1ASTR 1020: Stars & GalaxiesMarch 19, 2008• Reading: Chapter 20, sections 20.3.• Meet Friday at Fiske Planetarium –Hubble’s Expanding Universe (bring your clicker)Astronomy in the NewsJill HulleyToday’s ClassChapter 20:• Galaxies• Mapping the Universe: measuring distances to galaxiesA Universe Full of Galaxies• Galaxies are classified into basic types• Use both shapes and star properties• Sizes range from giants, through biggish (like the Milky Way), through dwarfsSpirals ~80% of galaxies• Disks (spiral arms)AND• Spheroids (central bulges + halos)• Some have “bars”across the centers2Elliptical ~15% of galaxies• Round or slightly flattened• Very little cold gas (no 21-cm emission), dust, or young stars• Reddish/yellow color = old stars (red giants, red main sequence)Dwarf ellipticalsMost common type of galaxy?• Only know nearby ones (faint!)Irregulars• Galaxies in formation?• Or Transition?• Often LOTS of star formation!Where they live• Spirals– mostly in groups (3-10 galaxies)• Ellipticals– more often in dense clusters of galaxies (100’s – 1000’s)• Why? Chapter 21…A dense Galaxy ClusterThe Big Picture- the universe is filled with a network of galaxies in groups and clusters3Mapping the Universe: We need Distances to Galaxies!So far– ParallaxNew methods: standard candles1.) Make some measure of an object which identifies its luminosity2.) Use this luminosity and measure apparent brightness to infer distance to it1.) Main sequence fitting• Start with a cluster distance known via parallax (upper)• Compare with other clusters (lower)• Which is more distant- the upper or lower?2.) Cepheid Stars• Region on the HR diagram with large, bright stars• Outer regions are unstable and tend to pulsate• See Chapter 15: pulsating variable stars• Star expands and contracts, getting brighter and fainter • Period = one whole cycleCepheid Stars• Period-luminosity relationship• Overall brighter Cepheids have longer periods(elephants and hummingbirds)• Clicker Question: Two Cepheid stars, Fred and Barney, have the same apparent brightness. Fred has a period of 5 days, and Barney of 10 days. Which is closer?a) Fredb) Barney4• A) Fred• Fred has a shorter period and so must be less luminous (hummingbird)• Less luminous but the same apparent brightness means that Fred is closer to usCepheids as Standard Candles• Measure period of variability• From period-luminosity relation, infer the luminosity• Compare with apparent brightness and determine
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