WE had occasion not long ago to speak of the interesting observation of Prof. Stebbins of the University of Illinois, who devotes his time to making very accurate measures of the brightness of the stars, with an apparatus depending upon the change in the electrical resistance of selenium when light falls on it. Not content with the study of known variable stars, Prof. Stebbins has taken up the work of discovery, with conspicuous success, finding two new variables in the first two stars observed. As, on the average, perhaps one star in a hundred, at most, varies in brightness, some principle must have guided such a successful search. Spectroscopic observation tells us that many bright stars are really close doubles, far too near together to be separated by the most powerful telescopes, but revealed by the approael or recession of the star as it swings in its orbit about the center of gravity of itself and a companion. In a few cases, such as Algol, the faint and relatively dark companion comes in front of the brighter star at every revolution and eclipses most of its light, causing conspicuous changes of its apparent brightness. The amount of light variation in such a case obviously depends on the character of the eclipse. If the companion passes almost squarely in front of the primary, it will eclipse the greater part of its area, and, indeed, may completely hide it if, as often happens, the former star is the larger. But if it passes apparently to one side of the brighter star, and, even at most, covers up but a small portion of the latter, only a little light will be lost by the eclipse, and the apparent variation will be small, and can be detected only by very accurate measures. Observing the brightness of selected spectroscopic binaries of this sort, Prof. Stebbins has found that the bright stars Beta Aurigae and Delta Orionis are eclipsing variables of the type just described. He defers full discussion of the latter until more observations have been secured; but his conclusions regarding the former are presented in definite form. Beta Aurigse was one of the first spectroscopic binaries discovered. Careful study by several observers has shown that the system consists of two stars of very nearly equal mass and brightness, revolving at equal distances on opposite sides of their common center of gravity, in a circular orbit, within a period of about an hour less than four days. Prof. Stebbins finds that, just at the times when, according to the spectroscopic data, either star ought to be in front of the other, the apparent brightness of the system diminishes by about 7 per cent. This shows that a partial eclipse takes place, one star covering up about one-seventh of the other, cutting off that fraction of its length, and hence one-fourteenth of the total light of the pair. These eclipses last a little less than six hours. Between the eclipses the brightness of the star is not perfectly constant, but increases by about 1 per cent until the time half-way between eclipses, and then falls again to its original value, just before the next eclipse. This may be explained by assuming that the mutual attraction of the two stars causes them to be slightly elongated toward one another, like two eggs with their narrow ends pointing at each other. When the stars are just out of eclipse, we see them end on. A day later, midway between eclipses, they are broadside on, and look correspondingly brighter. The amount of this elongation is, however, only 1 per cent of the whole diameter, which could hardly be detected on a drawing to scale. From the length and depth of the eclipses, it is possible to find the inclination of the orbit plane, and the diameters of the two stars, in terms of that of the orbit. The spectroscopic observations then furnisli us with a reliable scale of miles, and enable us to calculate the actual dimensions and mass of the system. What is more, in this case the parallax of the star is fairly well determined, both by direct measures and because it belongs to the group of stars moving together in space, of which the principal stars in Ursa Major are members. He can, there*-fore, compute the actual brightness of the stars, compared with the sun. At V/i o'clock: December 30. NIGHT SKY: DECEMBER AND JANUARY Combining all these facts, we reach a knowledge of the system more extensive than we at present possess for any other star. We may summarize it as follows: Beta Aurigae consists of a pair of stars of equal size and brightness and very nearly equal mass, which revolve about their common center of growth in circular orbits lying in a plane inclined 12 degrees 49 minutes to the line of sight, with a period of 3 days 23 hours 2 minutes and 30 seconds, and mutually eclipse one another at every revolution. The two stars are elongated toward one another by their mutual attraction to the extent of 1 per cent of their diameters, and, owing to the influence of the tides raised by each on the other, the period of revolution is increasing by seven-eighths of a second every year. The actual distance between the centers of the two stars is 7,630,000 miles, and each star is'2,200,000 miles in diameter. The more massive component has 2.38 times the