Essay on The Stars And Galaxies

The Stars And Galaxies

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 luminous body visible in the sky especially at night.

A self-luminous gaseous celestial body of great mass that 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 is found throughout the universe. In 1802, William Wollaston noted that the spectrum of sunlight did not appear to be a continuous 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 wavelength 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 categorize stars. By observing spectra, astronomers realized that large numbers of stars exhibit a small number of distinct patterns in their spectral lines. Classification by 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 have 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 the ninth magnitude. The scheme is based on lines that are mainly sensitive to stellar surface temperatures rather than actual composition differences, gravity, or luminosity. Important lines

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