lect 17 part 1

lect 17 part 1 - Galaxies Chapter 24 When galaxies were...

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1 Galaxies Chapter 24 When galaxies were first discovered, it was not clear that they lie far beyond the Milky Way A century ago, astronomers thought that the entire universe was only a few thousand light-years and nothing was beyond the Milky Way Galaxy In 1755, Kant suggested that vast collections of stars lie outside of the Milky outside of the Milky Way… What he called “island universes” In 1845, William Parsons built the largest telescope of the 19th century (1.8 m in diameter) Sketch of spiral galaxy M51 (telescope had no photographic equipment)
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2 A modern view of the Spiral Galaxy M51 Note the glowing HII regions (sites of star formation). One arm extends toward the companion galaxy. The “Discovery” of Galaxies At the beginning of the 20 th century, what we now call spiral galaxies were referred to as “spiral nebulae” and most astronomers believed them to be clouds of gas and stars associated with our own Milky Way. The breakthrough came in 1924 when Edwin Hubble was able to measure the distance to the “Great Nebula in Andromeda” (M 31) and found its distance to be much larger than the diameter of the Milky Way. Edwin P. Hubble (1889-1953) This meant that M 31, and by extension other spiral nebulae, were galaxies in their own right, comparable to or even larger than the Milky Way. Hubble demonstrated that the spiral nebulae are far beyond the Milky Way Edwin Hubble used Cepheid variables (period-luminosity relationship) to show that spiral nebulae were actually immense star systems far beyond our Milky Way Galaxy Cepheids are ~10 4 L Sun (very luminous) so for them to appear so dim, they must be extremely distant!
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3 Galaxies can be grouped into four major categories: spirals , barred spirals , ellipticals , and irregulars according to their appearance Lenticular galaxies are intermediate between spiral and elliptical galaxies Spiral galaxies classification Sa : smooth broad spiral arms and fat central bulge Sb : moderate central size bulge Sc : well defined spiral arms and a tiny central bulge Spiral Galaxies: Stellar Birthplaces • The spiral arms contain young, hot, blue stars and associated HII regions indicating ongoing star formation • Therefore, the spiral arms will be rich in metal (Population I stars), indeed the visible-light spectrum of the disk of a spiral galaxy has strong metal absorption lines By contrast there is little star formation in the central By contrast there is little star formation in the central bulges of spiral galaxies, dominated by Population II that has low metal content (central bulges has a yellowish or reddish color) • The differences between Sa, Sb, Sc might be related to the amount of gas and dust they contain 4%, 8% and 25% • Therefore, Sc type has a greater mass involved in star formation than types Sa or Sb.
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4 Barred Spiral Galaxies classification SBa : large central bulge, thin tightly wound spiral arms SBb : moderate central bulge, moderately wound spiral arms SBc : tiny central bulge; lumpy, loosely wound spiral arms
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lect 17 part 1 - Galaxies Chapter 24 When galaxies were...

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