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MEGAN JACKSON

MS. KEYS

SCIENCE

DECEMBER 5,2000

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

magnitude.

The scheme is based on lines which are mainly sensitiveto

stellar surface tempertures rather than actual composition

differences,gravity, or luminosity. Inportant lines


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