HIBTOET OP PHYSICAL ASTROKOHV.

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Various new stars are recorded in history, which after having shonewith great splendour for a short time, then gradually faded away untilthey ceased to be visible. Thus Pliny asserts, that the appearance of anew star in the time of Hipparchus , about the year 180 a.d., was theoccasion of inducing that astronomer to construct his famous catalogueof the stars. A new star is said to have appeared in the reign of theEmperor Honorius , about the year 390 a.d. ; also one in. the reign of theEmperor Otlio, about the year 945 ; and again a phenomenon of a similarnature in the year 1264.

t Tn the year 1572, there appeared a splendid new star in the constel-lation of Cassiopeia, which is memorable for being the earliest of suchphenomena respecting which any authentic particulars have been recordedby astronomers. This star was observed by Tycho Brahe , who, in a longwork, written expressly on the subject, has given a detailed account ofthe various circumstances connected with its appearance; including acritical discussion of the speculations of his contemporaries on its pro-bable origin. It was first seen by him on the evening of the 11th ofNovember, 1572. It then surpassed in lustre the brightest of the fixedstars, and was even more brilliant than the planet Jupiter , althoughthen in opposition and near perihelion. It almost rivalled Venus ,and, like that planet, was seen by some persons even in the daytime.During the remaining part of November, it continued to shine withundiminished lustre ; but it subsequently began to decline, and it gra-dually grew fainter, until at length, in the month of March, 1574, itceased to be visible. The colour of this extraordinary object underwent asuccession of changes during the period of its appearance. When it firstbecame visible, it shone with a bright white light, like Venus or Jupiter .It then acquired a yellowish tinge; afterwards became ruddy, like Mars or Aldebaran; and finally exhibited a leaden hue, like the planet Saturn *.Tycho Brahe supposed it to be generated from the ethereal substance ofwhich he imagined the Milky Way to be composed, and to have beenafterwards dissipated by the light of the sun and the other stars, or tohave dissolved spontaneously from some internal causef. A more modernhypothesis has referred the origin of the phenomenon to some vast com-bustion, an explanation which receives some degree of support from thegradual change of colour which the light of the star exhibited.

A few years after the close of the sixteenth century, another splendidnew star burst forth in the constellation of Serpentarius. In this instancethe phenomenon was witnessed by many eminent astronomers, includingKepler , who wrote an interesting dissertation on the subject of its appear-ance It was first seen by that astronomer on the 17th of October,1604. It surpassed in brightness the stars of the first magnitude, aswell as the planets Mars, Saturn, and Jupiter , all of which were in itsvicinity. Like the star of 1572, it began to decline soon after its appear-ance, and finally ceased to be visible between October, 1605, and February',1606. Kepler was of opinion that it was generated from an etherealsubstance, not confined exclusively to the region of the Milky Way , asTycho Brahe had supposed in the case of the star of 1572, but pervadingall space. In connexion with this explanation of the origin of the star,he remarked, that the luminous ring observed around the dark body of

* Progymnasmata, p. 297. 1- Ibid., p. 795.

$ “ De Stella Nova in Pede Serpentarii,” 4to, Praga ), 1606.