1 Announcements There will be a star map on the exam. I will not tell you in advance what month. Grades are not yet posted, sorry. They will be posted by exam time on Wednesday. Grades including the 3rd midterm will be posted by Monday 5/3. Final is optional. I will announce the room Wednesday at exam time.

2 Life History of a Star Loss of Energy to Space Gravitational Contraction of Core Contraction is halted temporarily by nuclear fusion Energy generation in core

3 HOTCOOL BRIGHT FAINT HR Diagram

4 Star Birth

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6 Small Mass Stars become RED GIANTS

7 Large Mass Stars become RED GIANTS

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10 Small mass stars can not get hot enough to fuse Carbon

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13 White Dwarfs & Neutron Stars White Dwarfs –Supported by pressure of degenerate electrons –About the size of the Earth –Mass < 1.4 Msun Neutron Stars –Supported by pressure of degenerate neutrons –About the size of Lansing –Mass < 3 Msun

14 Degenerate Pressure Pressure due to motion caused by squeezing particles very close together Depends only on density, not on temperature Uncertainty Principle  location x  speed ~ h/mass  Means uncertainty in h is a small constant number

15 Formation of Black Holes If the collapsing core of a massive star which produces the supernova explosion has more mass than the pressure of degenerate neutrons can support (> 3 M sun )  Nothing can stop its collapse  The escape velocity reaches the speed of light  Nothing can go faster than the speed of light  Black Hole

16 Surface of a Black Hole Surface where escape velocity = speed of light is surface of a Black Hole, called Event Horizon Outside Event Horizon can escape, inside can not

17 If nothing can escape from a BH, How do we know its there? If gas falls into a BH  BH gravity makes it speed up  Conservation of Angular Momentum makes it form an Accretion Disk, orbiting at nearly the speed of light  Friction makes it very hot  Emits X-Rays

18 What can we know about Black Holes? Nothing can escape from inside an Event Horizon Long range forces can exert influence outside Event Horizon: 1.Gravity 2.Electric Force Can know: 1.Mass 2.Charge 3.Spin

19 HR Diagram for brightest northern hemisphere stars

20 Test: Cluster HR Diagrams Same Distance Same Age

21 HR Diagram for clusters of different ages

22 Gas - Star - Gas Cycle Interstellar Gas H, He, C, N, O, Fe, etc. Star SN or Wind Fusion of He, C, N, O & heavier atoms Gas cloud contracts Return of gas enriched in heavy atoms SN fuse atoms > iron

23 Halo Stars: % heavy elements (O, Fe, …) only old stars Disk Stars: 2% heavy elements stars of all ages

24 Milky Way Cartoon

25 Density Excess? Higher density proto-galactic clouds form stars more rapidly, use up all their gas before it can form a disk.

26 Rotation? Larger rotation produces more disk-like distribution of matter.

27 Galaxies are close together Mergers may make Ellipticals Burst of star formation

28 (hundreds to thousands of galaxies) 1. Denser cloud 2. More collisions Elliptical galaxies are much more common in huge clusters of galaxies

29 Distance & Age Universe opaque space time light Here & Now

30 Fig 22.18

31 Relic Radiation Universe was once hot and opaque, see radiation from that time (as from surface of a star) –Comes at us from all directions (inside fog bank) –Has a thermal spectrum –Cold now, expansion cools

32 Spectrum is Thermal, T=2.7 K

33 CMB Radiation Radiation is nearly the same from all directions, Doppler Shift due to motion of the Milky Way  T/T ~ After subtracting emission from MW, see Primoridial fluctuations from when universe became transparent,  T/T ~ 10 -5

34 Relic Elements Theory Observations Universe is 75% H 25% He Deuterium abundance constrains density of ordinary matter

35 What do we know about Dark Energy? Constitutes 2/3 of energy in universe Is smoothly distributed and invisible Doesn’t clump into galaxies like Matter, visible or dark Has negative pressure produces Acceleration

36 Problems with the Big Bang Model 1. How can two pieces on opposites sides of the universe have the same temperature at the time the universe became transparent? They are too far apart to have communicated with each other within the age of the universe, since light from them has just now reached us half way between.

37 Problems with the Big Bang Model 2. Why is the space-time geometry of the universe so nearly flat, equivalent to the sum of the Ordinary Matter, Dark Matter and Dark Energy = Critical Density?

38 Inflation At very beginning of Big Bang, the Universe underwent a tremendous expansion (inflation).

39 Inflation Before Inflation the two parts of the universe were close enough together to communicate with each other Fig 23.14

40 Inflation Expansion smoothes out fluctuations and makes things appear flatter (e.g. blowing up a balloon).