Final Exam!!! The final is Thursday, July 2nd in class. Don t be late! The test will cover Chapters 1-16 and 18-19 with a STRONG EMPHASIS on Chapters 9-16, 18, and 19. It will consist of 50 questions and you will have until 12 o clock to complete it. The test is true-false and multiple choice. Make sure you have a SCAN-TRON 882 form and a #2 pencil. I will NOT have these available in class. Bring your student ID. The test is closed book, closed notes, and no calculators. There will be a sheet of equations available. A practice final is available. Take the practice test for 1.5 hours, and see how you do. EXTRA CREDIT!!! Get 1 extra percentage point for each mistake you find on the practice test!
The Distance and Evolution of Galaxies Today s Lecture: Galaxy Redshifts (Chapter 16, pages 383-397) Distances to Galaxies Galaxy Evolution Expanding Universe (Chapter 18, pages 422-428) Olber s Paradox Hubble s Law
Cosmology! The study of the shape and evolution of the Universe as a whole. How big is the Universe? What is the shape of the Universe? What is the fate of the Universe? How did the structure of the Universe (like galaxies) form? How does this structure and galaxies evolve with time?
Expansion of the Universe Vesto Slipher: noticed that spectra of spiral nebulae have large redshifts. Edwin Hubble - 1929 - Used newly derived distances (Cepheids, etc.) to find that: REDSHIFT IS PROPORTIONAL TO DISTANCE!
Galaxy Redshifts Brightness Hγ Hβ Hα 4341Å 4861Å 6563Å λ Hydrogen Balmer lines with NO SHIFT (v=0) Brightness λ Redshifted lines (v is away from us) z is called the redshift
Velocity increases with distance! By fitting a line we find that
Hubbles s Law V = H 0 d (H 0 = H-naught = Hubble s constant now, but it changes with time) H 0 = 71 ± 4 km/s/mpc For example - If a galaxy is 10 Mpc away, then it is receding from us at 710 km/s. If a galaxy is 20 Mpc away, then it is receding at 1420 km/s.
Use Hubble s law to get galaxy distances 1) Obtain a spectrum 2) Measure the redshift 3) Compute recession speed V, from z = V/c 4) Use Hubble s Law, V=H 0 d, to compute d Note that z = V/c is an approximation that is accurate for V<0.2c This is a more accurate approximation
Redshifts and Lookback times When we see an object at a given redshift, z, how far back in time are we seeing? Often called a distance (for example, 4.3 billion light years for z=0.4), but strictly speaking this is not correct because the Universe expanded while light traveled. Also z = 1000 corresponds to 13.3 billion years ago.
Galaxies Long Ago How did galaxies form? How did they evolve with time? What did typical galaxies look like long ago? Hubble Space Telescope observations of galaxies at progressively larger distances provide some answer. WE CAN LOOK AT LARGE DISTANCES TO LOOK BACK IN TIME TO SEE HOW GALAXIES EVOLVED!
Galaxy Formation and Evolution Main results: Most elliptical galaxies formed very early. Some ellipticals formed from the merger of spiral galaxies. Some (many?) galaxies formed from the agglomeration of numerous small subunits of stars. There used to be far more small, blue, irregular galaxies. Some probably merged together. Perhaps others faded and are very faint now. Most spirals were peculiar ( train wrecks )
Active Galaxies (Active Galactic Nuclei - AGN - Chapter 17) Radio galaxies lobes of radio emission, far from the nucleus. Jets of particles and radiation shot out over 1 Mpc size and gas with huge speeds (>10 4 km/s) These are supermassive BHs (10 6-10 9 M sun ), actively accreting matter, powering these galaxies. Quasars: especially bright, high redshift versions ~12 billion years ago there was an era where quasars were especially prominent. Nearby active and normal galaxies today may have been quasars in the past.
Olber s paradox Why is the sky dark at night? If the Universe is infinite in size and age, and uniformly filled with stars (on average), then every line of sight should intersect a star. The sky should be bright! Like looking through a forest!
Olber s paradox Why is the sky dark at night?
Surface Brightness of Stars Distance stars look dim b 1/d 2 (inverse-square law) θ Distant stars have small angular size A = πθ 2 1/d 2 Surface brightness = brightness per unit angular area It is independent of distance!!!
Possible Solutions (each profound!) 1. Universe has a finite size. 2. Universe has an infinite size, but few stars far away. 3. Universe has finite age, so that the light from most stars has not yet reached us. 4. Other possibilities can be ruled out or violate Copernican principle (for example, the stars could all be aligned on spokes, but this would require the Earth to have a special location in the Universe).
Olber s Paradox: Solution It turns out that option (3) is the main solution for our Universe. The Universe has a finite age, and light from the most distant stars has not reached us, so the sky is dark.
If the Universe has a finite age, it must have a beginning It is possible that that the entire Universe was plopped down at once and we can only see a certain distance because of the finite age. But remember that all the galaxies are expanding away from one another. In the past, everything must have been closer together: The Big Bang Remember: we re not expanding, stars aren t expanding, galaxies aren t expanding. It is the space between non-bound objects that is expanding!
Hubbles s Law V = H 0 d (H 0 = H-naught = Hubble s constant now, but it changes with time) H 0 = 71 ± 4 km/s/mpc Hubble s constant is for now, and doesn t tell you anything about the past or future. This does NOT imply that the speed of a given galaxy increases with time.
All galaxies are moving away from us. Are we the center of the Universe s expansion?
NO! There is no unique center! Simple analogy: a rubber band with dots drawn on it. 1 cm 2 cm 2 cm 4 cm Now stretch the rubber band. For any given dot, it appears as if everything is expanding from it. Another good analogy is the expansion of a balloon with stickers on it. What is the universe expanding into? A 4th physically inaccessible dimension!