Note 3 — Chapter II · THE RIDDLE OF THE MILKY WAY

Up to a short time ago Alpha Centauri was considered the fixed star nearest to the Sun. By comparing old photographs of the firmament with such of recent date the renowned astronomer Barnard found that a very small fixed star—of the magnitude 10.5 and therefore far from visible to the naked eye—in the constellation Ophiuchus (Right ascension 17 h. 58 m. 44 s., North declination 4° 27´.4 January 1, 1917) possesses a very large proper motion. It traverses in a year 10.3 seconds on the firmament. The distance to this star, which has the largest proper motion so far known, was later determined to be 3.3 light years or 3/4 of the distance from Alpha Centauri to our Sun. Hence its velocity at right angle to the line of vision is computed to be 49 km. (32 miles) per second. Spectroscopic measurements show that it approaches us with a velocity of 91 km. (56.5 miles) per second along the line of vision. The combined velocity, therefore, is 103 km. (63 miles) per second, an unusually high value. The value of 3.3 light years used in this calculation was determined by the French astronomer Gonnessiat, who found it by the study of old photographs from Algeria. He also calculated the parallax of this star to be 1 second. According to later measurements, given in the Harvard Bulletins 616 and 617, its parallax is only 0.7 seconds and consequently its distance 4.6 light years and its speed perpendicularly to the line of vision 70 km. (43.5 miles) per second. Campbell, in the Lick Observatory, had determined its radial velocity and found that it approaches the Sun at the rate of 128 km. (79 miles) per second. Its total velocity is, according to these two last determinations, 146 km. (91 miles) per second.

It is by no means improbable that similar discoveries will be made in the future, so that the Sun will be found to have more stars in its “immediate” vicinity than previously assumed.

4 American and French numeration; the Swedish and English usage is billion.