The Properties of Galaxies | Shapes | Colors | Sizes

1. Properties of Galaxies

Galaxies are interesting for two major reasons. Containing billions of stars, galaxies are fascinating ecosystems in which the lifecycles of stars are linked to the evolution of each galaxy in much the same way that life on Earth is linked to the changing Earth. Furthermore, galaxies trace out the largest known structures in the universe. Studying the distributions of galaxies in space has provided astronomers with valuable information about the shape of the universe, its history and its destiny. Before 1924, we thought that the entire universe consisted of our Galaxy. At least, that was the prominent view among astronomers at the time. Faint, nebulous objects were seen in the sky since the invention of the telescope.  Although the nature of these nebulae was not understood until later, catalogues of the objects were made by many of the prominent observers of the eighteenth and nineteenth centuries.   The spiral structure of some of the nebulae was discovered in the 19th century.  By 1908, positions and descriptions of nearly 15,000 nebulae and star clusters were known, but the true nature of the nebulae was still a mystery. The question ultimately reduced to the problem of finding their distances. If these distances were small compared with the size of our own galaxy, then these nebulae were part of our galaxy. If the distances were very large, then they were probably systems of stars in their own right.

These questions culminated in what was romantically known as 'The Great Debate'. The major players in this debate in the United States were H.D.Curtis (believed the nebulae were extra-galactic) and Harlow Shapley (did not believe his own calculations! And did not agree with the extra-galactic hypothesis). A synopsis of this debate is available at this website, http://antwrp.gsfc.nasa.gov/diamond_jubilee/debate20.html. It is interesting history, but not required for the lab. The debate was settled in 1924 by Edwin Hubble of the Mount Wilson Observatory, who was the first to determine distances to the spiral nebulae, thus finding that they must be beyond our galaxy. A short biography of Hubble is available at this website, http://antwrp.gsfc.nasa.gov/diamond_jubilee/hubble_nas.html and you really should visit his website at http://astro.caltech.edu/computers/homepage.template.html.