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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
Astronomy 114
Lecture 12: How Stars Work (continued)
Martin D. Weinberg
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, weatherreport 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, weatherreport 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 balancedby 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 forbillions of years
Know that the energy source is nuclear fusiondeep inside Sun
How does energy get to surface?
Inner Sun, fusion temperatures of 10 milliondegrees
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 withmatter give up some of their energy, replenishinglocal heat supply
Random walk [demo]
Takes 104 (ten thousand) years for photon
generated to get out!
Photons can also provide some of the pressure tosupport 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 belowto cooler regions above by bulk buoyant motions ofthe 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 belowto cooler regions above by bulk buoyant motions ofthe 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 belowto cooler regions above by bulk buoyant motions ofthe 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 cmthrough a 1 volt field
14 electron volts to remove an electron from ahydrogen atom
Energy equivalent of one atomic mass unit (mass ofproton & neutron) is 931.4 MeV
An electron has a mass which is roughly 1/1870 thatof the proton
If an electron and a positron annihilate the combinedmass 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 havemasses less than their sum of parts
Can liberate energy by assembling a more massivenucleus
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) persecond 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 belowiron (Fe) can fromby fusion
Elements belowiron (Fe) cannot(radioactive decay)
Most elements heavier than helium are made instars!
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
