Over the years, however, more and more stars have been found to vary in brightness. Novae 1054, the source of M1, the Crab Nebula is the most famous. This did not include the 130 other novae that had been detected by that time. Surprisingly, by the year 1900, the total number discovered had only increased to 161. Since then, it has been monitored often enough that not a single maximum has been missed.
Good charts are essential for variable star work.
Careful brightness measurements are also essential. The brightest stars in the sky he called 1st mag, and the dimmest stars visible to the human eye, 6th magnitude. Observations were started by J. Herschel, and Argelander made some 15,000 visual brightness estimates. Magnitudes for 36,682 fainter stars, covering the entire sky, were published in other volumes. The advent of the photomultiplier in World War II led to a more sensitive electric photometer.