Portion of the Sun’s Surface. Sunspot nearly 60,000 Miles Across
It is a curious fact that Al-Sûfi, the Persian astronomer, in his Description of the Heavens, written in the Tenth Century, speaks distinctly of Algol as a red star (étoile, brillant; d’un éclat, rouge), while at present it is white or at the most of a yellow color. A similar change of color is supposed to have taken place in the case of Sirius, but the change in Algol seems more certain, as Al-Sûfi’s descriptions are generally most accurate and reliable.
Stars of the Algol type of variable are very rare objects, only a dozen or so having been hitherto discovered in the whole heavens. Those visible to the naked eye, when at their normal brightness, are: Algol, Lambda Tauri, Delta Libræ, R Canis Majoris, and U Ophiuchi.
A remarkable peculiarity about the variable stars in general is that none of them has any considerable proper motion. As a large proper motion is generally considered to indicate proximity to the earth, we may conclude, with great probability, that the variable stars, as a rule, lie at a great distance from our system. In other words, it appears that the sun does not lie in a region of variable stars, and, with the exception of Alpha Cassiopeiæ and Alpha Herculis, a measurable parallax has not yet been found, so far as I know, for any known variable star.
We now come to the interesting and mysterious class of objects known as “new” or “temporary” stars. These phenomena are of very rare occurrence, and but few undoubted examples of the class are recorded in the annals of astronomy. Possibly in some cases they have been merely variable stars, of irregular period and fitful variability; but others may have been due to a real catastrophe, such as the collision of two dark bodies in space, or, possibly, the passage of a bright or dark body through a gaseous nebula.
The earliest temporary star of which we have any reliable information seems to be one which is recorded in the Chinese annals of Ma-tuan-lin, as having appeared in the year 134 B. C. in the constellation Scorpio. Its position seems to have been somewhere between the stars Beta and Rho of Scorpio. Pliny informs us that it was the sudden appearance of a new star which induced the famous astronomer Hipparchus to form his catalogue of stars, the first ever constructed. As the date of Hipparchus’s catalogue is 125 B. C., it seems highly probable that the new star referred to by Pliny was the same as that recorded by the Chinese astronomer as having appeared nine years previously.
A new star is said to have appeared in the year 76 B. C. between the stars Alpha and Delta in the Plow, but the accounts are vague.
In 101 A. D., a small “yellowish-blue” star is said to have appeared in the “sickle” in Leo, but its exact position is not known. In 107 A. D., a new star is mentioned near Delta, Epsilon and Eta in Canis Major, three bright stars southeast of Sirius. In 123 A. D., another new star is recorded by Ma-tuan-lin to have appeared between Alpha Herculis and Alpha Ophiuchi.
The Chinese annals record that on December 10, 173 A. D., a brilliant star appeared between Alpha and Beta Centauri in the Southern Hemisphere. It remained visible for eight months, and is described as resembling “a large bamboo mat!”—a curious description. There is at present, close to the spot indicated, a known variable star—R Centauri—of which the period seems to be long and the variation of light irregular. Possibly an unusually bright maximum of this variable star formed the star of the Chinese annals, or perhaps the variable star is the remnant of the outburst which took place in the First Century. The variable is a very reddish star, and at present varies from about the sixth to the tenth magnitude.