History of Astronomy - George Forbes (top 20 books to read .TXT) 📗

806 double stars, counted the stars in 3,400 guage-fields, and compared the principal stars photometrically.

Some of the things for which he is best known were results of those accidents that happen only to the indefatigable enthusiast. Such was the discovery of Uranus, which led to funds being provided for constructing his 40-feet telescope, after which, in 1786, he settled at Slough. In the same way, while trying to detect the annual parallax of the stars, he failed in that quest, but discovered binary systems of stars revolving in ellipses round each other; just as Bradley’s attack on stellar parallax failed, but led to the discovery of aberration, nutation, and the true velocity of light.

Parallax.—The absence of stellar parallax was the great objection to any theory of the earth’s motion prior to Kepler’s time. It is true that Kepler’s theory itself could have been geometrically expressed equally well with the earth or any other point fixed. But in Kepler’s case the obviously implied physical theory of the planetary motions, even before Newton explained the simplicity of conception involved, made astronomers quite ready to waive the claim for a rigid proof of the earth’s motion by measurement of an annual parallax of stars, which they had insisted on in respect of Copernicus’s revival of the idea of the earth’s orbital motion.

Still, the desire to measure this parallax was only intensified by the practical certainty of its existence, and by repeated failures. The attempts of Bradley failed. The attempts of Piazzi and Brinkley,[1] early in the nineteenth century, also failed. The first successes, afterwards confirmed, were by Bessel and Henderson. Both used stars whose proper motion had been found to be large, as this argued proximity. Henderson, at the Cape of Good Hope, observed alpha Centauri, whose annual proper motion he found to amount to 3”.6, in 1832-3; and a few years later deduced its parallax 1”.16. His successor at the Cape, Maclear, reduced this to 0”.92.

In 1835 Struve assigned a doubtful parallax of 0”.261 to Vega (alpha Lyræ). But Bessel’s observations, between 1837 and 1840, of 61 Cygni, a star with the large proper motion of over 5”, established its annual parallax to be 0”.3483; and this was confirmed by Peters, who found the value 0”.349.

Later determinations for alpha2 Centauri, by Gill,[2] make its parallax 0”.75—This is the nearest known fixed star; and its light takes 4 1/3 years to reach us. The light year is taken as the unit of measurement in the starry heavens, as the earth’s mean distance is “the astronomical unit” for the solar system.[3] The proper motions and parallaxes combined tell us the velocity of the motion of these stars across the line of sight: alpha Centauri 14.4 miles a second=4.2 astronomical units a year; 61 Cygni 37.9 miles a second=11.2 astronomical units a year. These successes led to renewed zeal, and now the distances of many stars are known more or less accurately.

Several of the brightest stars, which might be expected to be the nearest, have not shown a parallax amounting to a twentieth of a second of arc. Among these are Canopus, alpha Orionis, alpha Cygni, beta Centauri, and gamma Cassiopeia. Oudemans has published a list of parallaxes observed.[4]

Proper Motion.—In 1718 Halley[5] detected the proper motions of Arcturus and Sirius. In 1738 J. Cassinis[6] showed that the former had moved five minutes of arc since Tycho Brahe fixed its position. In 1792 Piazzi noted the motion of 61 Cygni as given above. For a long time the greatest observed proper motion was that of a small star 1830 Groombridge, nearly 7” a year; but others have since been found reaching as much as 10”.

Now the spectroscope enables the motion of stars to be detected at a single observation, but only that part of the motion that is in the line of sight. For a complete knowledge of a star’s motion the proper motion and parallax must also be known.

When Huggins first applied the Doppler principle to measure velocities in the line of sight,[7] the faintness of star spectra diminished the accuracy; but Vögel, in 1888, overcame this to a great extent by long exposures of photographic plates.

It has often been noticed that stars which seem to belong to a group of nearly uniform magnitude have the same proper motion. The spectroscope has shown that these have also often the same velocity in the line of sight. Thus in the Great Bear, beta, gamma, delta, epsilon, zeta, all agree as to angular proper motion. delta was too faint for a spectroscopic measurement, but all the others have been shown to be approaching us at a rate of twelve to twenty miles a second. The same has been proved for proper motion, and line of sight motion, in the case of Pleiades and other groups.

Maskelyne measured many proper motions of stars, from which W. Herschel[8] came to the conclusion that these apparent motions are for the most part due to a motion of the solar system in space towards a point in the constellation Hercules, R.A. 257°; N. Decl. 25°. This grand discovery has been amply confirmed, and, though opinions differ as to the exact direction, it happens that the point first indicated by Herschel, from totally insufficient data, agrees well with modern estimates.

Comparing the proper motions and parallaxes to get the actual velocity of each star relative to our system, C.L. Struve found the probable velocity of the solar system in space to be fifteen miles a second, or five astronomical units a year.

