Astronomy 114 - University of Massachusetts Amherstweinberg/a114/lectures/... · 2007. 3. 5. ·...

Astronomy 114

Lecture 13: Energy generation, magnetic fields

Martin D. Weinberg

weinberg@astro.umass.edu

UMass/Astronomy Department

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—1/17

Announcements

PS #4 will be posted on Wednesday or Friday, duebefore Spring Break (Friday 16 March)

Quizzes returned (and discussed) on Wednesday

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—2/17

Announcements

PS #4 will be posted on Wednesday or Friday, duebefore Spring Break (Friday 16 March)

Quizzes returned (and discussed) on Wednesday

Today:

Energy generation & magnetic fields

Our Star, the Sun, Chap. 18

Properties of Stars

The Nature of Stars, Chap. 19

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—2/17

Mass, energy and fusion (1/2)

Mass deficit: 0.03035 u

E = 0.03035 u × 931.494 MeV/u = 28.27 MeV

Or: 7.07 MeV per nucleon

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—3/17

Mass, energy and fusion (1/2)

Mass deficit: 0.03035 u

E = 0.03035 u × 931.494 MeV/u = 28.27 MeV

Or: 7.07 MeV per nucleon

How many H atoms in one gram?

Avogadro’s number: N = 6.02 × 1023

For each MeV, we get: N × 1.6 × 10−16

= 3.1 × 107 J

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—3/17

Mass, energy and fusion (1/2)

Mass deficit: 0.03035 u

E = 0.03035 u × 931.494 MeV/u = 28.27 MeV

Or: 7.07 MeV per nucleon

How many H atoms in one gram?

Avogadro’s number: N = 6.02 × 1023

For each MeV, we get: N × 1.6 × 10−16

= 3.1 × 107 J

Fusing 1 g of hydrogen (H) into helium (He) persecond generates 681 mega Watts!

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—3/17

Mass, energy and fusion (2/2)

Elements belowiron (Fe) can fromby fusion

Elements belowiron (Fe) cannot(radioactive decay)

Most elements heavier than helium are made instars!

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—4/17

P-P fusion reaction

3 steps in a fusion reaction

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—5/17

P-P fusion reaction

3 steps in a fusion reaction

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—5/17

P-P fusion reaction

3 steps in a fusion reaction

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—5/17

Solar model (1/2)

Fusion: radii < 0.25R⊙

Radiative diffusion:0.25R⊙ < radii < 0.71R⊙

Convection:0.71R⊙ < radii < 1.00R⊙

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—6/17

Solar model (2/2)

Theoretical computer models

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—7/17

Solar model (2/2)

Theoretical computer models

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—7/17

Testing the theory

Neutrinos

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—8/17

Testing the theory

Neutrinos

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—8/17

Testing the theory

Neutrinos

Helioseismology

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—8/17

Surface of the Sun

Three regions:

Photosphere— most of Sun’s luminosity

Chromosphere— Above the photosphere

Corona— Superheated region above theChromosphere

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—9/17

Surface of the Sun

Three regions:

Photosphere— most of Sun’s luminosityAbove: photons stream without interacting

Below: photons interact with solar material

Chromosphere— Above the photosphere

Corona— Superheated region above theChromosphere

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—9/17

Surface of the Sun

Three regions:

Photosphere— most of Sun’s luminosityAbove: photons stream without interacting

Below: photons interact with solar material

Chromosphere— Above the photosphereLower density, tenuous “atmosphere”. Slightlycooler.

Corona— Superheated region above theChromosphere

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—9/17

Surface of the Sun

Three regions:

Photosphere— most of Sun’s luminosityAbove: photons stream without interacting

Below: photons interact with solar material

Chromosphere— Above the photosphereLower density, tenuous “atmosphere”. Slightlycooler.

Corona— Superheated region above theChromosphereOrigin of the solar wind, protons that have escaped theSun.

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—9/17

Limb darkening

Photosphere–Sun appears darker around the edge

Photons from limb from greater height in atmosphere

Upper photosphere/lower chromosphere is cool

(F = σT4)

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—10/17

Limb darkening

Photosphere–Sun appears darker around the edge

Photons from limb from greater height in atmosphere

Upper photosphere/lower chromosphere is cool

(F = σT4)

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—10/17

Corona

Image of Sun during total eclipse

Material streams away from Sun

Narrow transition between chromosphere and corona

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—11/17

Sunspots (1/2)

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—12/17

Sunspots (1/2)

[movie]

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—12/17

Sunspots (2/2)

Sunspots are regions of high magnetic field

Zeeman splitting

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—13/17

Consequences: magnetic fields!

Generated by moving charges: electric currents

Continuous loops of lines of force that have bothtension and pressure (like rubber bands)

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—14/17

Consequences: magnetic fields!

Generated by moving charges: electric currents

Continuous loops of lines of force that have bothtension and pressure (like rubber bands)

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—14/17

Consequences: magnetic fields!

Generated by moving charges: electric currents

Continuous loops of lines of force that have bothtension and pressure (like rubber bands)

Can be strengthened by stretching them, twistingthem, and folding them back on themselves.

This stretching, twisting, and folding is done by thefluid flows within the Sun!

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—14/17

Solar dynamo (1/2)

Magnetic field is trapped in ionized gas (plasma)

Rotating Sun winds up the field lines

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—15/17

Solar dynamo (1/2)

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—16/17

Solar dynamo (1/2)

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—16/17

Solar dynamo (2/2)

TRACE solar

mission

(NASA)

UV image

A114: Lecture 13—05 Mar 2007 Read: Ch. 19 Astronomy 114—17/17