He observed the Andromeda "Nebula" (M31) in 1661, but according to Allen (1899/1963), he expressedly mentioned that this "Nebula" had been observed 150 years earlier by some anonymous but expert astronomer. Nevertheless, Edmond Halley accounted the discovery of this object to him in his 1716 treat of Nebulae.

Because the "nebula" had not been documented by so many famous astronomers, Boulliau suspected that it might been variable, similar to Mira Ceti which he also studied, and attempted to calculate a period of variation - of 333 days, compared to the modern value of 332.

He died in 1694 in Paris.

The astronomical community has named a Moon crater to his honor: Bullialdus (20.7S, 22.2W, 60 km diameter, in 1935).

References

• Allen, Richard Hinckley, 1899. Star-Names and their Meaning. G.E. Stechert. Republished corrected in 1963 as Star Names, Their Lore and Meaning, Dover, New York. Here: p. 39.
• Halley, Edmond, 1716. An Account of several Nebulae or lucid Spots like Clouds, lately discovered among the Fixt Stars by help of the Telescope. Phil. Trans., Vol. 29 (1714-1716), Num. 347 (Jan-Mar 1716), p. 390-392.
• Kenneth Glyn Jones, 1968. The Search for the Nebulae -- II. Journal of the British Astronomical Association, Vol. 78, No. 5 (1968), p. 360-368. Section on Bouillaud: p. 364-366. Reprinted in: The Search for the Nebulae. Chalfont St. Giles, 1975.

• Project Boulliau. Short biography, Correspondence Network, letters, and other topics, by R.A. Hatch
• Biography and References (Biographies of Mathematicians)
• Ismael Boulliau Biography (Galileo Project)