A new star appeared in the centre of the great nebula in Andromeda in August 1885. The first announcement of the discovery was by Karl Ernst Albrecht Hartwig (born 1851), who observed the new star on August 31; but it had been previously seen by several other observers. On September 1 it was of the seventh magnitude, and by March of the following year had fallen to the sixteenth. Observed by Vogel, Young, and Hasselberg, the new star gave a continuous spectrum, but Huggins and Copeland succeeded in discerning bright lines. Hall, at Washington, undertook a series of measures to detect the parallax of Nova Andromedæ, but his efforts were unsuccessful.
The discovery of the next temporary star was announced February 1, 1892, by the Rev. Thomas D. Anderson, a Scottish amateur astronomer, in a post-card to the Astronomer-Royal of Scotland. The star was situated in the constellation Auriga. An examination of photographs, taken at Harvard Observatory, showed that the new star had appeared December 10, 1891, and had risen to a maximum of the fourth magnitude ten days later. On a photograph taken by Max Wolf on December 8 the new star was not visible. After Anderson’s visual discovery, the spectrum of the new star was studied by Copeland, Huggins, Lockyer, Vogel, Campbell, and others. Bright hydrogen lines were visible in the spectrum, which appeared to be actually double, giving support to the theory that the outburst was the result of a collision between two dark bodies; and this was confirmed by the measurements of radial motion by the Potsdam astronomers.
After March 9, 1892, the new star steadily faded, falling to the sixteenth magnitude on April 26. But on August 17 Edward Singelton Holden (born 1846), director of the Lick Observatory, and his assistants, Schaeberle and Campbell, found it of the tenth magnitude. On August 19 Barnard found it transformed into a planetary nebula: while various spectroscopic observations of the revived Nova revealed the nebular lines. By the end of 1894 the new star had faded to the eleventh magnitude, and early in 1901 was observed as a minute nebula.
After 1892 several new stars appeared, and were detected on photographic plates by Mrs Fleming (born 1857), of Harvard Observatory. The first of these, in the southern constellation Norma, was discovered in 1893 by its peculiar spectrum on a Draper spectrographic plate taken at Harvard. But the new star rose only to the seventh magnitude. Other new stars were discovered in Carina (Argo) in 1895, in Centaurus in 1895, in Sagittarius in 1898, and in Aquila in 1900. Nova Sagittarii was, at its brightest, fully equal to Nova Aurigæ, and was plainly visible to the naked eye, but was never observed visually.
A temporary star, appropriately designated “the new star of the new century,” blazed out in Perseus on the night of February 21, 1901. It was discovered independently by several observers: on February 21, by Borisiak, a student at Kiev, in Russia; on the following morning, by Anderson in Edinburgh; and on the next evening, by Gore at Dublin, Nordvig in Denmark, Grimmler at Erlangen, and other observers. When first seen by Anderson, it was equal to Algol, of the second magnitude. A photograph by Williams at Brighton showed that it must have been fainter than the twelfth magnitude on February 20. On the evening of February 23 the star was brighter than Capella, and was then the brightest star in the northern hemisphere. On February 25 it fell to the first magnitude; on March 1 to the second, and on March 6 to the third. During the spring and summer the light fluctuated considerably, but in September and October faded to the 6·7 magnitude. In March 1902 it was of the eighth magnitude, and in July 1903 of the twelfth.
The spectrum of Nova Persei was found by Pickering to be of the Orion type on February 22 and 23. On February 24 the spectrum had become one of the bright and dark lines, and the hydrogen lines indicated a velocity of 700 to 1000 miles a second. Measures of the sodium and calcium lines, by Campbell and others, indicated a velocity of only three miles a second, so that the displacements of the hydrogen lines may have been due to an outburst of hydrogen in the star. The spectrum was carefully studied during the spring and summer by Pickering, Lockyer, Huggins, Vogel, and others. On June 25 Pickering reported that the spectrum was slowly changing into that of a gaseous nebula. In August and September 1901 the nebular spectrum became more apparent.
In August 1901 Wolf at Heidelberg discovered a faint trace of nebula near the nova. On September 20 this nebula was photographed by George Ritchey at the Yerkes Observatory, and was seen to be of a spiral form. This was confirmed by Perrine, who also found, from plates taken in November, that the nebula was moving at the rate of eleven minutes of arc a year. This extraordinary velocity was exceedingly puzzling to astronomers, and at length Kapteyn suggested that the nebula shone only by reflected light from the new star, and that the apparent motion was an illusion caused by the flare of the explosion travelling out from the nova.
On March 16, 1903, Herbert Hall Turner (born 1861), Professor of Astronomy at Oxford, discovered a new star of the seventh magnitude in the constellation Gemini, from an examination of photographic plates. Photographs taken at Harvard showed that on March 1 it must have been fainter than the twelfth magnitude, while five days later it was of the fifth. In August 1903 Pickering found its spectrum nebular. In August 1905 another small nova was found by Mrs Fleming on the Harvard photographs, situated in Aquila.
Many theories have been advanced to account for temporary stars. Flammarion has shown that a body surrounded by a hydrogen atmosphere, on grazing a dark body enveloped in oxygen, would produce a tremendous explosion. In 1892 Huggins suggested that the outburst of Nova Aurigæ was due to the near approach of two bodies with large velocities, disturbances of a tidal nature resulting and producing enormous outbursts. Vogel suggested that the new star was due to the encounter of a dark star with a worn-out system of planets; while Lockyer believes all new stars to be due to the collision of swarms of meteors. Perhaps the most probable theory is that of Seeliger, which attributes these outbursts to the movement of a dark body through nebulous matter, which is extensively diffused throughout space. This theory explains the changes in the spectra as well as the revivals of brightness which characterised Nova Aurigæ and the fluctuations of Nova Persei. In a paper read to the Royal Society of Edinburgh in November 1904, the German astronomer, Jacobus Halm, of the Royal Observatory, Edinburgh, extended and developed Seeliger’s theory, showing also that the necessary consequence of such an encounter as the theory assumes is the formation of an atmosphere of incandescent gases, followed by that of a revolving ring of nebulous matter. In the hands of Halm, therefore, Seeliger’s theory leads to the nebular hypothesis as advanced by Laplace and Herschel.