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| (b. Nov. 15, 1738, Hanover, Ger.—d. Aug. 25, 1822, Slough, Buckinghamshire, Eng.) |
Uranus
German-born British astronomer Sir William
Herschel , was the founder of sidereal astronomy for the sys tematic observation of
the heavens. He discovered the planet Uranus, hypothesized that nebulae are composed of stars, and
developed a theory of stellar evolution. He was knighted in 1816.
Discovery of Uranus
The intellectual curiosity that Herschel acquired
from his father led him from the practice to the theory of music, which he
studied in Robert Smith’s Harmonics. From this book he turned to Smith’s A
Compleat System of Opticks, which
introduced him to the techniques of telescope con struction. Herschel soon
began to grind his own mirrors. They were ground from metal disks of copper,
tin, and antimony in various proportions. He later produced large mirrors of
superb quality—his telescopes proved far supe rior even to those used at the
Greenwich Observatory. He also made his own eyepieces, the strongest with a magni fying power of
6,450 times. Herschel’s largest instrument, too cumbersome for regular use, had
a mirror made of speculum metal, with a diameter of 122 centimetres (48 inches)
and a focal length of 12 metres (40 feet). Completed in 1789, it became one of
the technical wonders of the 18th century.In 1781, during his third and most
complete survey of the night sky, Herschel came upon an object that he real
ized was not an ordinary star. It proved to be the planet Uranus, the first
planet to be discovered since prehistoric times. Herschel became famous almost
overnight. His friend Dr. William Watson, Jr., introduced him to the Royal
Society of London, which awarded him the Copley Medal for the discovery of
Uranus, and elected him a Fellow. He was subsequently appointed as an
astronomer to George III.Herschel’s big telescopes were ideally suited to study
the nature of nebulae, which appear as luminous patches in the sky. Some
astronomers thought they were nothing more than clusters of
innumerable stars the light of which blends to form a milky appearance. Others held that some nebulae were composed of a
luminous fluid. However, Herschel found that his most powerful telescope could resolve into stars several nebulae that appeared
“milky” to less well equipped observers. He was convinced that other nebulae would
eventually be resolved into individual stars .
Theory of the Evolution of Stars
In order to
interpret the differences between these star clusters, Herschel emphasized
their relative densities by contrasting a cluster of tightly packed stars with
others in which the stars were widely scattered. These formations showed that
attractive forces were at work. In other words, a group of widely scattered
stars was at an earlier stage of its development than one whose stars were
tightly packed. Thus, Herschel made change in time, or evolution, a fun
damental explanatory concept in astronomy.In 1785 Herschel developed a
cosmogony—a theory concerning the origin of the universe: the stars originally
were scattered throughout infinite space, in which attrac tive forces gradually
organized them into even more fragmented and tightly packed clusters. Turning
then to the system of stars of which the Sun is part, he sought to determine
its shape on the basis of two assumptions:
(1) that
with his telescope he could see all the stars in the sys tem, and (2) that
within the system the stars are regularly spread out. Both of these assumptions
he subsequently had to abandon. But in his studies he gave the first major
example of the usefulness of stellar statistics in that he could count the
stars and interpret this data in terms of the extent in space of the Galaxy’s
star system.
Theory of the Structure of Nebulae
On Nov. 13,
1790, Herschel observed a remarkable nebula, which he was forced to interpret
as a central star sur rounded by a cloud of “luminous fluid.” This discovery
contradicted his earlier views. Hitherto Herschel had rea soned that many
nebulae that he was unable to resolve (separate into distinct stars), even with
his best telescopes, might be distant “island universes” (such objects are now
known as galaxies). He was able, however, to adapt his ear lier theory to this
new evidence by concluding that the central star he had observed was condensing
out of the surrounding cloud under the forces of gravity. In 1811 he extended
his cosmogony backward in time to the stage when stars had not yet begun to form
out of the fluid.In dealing with the structural organization of the heavens,
Herschel assumed that all stars were equally bright, so that differences in
apparent brightness are an index only of differences in distances. Throughout
his career he stubbornly refused to acknowledge the accumu lating evidence that
contradicted this assumption. Herschel’s labours through 20 years of systematic
sweeps for nebulae (1783–1802) resulted in three catalogs listing 2,500 nebulae
and star clusters that he substituted for the 100 or so milky patches
previously known. He also cata loged 848 double stars—pairs of stars that
appear close together in space, and measurements of the comparative brightness
of stars. He observed that double stars did not occur by chance as a result of
random scattering of stars in space but that they actually revolved about each
other. His 70 published papers include not only studies of the motion of the
solar system through space and the announcement in 1800 of the discovery of
infrared rays but also a succession of detailed investigations of the plan ets
and other members of the solar system.
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