<P> William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky . During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight . From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core . His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction . In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems . </P> <P> The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi . By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines--the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies . In 1865, Secchi began classifying stars into spectral types . However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s . </P> <P> The first direct measurement of the distance to a star (61 Cygni at 11.4 light - years) was made in 1838 by Friedrich Bessel using the parallax technique . Parallax measurements demonstrated the vast separation of the stars in the heavens . Observation of double stars gained increasing importance during the 19th century . In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion . Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104 - day period . Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S.W. Burnham, allowing the masses of stars to be determined from computation of orbital elements . The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827 . The twentieth century saw increasingly rapid advances in the scientific study of stars . The photograph became a valuable astronomical tool . Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude . The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals . In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory . </P> <P> Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century . In 1913, the Hertzsprung - Russell diagram was developed, propelling the astrophysical study of stars . Successful models were developed to explain the interiors of stars and stellar evolution . Cecilia Payne - Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis . The spectra of stars were further understood through advances in quantum physics . This allowed the chemical composition of the stellar atmosphere to be determined . </P>

Characteristic that is related to a star's temperature