H205 Cosmic Origins  Today: Galaxies (Ch. 20)  Wednesday: Galaxy Evolution (Ch. 21)  EP 4 & Reflection 1 on Wednesday APOD

Main Types of Galaxies Spirals Ellipticals Irregulars

Hubble Deep Field Our deepest images of the universe show a great variety of galaxies, many of them billions of light-years away

Irregular Galaxies Hubble Ultra Deep Field Spiral Galaxy Elliptical Galaxy

Spiral Galaxy disk bulge halo

Spheroidal Component: bulge & halo, old stars, few gas clouds Disk Component: stars of all ages, many gas clouds

Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge & halo, old stars, few gas clouds Blue-white color indicates ongoing star formation Red-yellow color indicates older star population

Properties of Elliptical Galaxies Round or elliptical in shape Contain no visible gas or dust No young stars or star- forming regions The largest galaxies are ellipticals (and also some of the smallest)

Elliptical Galaxy: All spheroidal component, virtually no disk component Red-yellow color indicates older star population

Properties of Irregular Galaxies Chaotic mix of stars, gas and dust No bulge or spiral arms May contain star forming regions Usually small galaxies Blue-white color indicates ongoing star formation

Galaxy Classification Hubble “tuning fork” diagram

Famous Galaxies! The Local Group – about 3 dozen galaxies –Milky Way –Large and Small Magellanic Clouds –Andromeda –M33 –Dwarfs  About four times smaller than our Milky Way Galaxy  Near M31  more than twice the angular size of the full moon  visible with a good pair of binoculars Messier 33

The Whirlpool Messier million light years away

Messier 87 Giant elliptical galaxy at the center of the Virgo Cluster of galaxies ~60 million light years away note –globular clusters –jet –other galaxies

Messier 82 About 12 Mly very disturbed galaxy

The Mice ~ 300 Mly

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

How did Hubble prove that galaxies lie far beyond the Milky Way? Standard Candles! Pulsating Stars Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles

The Nearest Stars Determine distances of stars out to a few hundred light-years using parallax Distances

Identifying the Main Sequence Apparent brightness of star cluster’s main sequence tells us its distance

Knowing a star cluster’s distance, we can determine the luminosity of each type of star within it, including the distances to Cepheids Establishing the brightness of stars

Steps to the Distance Scale How do astronomers measure distances to objects? Brightness alone does not provide enough information to measure distance Start with nearby objects, move to greater distances

Distances of Galaxies  Galaxies are too far away for parallax technique  Use “standard candles” (INVERSE SQUARE LAW)  Cepheid variable  supergiant stars  planetary nebulae  supernovae  Image “graininess” – The smoother the distribution of stars in a galaxy the farther away it is

Cepheid Variable Stars Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles Period = 3 days

Cepheid variable stars with longer periods have greater luminosities

The Distance to Messier 100 Find the distance to the galaxy M100 using Cepheid variables discovered with Hubble…

Other Distance Methods What clues give you information about distance?

Using the apparent diameters of spiral galaxies More distant galaxies appear smaller Assume they really are the same size – 22 Kpc Measure the angular diameter Determine the distance

Surface Brightness Fluctuations

The Distance Scale  Combination of methods allows us to measure distances to nearby galaxies, and then to further and further distant galaxies. Andromeda – about 2,500,000 LY distant Virgo Cluster – about 50,000,000 LY distant Most distant galaxies –

By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way

The Doppler Shift Police Siren Doppler Pumpkin Johan Christian Doppler

The Doppler Effect How does light tell us the speed of a distant object? How does light tell us the rotation rate of an object? Doppler Principles (Police Traffic Radar Handbook) The Doppler Shift of Sound Waves What's Happening?

The Doppler Shift in YOUR Life

The Doppler Shift for Light Astronomers us the Doppler effect to measure the “radial” velocities of astronomical objects Radial velocities are motions toward or away from us Emission from a moving cloud of gas Absorption by a moving cloud of gas Identifying shifted spectral lines

Measuring the Shift Stationary Moving Away Away Faster Moving Toward Toward Faster Doppler shift tells us ONLY about the part of an object’s motion toward or away from us The amount of blue or red shift tells us an object’s speed toward or away from us

“Redshift” of Galaxies  The spectral lines of galaxies are redshifted, i.e. galaxies are moving away from us.  Plot the velocity of recession against the distance to the galaxy: the farther away a galaxy is, the faster it recedes from us!

Hubble Law

Hubble’s Law The correlation discovered by Hubble was reported in 1929 as the formula: Velocity of Recession = H o x Distance H o is now know as the Hubble constant, and is measured in kilometers per second per megaparsec o

The Hubble Law Galaxies are far away –Hubble measured the distance to the Andromeda galaxy using Cepheid variable stars as standard candles Galaxies are moving away from us –Doppler shift The Hubble Law –The faster a galaxy is moving away from us, the greater its distance: velocity = H 0 x distance

Hubble’s Law … The Hubble Space Telescope was launched to the distance-redshift relation Why is Hubble’s law so important? Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies Age = ___________~ 1 / H 0 Distance Velocity

The Hubble Law and the AGE of the Universe 1/H o is a very simple model for the expansion of the universe A better model would account for the deceleration caused by gravity. Models like this predict the age of the universe to be: t = (2/3)H o What is the age of the Universe?

For Wednesday  Chapter 21 – Galaxy Evolution  EP 4  Hand in 1 st Reflection