The northern of the two “pointers” in the Plough (so called because they nearly point to the Pole Star) is about the 2nd magnitude, as Al-Sufi rated it. It was thought to be variable in colour by Klein, Konkoly, and Weber; and M. Lau has recently found a period of 50 days with a maximum of “jaune rougeâtre” on April 2, 1902.
The famous variable star η Argus did “not exceed the 8th magnitude” in February, 1907, according to Mr. Tebbutt.[342] This is the faintest ever recorded for this wonderful star.
It is stated in Knowledge (vol. 5, p. 3, January 4, 1884) that the temporary star of 1876 (in the constellation of Cygnus) “had long been known and catalogued as a telescopic star of the 9th magnitude with nothing to distinguish it from the common herd.” But this is quite erroneous. The star was quite unknown before it was discovered by Schmidt at Athens on November 24 of that year. The remark apparently refers to the “Blaze Star” of 1866 in Corona Borealis, which was known previously as a star of about the 9th magnitude before its sudden outburst on May 12 of that year.
This “new star” of 1866—T Coronæ, as it is now called—was, with the possible exception of Nova Persei (1901), the only example of a nova which was known to astronomers as a small star previous to the great outburst of light. It is the brightest of the novæ still visible. It was the first of these interesting objects to be examined with the spectroscope. It was observed by Burnham in the years 1904-1906 with the great 40-inch telescope of the Yerkes Observatory (U.S.A.). He found its colour white, or only slightly tinged with yellow. In August and September, 1906, he estimated its magnitude at about 9·3, and “it would seem therefore that the Nova is now essentially of the same brightness it was before the outburst in 1866.” It shows no indication of motion. Burnham found no peculiarity about its telescopic image. A small and very faint nebula was found by Burnham a little following (that is east of) the nova.[343]
The following details of the great new star of 1572—the “Pilgrim Star” of Tycho Brahé—are given by Delambre.[344] In November, 1572, it was brighter than Sirius, Vega, and Jupiter, and almost equal to Venus at its brightest. During December it resembled Jupiter in brightness. In January, 1573, it was fainter and only a little brighter than stars of the 1st magnitude. In February and March it was equal to 1st magnitude stars, and in April and May was reduced to the 2nd magnitude. In June and July it was 3rd magnitude; in September of the 4th, and at the end of 1573 it was reduced to the 5th magnitude. In February, 1574, it was 6th magnitude, and in March of the same year it became invisible to the naked eye.
From this account it will be seen that the decrease in light of this curious object was much slower than that of Nova Persei (1901) (“the new star of the new century”). This would suggest that it was a much larger body.
There were also changes in its colour. When it was of the brightness of Venus or Jupiter it shone with a white light. It then became golden, and afterwards reddish like Mars, Aldebaran, or Betelgeuse. It afterwards became of a livid white colour like Saturn, and this it retained as long as it was visible. Tycho Brahé thought that its apparent diameter might have been about 3½ minutes of arc, and that it was possibly 361 times smaller than the earth(!) But we now know that these estimates were probably quite erroneous.
Temporary stars were called by the ancient Chinese “Ke-sing,” or guest stars.[345]
A temporary star recorded by Ma-tuan-lin (Chinese Annals) in February, 1578, is described as “a star as large as the sun.” But its position is not given.[346]
About the middle of September, 1878, Mr. Greely, of Boston (U.S.A.), reported to Mr. E. F. Sawyer (the eminent observer of variable stars) that, about the middle of August of that year, he had seen the famous variable star Mira Ceti of about the 2nd magnitude, although the star did not attain its usual maximum until early in October, 1878. Mr. Greely stated that several nights after he first saw Mira it had faded to the 4th or 5th magnitude. If there was no mistake in this observation (and Sawyer could find none) it was quite an unique phenomenon, as nothing of the sort has been observed before or since in the history of this famous star. It looks as if Mr. Greely had observed a new or “temporary” star near the place of Mira Ceti; but as the spot is far from the Milky Way, which is the usual seat of such phenomena, this hypothesis seems improbable.