1 1 Astr 102 Lec 7: Classification of Stars, the Sun What prevents stars from collapsing under the weight of their own gravity? Text Why is the center of the Sun hot? What is the source of the Sun s energy? What are neutrinos & why do we care How does energy get from the inside to the outside of a star?
2 2 In general, for normal gases, if one increases, the others increase as well. Pressure Temperature Density P nt
3 3 Increasing the pressure increases the DENSITY of the gas.
4 4 Temperature is also closely related to pressure. Hits harder, more often! Boltzmann distribution The particles hit the walls of particle speeds with a faster speed More particles per second bang on the walls
5 5 A balloon dipped in liquid What happens? Blue: Purple: Green: Nothing Balloon shrinks Balloon expands
6 6 A balloon dipped in liquid Shrinkage! Particles in the balloon move much more slowly, and can t supply enough pressure to keep the balloon inflated.
7 7 A previously frozen balloon,
8 8 Even Density is related to Temperature: If you squeeze something, you are pushing the particles within it, which speeds them up
9 9 What about temperatures, pressures, & densities in the Sun? Why are the outer edges darker? A. (yellow) Lower density B. (blue) Lower temp C. (green) Less Hydrogen D. (pink) I have no idea Hint: Most of the Sun s light is thermal radiation
10 Limb darkening : Outer layers are cooler Lower surface brightness=cooler 10 Sees into hotter inner layers Sees only cooler outer layers
11 11 Absorption lines also suggest the outer regions are cooler Absorption lines = cool gas on surface Temperature Gradient (Energy source is probably in the center)
12 12 Temperature Pressure Density For the Sun: Temperature increases inwards, so - Density must increase inwards! - Pressure must increase inwards!
14 14 Line Strength depends upon Line is Strongest at this Temperature Too cool, few electrons in Level 2 Too hot, few bound electrons
15 15 When the temperature of a star varies: Spectral Type (OBAFGKM) varies Color varies Surface brightness varies Σ T 4 Σ = σt 4
16 16 Why doesn t the Sun collapse???? Gravity pulls the surface of the Sun in. What is pushing back??? Gravity The pressure of the gas layers below.
17 17 Gas Pressure keeps the Sun Pressure Gravity This applies to all layers of the Sun. Gravity pulls outer layers in, Gas Pressure pushes them out.
18 18 Hydrostatic Equilibrium Lots of large forces, but the star settles to a size, density, pressure, & temperature where everything balances out
19 19 Central pressures are HUGE!!!! Supports the whole weight of the star!
20 20 The Sun Density & Temperature increase towards the center of a star. 15 times denser than a lead brick! >10 million degrees! Fully ionized!
21 Hot gas Cool gas Energy leaks out of the surface all the time, due to thermal radiation Stars would all be shrinking if they couldn t keep their cores hot! 21 There must be an energy source that continually replaces the heat lost from the surface
22 22 High Temperature and Density allows nuclei to interact! Nuclei can FUSE together: Nuclear Fusion
23 23 Nuclear Fission is not Messy (i.e. radioactive!) Atomic bombs & power plants. Requires large complex nuclei to split apart. Clean. Future power plants? Requires only Hydrogen atoms.
24 24 Energy Production in Stars: The short form. 4 Hydrogen Atoms fuse to make 1 Helium Atom and a bunch of energy.
25 25 Why extra energy at the end? E = Mc 2!! Some mass converts to energy! (4 protons are slightly heavier than 2 protons + 2 neutrons)
26 26 How much mass? X kg X kg Mass Difference = X kg (0.7% of the original mass)
27 27 How much mass? Doesn t take much! 200lb 102 TA Releases 10% of the entire US energy consumption per year.
28 28 E = Mc 2 Energy released each second Mass Destroyed per second = must be due to mass destroyed each second 1 Solar Luminosity c 2 4 Billion kg Every Second!
29 29 This sounds like a lot, but it isn t Sun s Total Mass: 2 x kg Every second: 4 x 10 9 kg destroyed Tiny fraction! It would take years to fuse 0.7% of the Sun s mass. Whew. Q: Why can t the Sun burn more than 0.7% of its mass?
