Chapter 15 Galaxies and the Foundation of Modern Cosmology Copyright 2009 Pearson Education Inc Should the Bonfire return to A M A Yes B No C What s a bonfire Copyright 2009 Pearson Education Inc Was the Midterm too hard A Yes B No C What I missed the midterm Copyright 2009 Pearson Education Inc Copyright 2009 Pearson Education Inc 15 1 Islands of Stars Our goals for learning What are the three major types of galaxies How are galaxies grouped together Copyright 2009 Pearson Education Inc halo disk bulge Spiral Galaxy Copyright 2009 Pearson Education Inc Disk Component stars of all ages many gas clouds Spheroidal Component bulge and halo old stars few gas clouds Copyright 2009 Pearson Education Inc Disk Component stars of all ages many gas clouds Spheroidal Component bulge and halo old stars few gas clouds Copyright 2009 Pearson Education Inc Disk Component stars of all ages many gas clouds Spheroidal Component bulge and halo old stars few gas clouds Copyright 2009 Pearson Education Inc Blue white color indicates ongoing star formation Red yellow color indicates older star population Disk Component stars of all ages many gas clouds Spheroidal Component bulge and halo old stars few gas clouds Copyright 2009 Pearson Education Inc Blue white color indicates ongoing star formation Red yellow color indicates older star population Thought Question Why does ongoing star formation lead to a bluewhite appearance A There aren t any red or yellow stars B Short lived blue stars outshine others C Gas in the disk scatters blue light Copyright 2009 Pearson Education Inc Thought Question Why does ongoing star formation lead to a bluewhite appearance A There aren t any red or yellow stars B Short lived blue stars outshine others C Gas in the disk scatters blue light Copyright 2009 Pearson Education Inc Barred Spiral Galaxy Has a bar of stars across the bulge Copyright 2009 Pearson Education Inc Lenticular Galaxy Has a disk like a spiral galaxy but much less dusty gas intermediate between spiral and elliptical Copyright 2009 Pearson Education Inc Elliptical Galaxy All spheroidal component virtually no disk component Copyright 2009 Pearson Education Inc Elliptical Galaxy All spheroidal component virtually no disk component Red yellow color indicates older star population Copyright 2009 Pearson Education Inc Irregular Galaxy Neither spiral nor elliptical Copyright 2009 Pearson Education Inc Irregular Galaxy Neither spiral nor elliptical Copyright 2009 Pearson Education Inc Blue white color indicates ongoing star formation Spheroid Dominates Copyright 2009 Pearson Education Inc Hubble s galaxy classes Disk Dominates How are galaxies grouped together Copyright 2009 Pearson Education Inc Spiral galaxies are often found in groups of galaxies up to a few dozen galaxies per group Copyright 2009 Pearson Education Inc Elliptical galaxies are much more common in huge clusters of galaxies hundreds to thousands of galaxies Copyright 2009 Pearson Education Inc What have we learned What are the three major types of galaxies Spiral galaxies elliptical galaxies and irregular galaxies Spirals have both disk and spheroidal components ellipticals have no disk How are galaxies grouped together Spiral galaxies tend to collect into groups of up to a few dozen galaxies Elliptical galaxies are more common in large clusters containing hundreds to thousands of galaxies Copyright 2009 Pearson Education Inc 15 2 Distances of Galaxies Our goals for learning How do we measure the distances to galaxies What is Hubble s law How do distance measurements tell us the age of the universe Copyright 2009 Pearson Education Inc How do we measure the distances to galaxies Copyright 2009 Pearson Education Inc Brightness alone does not provide enough information to measure distance Are Bright Stars Nearby or Luminous Copyright 2009 Pearson Education Inc Step 1 Determine size of solar system using radar Radar Pulses Copyright 2009 Pearson Education Inc Step 2 Determine distances of stars out to a few hundred light years using parallax Copyright 2009 Pearson Education Inc Luminosity passing through each sphere is the same Area of sphere 4 radius 2 Divide luminosity by area to get brightness Copyright 2009 Pearson Education Inc The relationship between apparent brightness and luminosity depends on distance Brightness Luminosity 4 distance 2 We can determine a star s distance if we know its luminosity and can measure its apparent brightness Distance Luminosity 4 Brightness A standard candle is an object whose luminosity we can determine without measuring its distance Copyright 2009 Pearson Education Inc Step 3 Apparent brightness of star cluster s main sequence tells us its distance Copyright 2009 Pearson Education Inc Knowing a star cluster s distance we can determine the luminosity of each type of star within it Copyright 2009 Pearson Education Inc Thought Question Which kind of stars are best for measuring large distances A High luminosity stars B Low luminosity stars Copyright 2009 Pearson Education Inc Thought Question Which kind of stars are best for measuring large distances A High luminosity stars B Low luminosity stars Copyright 2009 Pearson Education Inc Cepheid variable stars are very luminous Copyright 2009 Pearson Education Inc Cepheid Variable Stars The light curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50 day period Copyright 2009 Pearson Education Inc Cepheid variable stars with longer periods have greater luminosities Copyright 2009 Pearson Education Inc Step 4 Because the period of a Cepheid variable star tells us its luminosity we can use these stars as standard candles Using Cepheid Variables as Standard Candles Copyright 2009 Pearson Education Inc White dwarf supernovae can also be used as standard candles Copyright 2009 Pearson Education Inc Step 5 Apparent brightness of a white dwarf supernova tells us the distance to its galaxy up to 10 billion lightyears Copyright 2009 Pearson Education Inc What is Hubble s law Copyright 2009 Pearson Education Inc The Puzzle of Spiral Nebulae Before Hubble some scientists argued that spiral nebulae were entire galaxies like our Milky Way while others maintained they were smaller collections of stars within the Milky Way The debate remained unsettled until someone finally measured their distances Copyright 2009 Pearson Education Inc
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