Astronomy 114 - University of Massachusetts Amherstweinberg/a114/lectures/... · 2007. 3. 5. · Quizzes returned (and discussed) on Wednesday Today: Energy generation & magnetic

Astronomy 114

Lecture 13: Energy generation, magnetic fields

Martin D. Weinberg

[email protected]

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


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


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




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


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




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


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!



Elements belowiron (Fe) can fromby fusion

Elements belowiron (Fe) cannot(radioactive decay)

Most elements heavier than helium are made instars!


P-P fusion reaction

3 steps in a fusion reaction


P-P fusion reaction



P-P fusion reaction



Solar model (1/2)

Fusion: radii < 0.25R⊙

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

Convection:0.71R⊙ < radii < 1.00R⊙


Solar model (2/2)

Theoretical computer models


Solar model (2/2)

Theoretical computer models


Testing the theory

Neutrinos


Testing the theory

Neutrinos


Testing the theory

Neutrinos

Helioseismology


Surface of the Sun

Three regions:

Photosphere— most of Sun’s luminosity

Chromosphere— Above the photosphere

Corona— Superheated region above theChromosphere


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



Surface of the Sun

Three regions:



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



Surface of the Sun

Three regions:



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

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


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)


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)


Corona

Image of Sun during total eclipse

Material streams away from Sun

Narrow transition between chromosphere and corona


Sunspots (1/2)


Sunspots (1/2)

[movie]


Sunspots (2/2)

Sunspots are regions of high magnetic field

Zeeman splitting


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!


Solar dynamo (1/2)

Magnetic field is trapped in ionized gas (plasma)

Rotating Sun winds up the field lines


Solar dynamo (1/2)


Solar dynamo (1/2)


Solar dynamo (2/2)

TRACE solar

mission

(NASA)

UV image


Documents

Astronomy 114 - University of Massachusetts Amherstweinberg/a114/lectures/... · 2007. 3. 5. · Quizzes returned (and discussed) on Wednesday Today: Energy generation & magnetic