Dark Matter. About 90% of the mass in the universe is dark matter Initial proposals: MACHOs: massive compact halo objects

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2 Dark Matter About 90% of the mass in the universe is dark matter Initial proposals: MACHOs: massive compact halo objects Things like small black holes, planets, other big objects They must be dark (so we cannot see them) WIMPs: weakly interacting massive particles These are particles that do not interact with light - like neutrinos, but heavier That means: they cannot be the stuff that atoms are made of 2

3 MACHOs These have been ruled out If there were lots of MACHOS around......we would expect Gravitational Microlensing 3

4 MACHOs These have been ruled out If there were lots of MACHOS around......we would expect Gravitational Microlensing NOT OBSERVED 3

5 When WIMPs collide: WIMPs they create other particles we can see that process through radiation but it is rare ( weakly interacting means rarely interacting) 4

6 Is there More? Let s go back to Hubble s diagram In matter only universe......hubble constant always drops with time...universe slows down Is this, in fact, what we observe? hint: it s not... 5

7 Luminosity Distances Standard candles: Suppose you know how much light is emitted by an object Measure how much light is received Compare received light to emitted light and calculate the distance! 6

8 Luminosity Distances All standard candles are relative measures: Compare two objects and determine their relative distance Examples of standard candles: Variable stars Supernovae Ordinary stars Galaxies Burning neutron star surfaces 7

9 Supernovae Why are supernovae good standard candles? They are bright! We kind of understand them! 8

10 The Sun s Structure With this much mass, the Sun has enormous gravity (30 times higher than Earth at surface) What keeps the Sun from collapsing? gravity Hydrostatic equilibrium! The Sun is hot and dense at its center It has larger pressure Pressure decreases towards surface Net outward force 9

11 The Sun s Structure With this much mass, the Sun has enormous gravity (30 times higher than Earth at surface) What keeps the Sun from collapsing? Hydrostatic equilibrium! The Sun is hot and dense at its center It has larger pressure Pressure decreases towards surface Net outward force This force balances Gravity 9

12 Solar Power Things that radiate cool down Sun constantly loses energy If Sun cools, pressure decreases gravity If pressure decreases, Sun must shrink With just gravity, Sun would slowly shrink pressure But: The Sun s size is constant 10

13 Thermo-Nuclear Fusion Einstein: E=mc2 mass is a form of energy! Let s add up the mass on the left and right side of the reaction: 0.7% of the mass released as energy! before + + after 0.7% difference 11

14 Thermo-Nuclear Fusion So, is this easy? No! Protons are positive charges Protons repel each other The closer they get, the more they repel each other (until the strong force takes over) Slow protons never get close enough You have to slam them into each other to stick Need hot gas - thus the thermonuclear 12

15 The Sun in Balance The sun is in a state of equilibrium: It stays at constant temperature and size This requires two things: The sun must......generate just as much energy in its core as it radiates away at its surface...be hydrostatic 13

16 Equilibrium can be Stable or Unstable? stable......or unstable 14

17 Equilibrium can be Stable or Unstable? stable......or unstable 14

18 Solar Equilibrium Suppose the Sun were somehow to increase in size... Larger radius = weaker gravity = weaker pressure Lower pressure and temperature = less power generation Larger radius = bigger surface area Radiation lost into space would increase The Sun would radiate more energy and generate less It would cool It would shrink back to its equilibrium size Solar equilibrium is stable (phew...) 15

19 Solar Equilibrium Suppose the Sun were somehow to increase in size... Larger radius = weaker gravity = weaker pressure Lower pressure and temperature = less power generation Larger radius = bigger surface area Radiation lost into space would increase The Sun would radiate more energy and generate less It would cool It would shrink back to its equilibrium size Solar equilibrium is stable (phew...) 15

20 Main sequence stars Most stars we see are burning hydrogen high mass This makes them all similar: Stellar structure mostly determined by stellar mass Mass determines temperature and luminosity Main sequence They evolve slowly!!! (not along the main sequence) The stellar main sequence : Determined by mass low mass 16

