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Astronomy 114 Lecture 12: How Stars Work (continued) Martin D. Weinberg [email protected] UMass/Astronomy Department A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—1/15 Announcements See the Lunar eclipse on Saturday! Special Orchard Hill Observatory Open House 6pm to 9pm (but check the web site, weather report is not promising . . . ) A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—2/15 Announcements See the Lunar eclipse on Saturday! Special Orchard Hill Observatory Open House 6pm to 9pm (but check the web site, weather report is not promising . . . ) Today: Energy generation and transport in stars Our Star, the Sun, Chap. 18 The Nature of Stars, Chap. 19 A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—2/15 How do stars work: Part I review Begin with a blob of gas Gravitational contraction of a “ball” of gas is balanced by pressure Energy of “falling” turned into heat Collapse stops when gravity is balanced by pressure Gravity “bottle” or self-gravity 73% of stars (and all matter in Universe) is Hydrogen Hydrogen is ionized Hot, charges moving ⇒ blackbody A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—3/15 Stellar nursery A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—4/15 How do stars work: Part II Equilibrium: balance of pressure and gravity Energy is radiated from surface of star Must be replenished or star will change quickly Good evidence that Sun has been stable for billions of years Know that the energy source is nuclear fusion deep inside Sun How does energy get to surface? Inner Sun, fusion temperatures of 10 million degrees Outer Sun, 5000 degrees A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—5/15 Energy transport (1/2) Three mechanisms for energy transport 1. Conduction: heat flows from hot to cold Energy transferred from atom to atom E.g. handle of frying pan on stove A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—6/15 Energy transport (1/2) Three mechanisms for energy transport 1. Conduction: heat flows from hot to cold Energy transferred from atom to atom E.g. handle of frying pan on stove 2. Radiative diffusion: high energy photons interact with matter give up some of their energy, replenishing local heat supply Random walk [demo] Takes 104 (ten thousand) years for photon generated to get out! Photons can also provide some of the pressure to support star against its own gravitational pull A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—6/15 Energy transport (2/2) 3. Convection: energy carried from hotter regions below to cooler regions above by bulk buoyant motions of the gas. Hot blobs of gas rise, release energy Cool blobs of gas fall Example: coffee cup with milk . . . A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—7/15 Energy transport (2/2) 3. Convection: energy carried from hotter regions below to cooler regions above by bulk buoyant motions of the gas. A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—7/15 Energy transport (2/2) 3. Convection: energy carried from hotter regions below to cooler regions above by bulk buoyant motions of the gas. [movie] A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—7/15 Mass is not always conserved. . . E = mc2 A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—8/15 Mass is not always conserved. . . E = mc2 electron volt: energy an electron gains if it moves 1 cm through a 1 volt field 14 electron volts to remove an electron from a hydrogen atom Energy equivalent of one atomic mass unit (mass of proton & neutron) is 931.4 MeV An electron has a mass which is roughly 1/1870 that of the proton If an electron and a positron annihilate the combined mass would be 2 x 1/1870 of a proton or 1.02 MeV A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—8/15 Mass, energy and fusion (1/3) Certain combinations of protons and nuclei have masses less than their sum of parts Can liberate energy by assembling a more massive nucleus A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—9/15 Mass, energy and fusion (2/3) Mass deficit: 0.00035 u A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—10/15 Mass, energy and fusion (2/3) Mass deficit: 0.00035 u E = 0.00035 u × 931.494 MeV/u = 0.33 MeV A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—10/15 Mass, energy and fusion (2/3) Mass deficit: 0.00035 u E = 0.00035 u × 931.494 MeV/u = 0.33 MeV 1 MeV = 1.6 × 10−16 J A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—10/15 Mass, energy and fusion (2/3) Mass deficit: 0.00035 u E = 0.00035 u × 931.494 MeV/u = 0.33 MeV 1 MeV = 1.6 × 10−16 J Need 2 × 1016 of these mass deficits to make one J (Joule). Seems like a tiny amount of energy in fusion . . . or is it? A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—10/15 Mass, energy and fusion (2/3) Mass deficit: 0.00035 u E = 0.00035 u × 931.494 MeV/u = 0.33 MeV 1 MeV = 1.6 × 10−16 J Need 2 × 1016 of these mass deficits to make one J (Joule). Seems like a tiny amount of energy in fusion . . . or is it? How many H atoms in one gram? A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—10/15 Mass, energy and fusion (2/3) Mass deficit: 0.00035 u E = 0.00035 u × 931.494 MeV/u = 0.33 MeV 1 MeV = 1.6 × 10−16 J Need 2 × 1016 of these mass deficits to make one J (Joule). Seems like a tiny amount of energy in fusion . . . or is it? How many H atoms in one gram? Avogadro’s number: N = 6.02 × 1023 A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—10/15 Mass, energy and fusion (2/3) Mass deficit: 0.00035 u E = 0.00035 u × 931.494 MeV/u = 0.33 MeV 1 MeV = 1.6 × 10−16 J Need 2 × 1016 of these mass deficits to make one J (Joule). Seems like a tiny amount of energy in fusion . . . or is it? How many H atoms in one gram? Avogadro’s number: N = 6.02 × 1023 N × 1.6 × 10−16 = 3.1 × 107 J A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—10/15 Mass, energy and fusion (2/3) Mass deficit: 0.00035 u E = 0.00035 u × 931.494 MeV/u = 0.33 MeV 1 MeV = 1.6 × 10−16 J Need 2 × 1016 of these mass deficits to make one J (Joule). Seems like a tiny amount of energy in fusion . . . or is it? How many H atoms in one gram? Avogadro’s number: N = 6.02 × 1023 N × 1.6 × 10−16 = 3.1 × 107 J Fusing 1 g of hydrogen (H) into helium (He) per second generates 30 mega Watts! A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—10/15 Mass, energy and fusion (3/3) Elements below iron (Fe) can from by fusion Elements below iron (Fe) cannot (radioactive decay) Most elements heavier than helium are made in stars! A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—11/15 P-P fusion reaction 3 steps in a fusion reaction A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—12/15 P-P fusion reaction 3 steps in a fusion reaction A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—12/15 P-P fusion reaction 3 steps in a fusion reaction A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—12/15 Solar model (1/2) Fusion: radii < 0.25R⊙ Radiative diffusion: 0.25R⊙ < radii < 0.71R⊙ Convection: 0.71R⊙ < radii < 1.00R⊙ A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—13/15 Solar model (2/2) Theoretical computer models A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—14/15 Solar model (2/2) Theoretical computer models A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—14/15 Testing the theory Neutrinos A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—15/15 Testing the theory Neutrinos A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—15/15 Testing the theory Neutrinos Helioseismology A114: Lecture 12—02 Mar 2007 Read: Ch. 18, 19 Astronomy 114—15/15