Chapter 20 Galaxies 20.1 Islands of Stars And the Foundation of Modern Cosmology • Our goals for learning • How are the lives of galaxies connected with the history of the universe? • What are the three major types of galaxies? • How are galaxies grouped together? How are the lives of galaxies Hubble Deep Field connected with the history of the universe? • Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away What are the three major types of Galaxies and Cosmology galaxies? • A galaxy’s age, its distance, and the age of the universe are all closely related • The study of galaxies is thus intimately connected with cosmology — the study of the structure and evolution of the universe 1
Hubble Hubble Ultra Ultra Deep Deep Field Field Hubble Hubble Ultra Ultra Deep Deep Field Field Spiral Galaxy Spiral Galaxy Hubble Hubble Ultra Ultra Deep Deep Field Field Elliptical Galaxy Elliptical Galaxy Elliptical Galaxy Elliptical Galaxy Spiral Galaxy Spiral Galaxy 2
Hubble Ultra Deep halo Field Elliptical Galaxy Elliptical Galaxy disk bulge Spiral Galaxy Irregular Galaxies Spiral Galaxy Disk Component: Disk stars of all ages, Component: many gas clouds stars of all ages, many gas clouds Spheroidal Component: bulge & halo, old stars, Spheroidal Component: few gas bulge & halo, old stars, clouds few gas clouds Disk Blue-white color Disk Blue-white color Component: Component: indicates ongoing indicates ongoing stars of all stars of all star formation star formation ages, ages, many gas many gas clouds clouds Spheroidal Spheroidal Component: Component: bulge & halo, bulge & halo, old stars, old stars, Red-yellow color Red-yellow color few gas few gas indicates older star indicates older star clouds clouds population population 3
Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical) Barred Spiral Galaxy : Has a bar of stars across the bulge Elliptical Galaxy: All spheroidal component, virtually no disk component Red-yellow color indicates Irregular Galaxy Blue-white color older star population indicates ongoing star formation How are galaxies grouped together? Hubble’s galaxy classes Spheroid Disk Dominates Dominates 4
Spiral Elliptical galaxies are galaxies are often found much more in groups of common in galaxies huge clusters of galaxies (up to a few dozen (hundreds to galaxies) thousands of galaxies) What have we learned? What have we learned? • How are the lives of galaxies connected • How are galaxies grouped together? with the history of the universe? – Spiral galaxies tend to collect into groups of up to a few dozen galaxies – Galaxies generally formed when the universe was young and have aged along with the – Elliptical galaxies are more common in large universe clusters containing hundreds to thousands of galaxies • 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 do we measure the distances 20.2 Measuring Galactic Distances to galaxies? • Our goals for learning • How do we measure the distances to galaxies? 5
Brightness alone does Step 1 not provide enough Determine size information of solar system to measure using radar distance Luminosity passing Step 2 through each sphere is the same Determine distances of stars out to a Area of sphere: few hundred light-years 4 π (radius) 2 using parallax Divide luminosity by area to get brightness The relationship between apparent brightness and Step 3 luminosity depends on distance: Apparent Luminosity Brightness = brightness of 4 π (distance) 2 star cluster’s main sequence We can determine a star’s distance if we know its tells us its luminosity and can measure its apparent brightness: distance Luminosity Distance = 4 π x Brightness A standard candle is an object whose luminosity we can determine without measuring its distance 6
Cepheid variable stars are very luminous Knowing a star cluster’s distance, we can determine the luminosity of each type of star within it Step 4 Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles Cepheid variable stars with longer periods have greater luminosities White-dwarf Step 5 supernovae can also be Apparent used as brightness of standard white-dwarf candles supernova tells us the distance to its galaxy (up to 10 billion light- years) 7
Tully-Fisher We measure galaxy distances using Relation a chain of interdependent techniques Entire galaxies can also be used as standard candles because galaxy luminosity is related to rotation speed What have we learned? 20.3 Hubble’s Law • How do we measure the distances to galaxies? • Our goals for learning – The distance-measurement chain begins with • How did Hubble prove that galaxies lie far beyond parallax measurements that build on radar the Milky Way? ranging in our solar system • What is Hubble’s Law? – Using parallax and the relationship between • How do distance measurements tell us the age of luminosity, distance, and brightness, we can the universe? calibrate a series of standard candles • How does the universe’s expansion affect our – We can measure distances greater than 10 distance measurements? billion light years using white dwarf supernovae as standard candles How did Hubble prove that galaxies lie far beyond the Milky The Puzzle of “Spiral Nebulae” Way? • 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 8
What is Hubble’s Law? Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way The spectral features of virtually all galaxies are redshifted ⇒ They’re all moving away from us Redshift of a galaxy tells us its distance through Hubble’s Law: distance = velocity H 0 Hubble’s Law: velocity = H 0 x distance 9
Distances of How do distance measurements farthest tell us the age of the universe? galaxies are measured from redshifts The expansion rate appears to be the same everywhere in space The universe has no center and no edge (as far as we can tell) One example of something that expands but has no center or edge is the surface of a balloon Hubble’s constant tells Cosmological Principle us age of universe because it The universe looks about the same no matter relates where you are within it velocities and distances of all • Matter is evenly distributed on very large scales galaxies in the universe • No center & no edges • Not proved but consistent with all observations to Distance date Age = Velocity ~ 1 / H 0 10
How does the universe’s Distances between expansion affect our distance faraway measurements? galaxies change while light travels distance? Distances between faraway galaxies change while light travels Astronomers think in terms distance? of lookback time rather than distance Expansion stretches photon wavelengths causing a cosmological redshift directly related to lookback time What have we learned? What have we learned? • How did Hubble prove that galaxies lie far • How do distance measurements tell us the beyond the Milky Way? age of the universe? – He measured the distance to the Andromeda – Measuring a galaxy’s distance and speed galaxy using Cepheid variable stars as allows us to figure out how long the galaxy standard candles took to reach its current distance • What is Hubble’s Law? – Measuring Hubble’s constant tells us that amount of time: about 14 billion years – The faster a galaxy is moving away from us, • How does the universe’s expansion affect the greater its distance: our distance measurements? velocity = H 0 x distance – Lookback time is easier to define than distance for objects whose distances grow while their light travels to Earth 11
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