30 30 Suppose there is a momentary drop in the rate of fusion in the center of the Sun. The temperature: A. Drops B. Increases The central Pressure: A. Decreases B. Increases
31 31 What happens if the central pressure drops? The star: A. Shrinks B. Expands The central density: A. Decreases B. Increases
32 32 What happens if the central density increases? The rate of fusion: A. Decreases B. Increases The central temperature: A. Decreases B. Increases
33 33 What happens if the central temperature increases? The pressure: A. Decreases B. Increases The star: A. Shrinks B. Expands
34 34 What happens if the central temperature increases? The pressure: A. Decreases B. Increases The star: A. Shrinks B. Expands Energy production in the Sun is stable, meaning it easily recovers from small changes
35 35 A Recap: Stars get denser and hotter towards the center! They are entirely gaseous & ionized. High density and temperature allows nuclei to collide and interact! FUSION!!!!
36 36 The Recipe for Fusion (the anti-matter version of an electron!) Some energy is carried off in the kinetic energy of the positrons. Some is carried off by the energetic gamma ray photons. 2% is carried off by neutrinos
37 37 What are neutrinos? Nearly massless. Chargeless. Interacts only by the very short range weak force. Predicted in Detected in 1956!
38 38 Neutrinos are a critical signal that we understand energy generation in the Sun We think we know how much He is made every second So we d better get 2x as many neutrinos out every second!
39 39 Neutrinos rarely interact with anything! Nearly massless. Chargeless. Interacts only by the very short range weak force. Moves fast No electromagnetic force Needs to run smack dab into a tiny nucleus for anything to happen! It would take a light-year of lead to be sure of stopping a neutrino!
40 40 Super K, a big honking neutrino detector Deep underground. Detects neutrinos which have burrowed THROUGH the Earth!
41 41 Sudbury Neutrino Observatory 12m across! 6800 ft underground! Filled with 1000 tons of heavy water (D 2 0)
42 42 Image of the Sun in neutrinos!
43 43 BUT.. Experiments like Super-K only detect a fraction of the neutrinos expected! Points to new neutrino physics!
44 44 Neutrinos are changing flavor! The detectors detect only one specific type of neutrino -- the kind we think the Sun is emitting. Some neutrinos that make it to Earth must have switched to a different flavor!
45 45 So, we think we know how energy is produced How does the energy get out????
46 46 So how does the energy get out? 1. radiative diffusion 2. convection Both methods can operate, but they tend to operate in different regions of different stars. Some stars only have 1.
47 47 Radiative Diffusion: Photon comes in with one energy - and leaves with a different energy scatters off a free electron Photon Scattering
48 48 Radiative Diffusion The photons diffuse outwards, heating the gas as they go. Ionized gas Photons scattering shares energy with the electrons The electrons get hotter
49 49 Convection Hot stuff floats. Cool stuff sinks.
50 50 Slight temperature variations!
51 51 Granules up close. These are regions where hot gas is floating up, cooling, and then sinking back down.
52 52 The sun is convective near the surface, radiative near the center. The photons we detect from the Sun are NOT the photons produced in the core, but are thermal photons emitted by the hot dense gas in the skin of the Sun!
53 53 Cross section of the Sun: We only see the outer 2000 km of the sun! The Core is the region where Hydrogen is fusing into Helium
54 54 The Corona: Very hot halo of gas surrounding the Sun. T > 10 6 K! (really only visible optically during a solar eclipse)
55 55 Sunspots: magnetic fields! Sunspots are regions of high magnetic field strength.
56 56 Stuff that interacts with magnetic fields tends to align along magnetic field lines Iron Charged particles
57 57 Overhead Powerful eruptions: dynamic Sun! view Side view
58 58 Coronal Mass Ejections: These regions of high magnetic field strength Solar Storms are probably responsible for sending plumes Affects telecommunications! of charged particles out into the solar system.
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