40 stars of the 5th magnitude to make one of the 1st.
100 stars of the 6th magnitude to make one of the 1st.
10,000 stars of the 11th magnitude to make one of the 1st.
1,000,000 stars of the 16th magnitude to make one of the 1st.
323. The Number of the Stars.—The total number of stars in the celestial sphere visible to the average naked eye is estimated, in round numbers, at five thousand; but the number varies much with the perfection and the training of the eye and with the atmospheric conditions. For every star visible to the naked eye, there are thousands too minute to be seen without telescopic aid. Fig. 364 shows a portion of the constellation of the Twins as seen with the naked eye; and Fig. 365 shows the same region as seen in a powerful telescope.
Fig. 364.
Fig. 365.
Struve has estimated that the total number of stars visible with Herschel's twenty-foot telescope was about twenty million. The number that can be seen with the great telescopes of modern times has not been carefully estimated, but is probably somewhere between thirty million and fifty million.