1. A105 Stars and Galaxies  This week’s units: 74, 75, 76, 78, 79  News Quiz Today  Galaxies homework due Thursday  Projects due Nov. 30 Today’s APODAPOD

2. Announcements… Kirkwood Obs. open Weds night 6:30-8:30 PM Rooftop Session on Thurs, Nov. 16 @ 8 PM Leonid Meteor Shower, Friday AM or PM

3. Main Types of Galaxies Spirals Ellipticals Irregulars

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

5. Irregular Galaxies Hubble Ultra Deep Field Spiral Galaxy Elliptical Galaxy

6. Spiral Galaxy disk bulge halo

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

8. 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

9. 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)

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

11. 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

12. Classifying Galaxies 1 2 3 14 A. Elliptical B. Spiral C. Irregular

13. Classifying Galaxies 24 22 28 30 A. Elliptical B. Spiral C. Irregular

14. Thought Question Why does ongoing star formation lead to a blue- white 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

15. 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

16. The Whirlpool Messier 51 23 million light years away

17. 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

18. Messier 82 About 12 Mly very disturbed galaxy

19. The Mice ~ 300 Mly

20. 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

21. 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

22. 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

23. Cepheid variable stars with longer periods have greater luminosities

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

25. 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

26. 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

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

28. 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

29. Other Distance Methods What clues give you information about distance?

31. 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 –

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

33. “Redshift” of Galaxies  Remember the Doppler Shift?  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!

34. Hubble Law

35. 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

36. Hubble’s Law … o 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

37.  Read Units 74, 75, 76, 78, 79  Galaxies homework due Thurs