Part 21

Double and multiple stars may be either optical or real. Optical double stars are those in which the component stars are merely apparently close together, owing to their being seen in nearly the same direction in space. Two stars may seem to be close together, while, in reality, one of them may be placed at an immense distance behind the other. Just as two lighthouses at sea may, on a dark night, appear close together when viewed from a certain point, whereas they may be really miles apart. In the case of double stars it is, of course, always difficult to determine whether the apparent closeness of the stars is real or merely optical. But when, from a long series of observations of their relative position, we find that one is apparently moving round the other, we know that the stars must be comparatively close, and linked together by some physical bond of union. These most interesting objects are known to astronomers as binary, or revolving double stars. The probable existence of such objects was predicted from abstract reasoning by Mitchell in the Eighteenth Century; but the discovery of their actual existence was made by Sir William Herschel, while engaged on an attempt to determine the distance of some of the double stars from the earth. Unlike the planetary orbits, which are nearly circular, at least those of the larger planets of the Solar System, it is found that the orbits of these double stars differ, in many cases, widely from the circular form, in some cases, indeed, approaching in shape more the orbit of a comet than a planet.

The binary stars are among the most interesting objects in the heavens. The number now known probably amounts to nearly one thousand. In most of them, however, the motion is very slow, and in only about seventy cases has the change of position, since their discovery, been sufficient to enable an orbit to be computed.

Savary, in 1830, was the first astronomer who attempted to compute the orbit of a binary star, namely, the star Xi Ursæ Majoris. This remarkable pair was discovered by Sir William Herschel in 1780, and as the period of revolution is about sixty-one years, a considerable portion of the ellipse had been described in 1830, when it was attacked by Savary.

The binary star with the shortest period known at present seems to be the fourth magnitude star Kappa Pegasi. It was discovered as a wide double star by Sir William Herschel in 1786, the companion star being of the ninth magnitude. In August, 1880, Mr. Burnham, the famous American double star observer, examining the star with the 18½-inch refractor of the Dearborn Observatory, found the brighter star to be a very close double, with a distance between the components of only a quarter of a second of arc. A few years’ observations showed that this pair were in rapid motion round each other (about eleven years).

Another binary star, with a period of about the same length, is Delta Equulei, which was discovered to be a close double by Otto Struve in 1851. Next in order of shortness of period comes the southern binary star Zeta Sagittarii, for which an orbit was first computed in the year 1886 by the present writer. The orbit of this star will, I think, require still further revision, but the period of about eighteen years is probably not far from the truth.

Another remarkably rapid binary star is 85 Pegasi. Next in order of rapidity of motion we have the southern binary star 9 Argûs.

The star 42 Comæ Berenices has a period of about 25¾ years, according to Otto Struve. The orbit is remarkable from the fact that its plane passes through, or nearly through, the earth, and is, therefore, projected into a straight line, the companion star oscillating backward and forward on each side of its primary.

The star Beta Delphini—the most southern of the four stars in the “Dolphin’s Rhomb”—is also a fast-moving binary, discovered by Burnham in 1873. Burnham thinks the period will prove to be about twenty-eight years. The spectrum of the light of Beta Delphini is similar to that of our sun, so that the two bodies should be comparable in intrinsic brilliancy.

Another remarkable binary star with a comparatively short period is Zeta Herculis. This pair have now performed three complete revolutions since their discovery in 1782 by Sir William Herschel. Several orbits have been computed, but Dr. See’s period of thirty-five years is probably the best. The companion is, however, rather faint, being only 6½ magnitude, while the primary star is of the third.

In the case of the binary star, Eta Coronæ Borealis, it was, some forty years ago, uncertain whether its period was forty-three or sixty-six years, but now that two complete revolutions have been performed since its discovery by Sir William Herschel in 1781, the question has been finally decided in favor of the shorter period.