The work of Herschel in this matter has been checked by comparing spectroscopic velocities in the line of sight which, so far as the sun’s motion is concerned, would give a maximum rate of approach for stars near Hercules, a maximum rate of recession for stars in the opposite part of the heavens, and no effect for stars half-way between. In this way the spectroscope has confirmed generally Herschel’s view of the direction, and makes the velocity eleven miles a second, or nearly four astronomical units a year.

The average proper motion of a first magnitude star has been found to be 0”.25 annually, and of a sixth magnitude star 0”.04. But that all bright stars are nearer than all small stars, or that they show greater proper motion for that reason, is found to be far from the truth. Many statistical studies have been made in this connection, and interesting results may be expected from this treatment in the hands of Kapteyn of Groningen, and others.[9]

On analysis of the directions of proper motions of stars in all parts of the heavens, Kapteyn has shown[10] that these indicate, besides the solar motion towards Hercules, two general drifts of stars in nearly opposite directions, which can be detected in any part of the heavens. This result has been confirmed from independent data by Eddington (_R.A.S., M.N._) and Dyson (_R.S.E. Proc._).

Photography promises to assist in the measurement of parallax and proper motions. Herr Pulfrich, of the firm of Carl Zeiss, has vastly extended the applications of stereoscopic vision to astronomy—a subject which De la Rue took up in the early days of photography. He has made a stereo-comparator of great beauty and convenience for comparing stereoscopically two star photographs taken at different dates. Wolf of Heidelberg has used this for many purposes. His investigations depending on the solar motion in space are remarkable. He photographs stars in a direction at right angles to the line of the sun’s motion. He has taken photographs of the same region fourteen years apart, the two positions of his camera being at the two ends of a base-line over 5,000,000,000 miles apart, or fifty-six astronomical units. On examining these stereoscopically, some of the stars rise out of the general plane of the stars, and seem to be much nearer. Many of the stars are thus seen to be suspended in space at different distances corresponding exactly to their real distances from our solar system, except when their proper motion interferes. The effect is most striking; the accuracy of measurement exceeds that of any other method of measuring such displacements, and it seems that with a long interval of time the advantage of the method increases.

Double Stars.—The large class of double stars has always been much studied by amateurs, partly for their beauty and colour, and partly as a test for telescopic definition. Among the many unexplained stellar problems there is one noticed in double stars that is thought by some to be likely to throw light on stellar evolution. It is this: There are many instances where one star of the pair is comparatively faint, and the two stars are contrasted in colour; and in every single case the general colour of the faint companion is invariably to be classed with colours more near to the blue end of the spectrum than that of the principal star.

Binary Stars.—Sir William Herschel began his observations of double stars in the hope of discovering an annual parallax of the stars. In this he was following a suggestion of Galileo’s. The presumption is that, if there be no physical connection between the stars of a pair, the largest is the nearest, and has the greatest parallax. So, by noting the distance between the pair at different times of the year, a delicate test of parallax is provided, unaffected by major instrumental errors.

Herschel did, indeed, discover changes of distance, but not of the character to indicate parallax. Following this by further observation, he found that the motions were not uniform nor rectilinear, and by a clear analysis of the movements he established the remarkable and wholly unexpected fact that in all these cases the motion is due to a revolution about their common centre of gravity.[11] He gave the approximate period of revolution of some of these: Castor, 342 years; delta Serpentis, 375 years; gamma Leonis, 1,200 years; epsilon Bootis, 1,681 years.

Twenty years later Sir John Herschel and Sir James South, after re-examination of these stars, confirmed[12] and extended the results, one pair of Coronæ having in the interval completed more than a whole revolution.

It is, then, to Sir William Herschel that we owe the extension of the law of gravitation, beyond the limits of the solar system, to the whole universe. His observations were confirmed by F.G.W. Struve (born 1793, died 1864), who carried on the work at Dorpat. But it was first to Savary,[13] and later to Encke and Sir John Herschel, that we owe the computation of the elliptic elements of these stars; also the resulting identification of their law of force with Newton’s force of gravitation applied to the solar system, and the force that makes an apple fall to the ground. As Grant well says in his History: “This may be justly asserted to be one of the most sublime truths which astronomical science has hitherto disclosed to the researches of the human mind.”

Latterly the best work on double stars has been done by S. W. Burnham,[14] at the Lick Observatory. The shortest period he found was eleven years (kappa Pegasi). In the case of some of these binaries the parallax has been measured, from which it appears that in four of the surest cases the orbits are about the size of the orbit of Uranus, these being probably among the smallest stellar orbits.

The law of gravitation having been proved to extend to the stars, a discovery (like that of Neptune in its origin, though unlike it in the labour and originality involved in the calculation) that entrances the imagination became possible, and was realised by Bessel—the discovery of an unknown body by its gravitational disturbance on one that was visible. In 1834 and 1840 he began to suspect a want of uniformity in the proper motion of Sirius and Procyon respectively. In 1844, in a letter to Sir John Herschel,[15] he