21 Main sequence stars Most stars we see are burning hydrogen high mass This makes them all similar: Stellar structure mostly determined by stellar mass Mass determines temperature and luminosity Main sequence They evolve slowly!!! (not along the main sequence) The stellar main sequence : Determined by mass low mass 16

22 Main sequence stars What happens when fuel runs out? For sun: tfuel ~ 10 billion years More massive stars burn more quickly Once hydrogen is gone, collapse? 17

23 net energy loss Main sequence stars We could burn 3 x Helium 1 x Carbon... We could burn 1 x Carbon + 1 x Helium 1 x Oxygen... Can we go on like this forever? net energy gain It becomes harder to fuse (requires more pressure) Gain energy until we hit iron Above iron: it takes net energy to fuse Iron is the end of the line! 18

24 Main sequence stars We could burn 3 x Helium 1 x Carbon... We could burn 1 x Carbon + 1 x Helium 1 x Oxygen... Can we go on like this forever? It becomes harder to fuse (requires more pressure) Gain energy until we hit iron Above iron: it takes net energy to fuse Iron is the end of the line! 18

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27 White Dwarf Stars After Helium burning: Sun will shrink quickly (25 million years) But something magical happens: Quantum mechanics! Electrons are anti-social pressure gravity They don t like to be close to each other 21

28 White Dwarf Stars After sufficient compression: Electrons become degenerate, (too close together) Degenerate matter (helium, carbon, oxygen) Normal gas (50 km thick) Resist further compression 5000 km No more contraction No more burning (stops at carbon or oxygen) Just cooling New equilibrium where gravity is!!!!!! balanced by degeneracy pressure a white dwarf 22

29 White Dwarf Stars Electron degeneracy : Pressure independent of temperature White dwarfs cool, but don t shrink Stable equilibrium: if star is compressed, its pressure goes up more quickly than gravity White dwarf stars are ~ Earth sized 23

30 White Dwarf Stars Electron degeneracy : Pressure independent of temperature White dwarfs cool, but don t shrink Stable equilibrium: if star is compressed, its pressure goes up more quickly than gravity White dwarf stars are ~ Earth sized 23

31 White Dwarf Stars Electron degeneracy : Pressure independent of temperature White dwarfs cool, but don t shrink Stable equilibrium: if star is compressed, its pressure goes up more quickly than gravity Size comparison with regular stars White dwarf stars are ~ Earth sized 23

32 Degeneracy resists compression White Dwarf Stars Center never gets hot and dense enough for O + He Ne burning No more burning... No more heating......no more shining 24

33 White Dwarf Stars A fun fact about white dwarf stars: The more mass they have, the smaller they are! The smaller they are, the faster the electrons move Above 1.38 solar masses: Electrons move close to the speed of light This changes the equation of state (pressure vs. energy) 25

34 White Dwarf Stars Such a star is unstable: When compressed, gravity rises faster than pressure This is the famous Chandrasekhar limit 1938 Nobel Prize Stars with core masses above 1.38 solar masses......cannot become white dwarfs! 26

35 Supernovae But stars under 1.38 M remain stable......unless we add some some Once mass reaches 1.38 M......collapse!...density increases...fusion reactions restart 27

36 Supernovae But stars under 1.38 M remain stable......unless we add some some Once mass reaches 1.38 M......collapse!...density increases...fusion reactions restart...boom! 27

37 Recipe for a supernova: Supernovae Take one 1.38 M white dwarf Add mass Take cover! 28

38 Supernovae How to add mass to a star... Most stars are in binaries Suppose our white dwarf has a companion Nothing will happen......until star #2 runs out of fuel Then......it will swell up...to become a red giant 29

39 Supernovae Red Giant Companion Star White Dwarf 30

40 Supernovae How does this make them good standard candles? 1.38 solar masses of thermonuclear fuel Produces a well defined amount of Nickel Luminosity: Radioactive decay from Nickel...Bingo! 31

41 Tycho Supernova Remnant 32

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