The 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
The scheme is based on lines which are mainly sensitiveto
stellar surface tempertures rather than actual composition
differences,gravity, or luminosity. Inportant lines