mass of our sun, and the other 2.34 times. The whole system is at a distance of about ten million times that of the sun—so great that light takes 160 years to travel Over it. Each of the two stars gives out about 130 times as much light as our sun, from a superficial area a little less than seven times as great, so that per square mile, these stars send out nearly twenty times as much light as does the sun. The sun itself, if in front of one of them, would appear as a rather small and very dark spot on the star's surface. Though there is no other individual star about which we know nearly as much as this, it is probable, from various lines of evidence, that Beta Aurigae is a fairly good representative of a considerable class of the brighter stars—'which emphasizes the fact that our mighty sun is, after all, one of the smaller members of the host of heaven. The Heavens. Turning our gaze overhead, we find Auriga close to the zenith. The brilliant Capella may serve as our landmark. Beta Aurigae is close by on the east, almost in the line toward Castor and Pollux, which are conspicuous at an altitude of about 45 degrees. Procyon shines on the right, lower down, and Regulus, with the Sickle of Leo, is rising a little north of east. In the southeast is the magnificent group of constellations centering in Orion. Sirius, still lower down, but surpassing all other stars in brilliancy, so outshines the other stars of Canis Minor that it is only when we hide him from view that we realize that some of them are as bright as the stars of Orion's belt. High up near the zenith is Taurus, now made conspicuous by the ruddy Mars, which exceeds even Sirius in brilliancy. The constellation of the Hare and the Dove, below Orion, though small, contains more stars of fair brightness than does the vast extent of - the sky to the westward, occupied by the winding course of Eridanus. Cetus, farther to the right, is more conspicuous. Due west from the zenith we find Perseus, then Andromeda, and finally Pegasus. Between these last and Cetus are the little triangle of. Aries and the extensive but inconspicuous constellation Pisces. This is the faintest of the zodiacal constellations, except perhaps Cancer, but makes up for this by some faint resemblance to the two fishes bound together by a cord, which are its traditional likeness. This cord, or rather cords, start at Alpha Pisceum, a fairly conspicuous star of the fourth magnitude, pointed out by a line from a and through y Ceti. From this one line of faint stars runs northward toward S Andromedae to the Northern Fish, which is represented by a group of faint stars south of /3 Andromedae. Another and rather more conspicuous line of stars runs west from a Pisceum for about 30 degrees to the Southern Fish, whose body is marked by an irregular polygon of small stars south of the great square of Pegasus. Our initial letter shows the general arrangement of the constellation. In the northern sky, Cassiopeia and Cepheus are on the left of the Pole, Ursa Minor and Draco below, and Ursa Major coming up on the right. The Planets. Mercury is evening star until the 25th, and afterward morning star. He is apparently farthest from the sun on the 7th, but, being very far south, is unfavorably placed, and sets about 5:40 P. M. Venus is morning star, very conspicuous and' brll'-liant, rising about 3:10 A. M. on the 1st, and 4'A M. on the 31st. She passes eastward and southward from Virgo almost into Scorpio during the month, and, telescopically, appears like the Moon two days past the first quarter. Mars is in Taurus, just past opposition, and very bright. He is south of the Pleiades at the beginning of the month, and moves slowly westward. Telescop-ically he shows a disk about 19 seconds in diameter, upon which much detail may be seen under favorable atmospheric conditions. By the end of the month bis distance has increased from 49 to 64 millions of miles, and he appears correspondingly fainter and smaller. Jupiter is morning star in Scorpio, rising about 5:30 A. M. in the middle >of the month. Saturn is in Aries, well past opposition, but still conspicuous in the evening sky. Uranus is in Caipricornus, too near the Sun to be observed. Neptune is in Gemini, approaching opposition, but too faint to be seen without a telescope. The Moon is full at 10 P. M. on the 5th, in her last quaver at 1 P. M. on the 12th, new at 10 A. M. on the 20th, and in her first quarter at 2 P. M. on the 28th. She is nearest the Earth on December Gth, and farthest away on December 21st. In her circuit of the skies she passes near Saturn on the night 'Of the 3rd, Mars at 11 P. M. on the 4th, Neptune on the 8th, Venus on the 16th, Jupiter on the 18th, Mercury on the 21st, Uranus i on the 22nd, Saturn again on the 31st, ! and Mars once more about three hours after the new year begins. The two conjunctions with Mars are close. As seen from Washington, the planet is about 15 minutes south of the Moon's limb on the 4th, and, about 20 minutes north on Neiw Year's Eve. Princeton University Observatory.
The Heavens in December, 1911
This article was originally published with the title "The Heavens in December, 1911" in Scientific American 105, 23, 500 (December 1911)