Variable stars had been known for 200 years when Laplace wrote Exposition du systeme du monde (1796). The first to be discovered was Mira in the constellation of Cetus. It was discovered by the German astronomer David Fabricius in 1596 and had a period of 331 days fluctuating between magniture 9 (well below being able to be seen with the naked eye) to magnitude 3 which is clearly visible to the naked eye. Stars which make a sudden appearance (a nova or supernova) were had been seen long before this. Supernovae were observed in 185, 393, 1006, 1054, 1181, 1572, and 1604. The supernova of 1054 appeared on 4 July of that year and was recorded by the Chinese astronomers. For three weeks it was so bright that it was visible in daylight and it remained visible until April 1056. Tycho Brahe observed a supernova on 11 November 1572. He wrote:-
Laplace on "new stars"
... directly overhead a certain strange star was seen, flashing its light with a radiant gleam.Again it was bright enough for a few days to be visible in daylight. It remained visible for about 16 months.
Laplace wrote about supernovae in Exposition du systeme du monde in 1796:-
... some stars have suddenly appeared, and then disappeared, after having shone for several months with the most brilliant splendour. Such was the star observed by Tycho Brahe in the year 1572, in the constellation. Cassiopeia. In a short time it surpassed the most brilliant stars, and even Jupiter itself. Its light then waned away, and finally disappeared sixteen months after its discovery. Its colour underwent several changes; it was at first of a brilliant white, then of a reddish yellow, and finally of a lead-coloured white, like to Saturn.