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Astronomers describe star brightness in terms of .

The first direct measurement of the distance to a star ( at 11.4 ) was made in 1838 by using the 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. discovered the first in 1899 when he observed the periodic splitting of the spectral lines of the star in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as and , allowing the masses of stars to be determined from computation of . 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 became a valuable astronomical tool. discovered that the color of a star and, hence, its temperature, could be determined by comparing the against the . The development of the allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 made the first measurements of a stellar diameter using an on the at .

The science of was pioneered by and . By comparing the spectra of stars such as to the Sun, they found differences in the strength and number of their —the dark lines in a stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865 Secchi began classifying stars into . However, the modern version of the stellar classification scheme was developed by during the 1900s.

The mass of a star affects surface temperature.

The closest star to the sun is Proxima Centauri, but its brighter neighbor Alpha Centauri, a double star, is so nearly the same distance that data about it are usually given. It is about 4 light years distance, or about 3.8x1016 m.

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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 . His son 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 systems.

Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. 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 . This allowed the chemical composition of the stellar atmosphere to be determined.