FREE ESSAY ON STARS AND GALAXIES

STARS AND GALAXIES

Have you ever looked up at the sky and wondered what are those bright and shiny things up
there. Stars: a natural luminious body visible in the sky especially at night. A
self-luminious gaseous celestial body ofgreat mass which produces energy by means of
nuclear fusion reactions, whose shape is usually spheroidal, and whose size may be as
small as the earth or larger than the earth's orbit. Galaxies: Any of the very large
groups of stars and associated matter that are found throughout the universe. In 1802,
William Wollaston noted that the spectrum of sunlight did not appear to be a continious
band of colors, but rather had a series of dark lines superimposed on it. Wollaston
attributed the lines to natural boundaries between colors. Joseph Fraunhofer made a more
careful set of observations of the solar spectrum in 1814 and found some 600 dark lines,
and he specifically measured the wavelenght of 324 of them. Many of the Fraunhofer lines
in the solar spectrum retain the notations he created to designate them. In 1864, Sir
William Huggins matched some of these dark lines in the spectra from other stars with
terrestrial substances, demonstrating that the stars are made of the same materials of
everyday material rather than exotic substances. This paved the way for modern
spectroscopy. Since even before the discovery of the spectra, scientists had tried to
find ways to catergorize stars. By observing spectra , astronomers realized that the
large numbers of stars exhibit a small number of distinct patterns in their spectral
lines. Classification by the spectral features quickly proved to be a powerful tool for
understanding stars. The current spectral classification scheme was developed at Harvard
Observatory in the early 20th century. Work was begun by Henry Draper who photographed
the first spectrum of Vega in 1872. After his death, his wife donated the equipment and a
sum of money to the Observatory to continue his work. The bulk of classification work was
done by Annie Jump Cannon from 1918 to 1924. The original scheme used capital letters
running alphabetically, but the subsequent revisions have reduced this as stellar
evolution and typing has become better understood. The work was published in the Henry
Draper Catalog and Henry Draper Extension which contained spectra of 225,000 stars down
to ninth magnitude. The scheme is based on lines which are mainly sensitiveto stellar
surface tempertures rather than actual composition differences,gravity, or luminosity.
Inportant lines