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J B Lester

AST 201S - Stars and Galaxies University of Toronto Mississauga Chapter 15 Summary Essential Cosmic Evolution, 5 Edition th 1. Measuring Astronomical Distances • Use parallax (geometry of triangles) for the nearest stars • Use Cepheid variable stars for larger distances – measure apparent brightness – use the period of variation to find the luminosity – assumed the luminosity is the same as nearby stars of the same period – solve for the distance • Use white-dwarf supernova explosions for largest distances – measure apparent brightness – assume explosions always happen the same way producing the same lumi- nosity – solve for the distance 2. Existence of other galaxies • Diameter of Milky Way = 100,000 ly (Chapter 14) • Cepheid variables with faint apparent brightnesses → other galaxies millions of ly away 3. Types of galaxies • Spiral (S) type – bulge size ranges from small to large – spiral arms range from loose to tight – form is “regular” or “barred” • Elliptical (E) type – shapes range from round to oval – sizes range from small to huge • Irregular (I) type – no regular form – large amounts of gas and dust – actively forming stars • Organize the various type using the “tuning fork” diagram, which is NOT a developmental sequence continued on page 2 AST201H5S – Chapter 15 Summary page 2 of 2 4. Distribution of Galaxies in Space • Small clusters - few dozen galaxies, 1 or 2 large S types with the remainder being small E and I types in a volume a few million ly across example = Local Group cluster to which the Milky Way
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