The brilliant star Sirius is also an interesting binary star. The companion, which is relatively very faint—about tenth magnitude—was discovered by Alvan Clark in 1862. The existence of some such disturbing body was previously suspected by astronomers, owing to observed irregularities in the proper motion of Sirius. Several orbits, giving periods of about fifty years, have been computed. The great brilliancy of Sirius, the brightest star in the heavens, naturally suggests a sun of great size. Recent investigations do not favor this idea. Its spectrum is, however, of the first type, and the star is therefore not comparable with the sun in brilliancy. The above result would indicate that stars of the first, or Sirian type, are intrinsically brighter than our sun.

Sirius is about eleven magnitudes brighter than its faint companion. This makes the light of Sirius about 25,000 times the light of the small star. The two bodies must, therefore, be differently constituted, and, indeed, the companion must be nearly a dark body. If Sirius has any planets revolving round it—like those of our solar system—they must forever remain invisible in our largest telescopes. This remark, of course, applies to all the fixed stars, single and double. They may possibly have attendant families of planets, like our sun, but if so, the fact can never be ascertained by direct observation.

The star Zeta Cancri is a well-known triple star, the close pair revolving in a period of about sixty years. Nearly two revolutions have now been completed since its discovery by Sir William Herschel in 1781. All three stars probably form a connected system, but the motion of the third star round the binary pair is very slow and irregular.

Another interesting binary star is Xi Ursæ Majoris. As already stated, this was the first pair for which an orbit was computed. More than a complete revolution has now been performed since its discovery by Sir William Herschel in 1780. The period has, therefore, been well determined, and seems to be about sixty years.

The bright southern star, Alpha Centauri, the nearest of all the fixed stars to the earth, so far as is known at present, is also a remarkable binary star. It seems to have been first noticed as a double star by Richaud in 1690.

Assuming my value of the sun’s stellar magnitude (about 27), I find that the sun, if placed at the distance of Alpha Centauri, would appear of about the same brightness as the star does to us. As, according to Professor Pickering, the spectrum of Alpha Centauri is of the second or solar type, it would seem that in mass, brightness, and physical condition the star closely resembles our sun.

We next come to another very interesting binary star, known to astronomers as 70 Ophiuchi. It is a very fine double star, the magnitudes of the components being about four and six, and the colors yellow and orange. More than a complete revolution has now been described by the components since its discovery by Sir William Herschel in 1779. Placed at the distance indicated by Krüger’s parallax, I find that our sun would be reduced to a star of about magnitude 3½, which shows that the sun and star are of about equal brightness. The spectrum is of the solar type, according to Vogel.

A very famous binary star is that known to astronomers as Gamma Virginis. Its history is a very interesting one. It lies close to the celestial equator, about one degree to the south and about fifteen degrees to the northwest of the bright star Spica (Alpha of the same constellation), with which it forms the stem of a Y-shaped figure formed by the brightest stars of the constellation Virgo, or the Virgin, Gamma being at the junction of the two upper branches. The brightness of Gamma Virginis is a little greater than an average star of the third magnitude. Variation of light has, however, been suspected in one or both components. The Persian astronomer, Al-Sûfi, in his description of the heavens, written in the Tenth Century, rates it of the third magnitude, and describes it as “the third of the stars of al-auvâ, which is a mansion of the moon,” the first and second stars of this “mansion” being Beta and Eta Virginis, the fourth star Delta, and the fifth Epsilon, these five stars forming the two upper branches of the Y-shaped figure above referred to. Gamma was called Zawiyah-al-auvâ, “the corner of the barkers!” perhaps from its position in the figure, which formed the thirteenth Lunar Mansion of the old astrologers. It was also called Porrima and Postvarta in the old calendars. The fact that Gamma Virginis really consists of two stars very close together seems to have been discovered by the famous astronomer, Bradley, in 1718. The rapid decrease in the apparent distance from 1780-1834 indicated that the apparent orbit is very elongated, and that possibly the two stars might “close up” altogether, and appear as a single star even in telescopes of considerable power. This actually occurred in the year 1836, or, at least, the stars were then so close together that the most powerful telescopes of that day failed to show Gamma Virginis as anything but a single star. Of course, it would not have been beyond the reach of the giant telescopes of our day. From the year 1836 the pair began to open out again.

