AST-1002 Chapter 16: AST Chapter 16
Document Summary
Despite the incredible number of galaxies, there is surprising consistency in their overall shapes. Edwin hubble began cataloging their appearance in the 1920s. The hubble classification of galaxies spirals, barred spirals, lenticulars, ellipticals, irregulars, and their subclasses is still used today. Hubble classification - a system of classifying galaxies according to their appearance into one of four broad categories: spirals, barred spirals, ellipticals, and irregulars. Spiral galaxies without bars (often called normal spiral galaxies) are characterized by a central bulge and arched lanes of stars and interstellar clouds, which appear as arms that spiral out from the bulge. Observations of many spiral galaxies reveal that the more tightly wound the spiral, the larger the central bulge. A few high-mass stars form in most open clusters, such as those created in spiral density waves. The answer is that the massive stars highlighting each density wave are short-lived and they explode before they finish passing through it.
