectrum, the predominating element being hydrogen, having a very elevated temperature and under relatively high pressure. The vapours of iron, sodium, magnesium, and other metals, are indicated as existing in small quantities.

The second class of stars is that to which our Sun belongs. They are of a yellow colour, and embrace two-thirds of the remaining stars. The most prominent examples of this type are Arcturus, Capella, Aldebaran, Procyon, and Pollux. Hydrogen does not predominate so much in these as in the Sirian stars, and their spectra resemble closely the solar spectrum, indicating that they are composed of elements similar to those which exist in the Sun.