Another interesting binary star is Eta Cassiopeiæ. Periods ranging from 149 to 222½ years have been found by different computers. The most recent computation makes it about 196 years.

The bright star Gamma Leonis, situated in the well-known “Sickle in Leo,” is also a binary star, but only a small portion of the orbit has been described since its discovery by Sir William Herschel in 1782. Dr. Doberck finds a period of 407 years. It is remarkable for its very high “relative brightness.” This pair forms a fine object for a small telescope.

The star known as 12 Lyncis is a triple star, the components being 5, 6, and 7½ magnitude. The close pair forms a binary system, for which an orbit has been computed by the present writer, who finds a period of about 486 years. Sir John Herschel predicted in 1823 that the angular motion of the pair would “bring the three stars into a straight line in 57 years.” This prediction was fulfilled in 1887, when measures by Tarrant showed that the stars were then exactly in a straight line.

The bright star Castor is a famous double star, and has been known since the year 1718, when it was observed by Bradley and Pond. It was also observed by Maskelyne in 1759, and frequently by Sir William Herschel from 1799 to 1803. Numerous orbits have been computed, with periods ranging from 199 years by Mädler and 1,001 years by Doberck. I find that the mass of the system of Castor is only 1/19th of the sun’s mass, a result which would imply that the components are masses of glowing gas! Dr. Bélopolsky has found, with the spectroscope, that the brighter component is a close binary star with a dark companion, like Algol. The period of revolution is about three days, and the relative orbital velocity about 20¾ miles a second. Dr. Bélopolsky’s observations show that the system is receding from the earth at the rate of about 4½ miles per second.

With reference to the colors of the components of binary stars, the following relation between color and relative brightness has been established:

(1) When the magnitudes of the components are equal, or approaching equality, the colors are generally the same, or similar.

(2) When the magnitudes of the components differ considerably, there is also a considerable difference in color.

A new class of binary stars has been discovered within the last few years by means of the spectroscope. These have been called “spectroscopic binaries,” and the brighter component of Castor, referred to above, is an example of the class. They are supposed to consist of two component stars, so close together that the highest powers of the largest telescopes fail to show them as anything but single stars. Indeed, the velocities indicated by the spectroscope show that they must be so close that the components must forever remain invisible by the most powerful telescopes which could ever be constructed by man. In some of these remarkable objects, the doubling of the spectral lines indicates that the components are both bright bodies, but in others, as in Algol, the lines are merely shifted from their normal position, not doubled, thus denoting that one of the components is a dark body. In either case, the motion in the line of sight can be measured by the spectroscope, and we can, therefore, calculate the actual dimensions of the system in miles, and thence its mass in terms of the sun’s mass, although the star’s distance from the earth remains unknown. Judging, however, from the brightness of the star, and the character of its spectrum, we can make an estimate of its probable distance from the earth.

The bright star Spica has also been found by the spectroscope to be a close binary star. Vogel finds a period of four days with a distance between the components of about 6¼ millions of miles, and assuming that the components have equal mass and are moving in a circular orbit, he finds the mass of the system about 2.6 times the mass of our sun. In addition to its orbital motion, Vogel finds that Spica is approaching the sun at the rate of over nine miles per second.

To ordinary observers, the light of the stars seems to be constant. Even to those who are familiar with the constellations, the stars appear to maintain their relative brilliancy unchanged. To a great extent this is, of course, true; the great majority of the stars remaining of the same brightness from day to day, and from year to year. There are, however, numerous exceptions to this rule. Many of the stars, when carefully watched, are found to fluctuate in their light, being sometimes brighter and sometimes fainter. These are known as “variable stars”—one of the most interesting class of objects in the heavens. Some of these have been known for a great number of years, and their variations having been carefully watched, the laws governing their light changes have been well determined.