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Lecture 30: The Sun

The Sun•The Sun is a typical star in terms of mass, radius, and composition –the Sun is a ball of hot, glowing gas

•The Sun contains no solid material, but it does have a visible surface, called the photosphere

•The chromosphere and corona lie above the photosphere:

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The Sun

•The Sun’s diameter is 100 times larger than the Earth’s, and 10 times larger than Jupiter’s

•Below the photosphere lie the convection zone, the radiation zone, and the solar core:

Regions within the Sun

core 15,000,000 K generated by

nuclear fusion

radiation zone 7,000,000 K transported by photons

convection zone 2,000,000 K transported by convective turnover

photosphere 6,000 K escapes as radiation

chromosphere 4,500 K (transition region)

corona 1,000,000 K X-rays produced

solar wind 2,000,000 K energetic particles stream away from the Sun

Name Temperature Energy

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Bulk Properties of the Sun•The Sun’s mass is 2 x 1033 g, or about 300,000 times the mass of the Earth

•The Sun’s radius is 7 x 1010 cm, or about 110 times larger than the Earth

•The average density of the Sun is 1.4 g/cm3, about the same as Jupiter

•The Sun rotates differentially, with a period of 27 days at the equator and 31 days at the poles

Bulk Properties of the Sun•Wien’s law tells us that the temperature at the photosphere is about 6,000 K, since the Sun emits mostly yellow light:

•The amount of energy radiated by the Sun each second is the enormous solar luminosity, L=3.9 x 1026 watts

•At the distance of the Earth, this gives a solar constant of 1,400 watts/meter2, which produces gentle heating

Solar Interior•We can’t directly observe the interior of the Sun

•Therefore we must use indirect techniques to learn about its internal structure

•This is similar to the situation with the Earth

•We can study the solar interior using helioseismology, which is the analysis of acoustic waves observed on the solar surface

•Only P -type (pressure) waves are observed since the Sun has no solid interior to support the propagation of S -type (shear) waves

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Energy Generation in the Sun•The solar core has a density of 150 g/cm3 and a temperature of 15,000,000 K

•When the thermal motion is fast enough to overcome the repulsiveelectrostatic force, the attractive strong force can cause two protons to fuse into helium:

•The core temperature in the Sun is high enough to drive nuclear reactions:

H HeH+ = ++ energy particles

H H

Electrostatic repulsion

HH

Strong force

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Energy Transport in the Sun•The energy from the nuclear reactions is carried outward from the core towards the surface:

•The energy is carried via radiation or convection within the various regions of the Sun

•In the radiation zone, the gas is ionized and photons can move freely without being absorbed

•At the bottom of the convection zone, the photons are absorbed by atoms, producing strong heating

Energy Transport in the Sun

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•The top of the convection zone is cooled by the emission of radiation into the vacuum of space

•This produces a strong temperature gradient between the bottom and top of the convection zone

•The temperature gradient drives convection, similar to the boiling of water in a kettle:

Energy Transport in the Sun

Convection cells

•The photosphere radiates a blackbody spectrum

•Absorption lines appear due to absorption by cooler, overlying material

Energy Transport in the Sun

Absorption Spectrum

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•The cause for the extreme temperature rise in the corona is not yet understood…

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Ultraviolet images of the Sun

reveal gas at a temperature of 1.5

million degrees

•The surface activity we observe on the Sun is driven by the solar magnetic field

•This produces sunspots, energetic flares, magnetic loops

Solar Magnetic Fields

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X-Ray images of the Sun reveal gas at a temperature

of 10 million degrees

Solar Activity in X-Rays

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Dark regions in the X-ray images are cool “coronal holes”

1994 total solar eclipse, showing solar wind

Solar streamers and the Sun-

plunging comet SOHO-6

Sun Grazing Comets 1

Sun Grazing Comets 2

Christmas Comet

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Comet Hyakutake (1996)

SOHO video of near-Sun passage

The Sun’s surface is very active, often producing huge flares

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Sunspots

•Sunspot patterns display several types of variability

Solar VariabilitySunspots and Solar activity exhibit a 16 year cycle of variability, as well as much longer timescale changes

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•The bulk properties of the Sun are:

radius = 7 x 1010 cm

mass = 2 x 1033 g

density = 1.4 g/cm3

•A white dwarf star has the mass of the Sun, but the size of the Earth, with a density of 1.4 x 106 g/cm3, or 1.5 tons/cm3

•A neutron star has the mass of the Sun, but the size of Washington, DC, with a density of 4.7 x 1014 g/cm3, or 500 million tons/cm3, or 10,000 Titanics/cm3!!!

•You will be learning more about these exotic objects next semester in ASTR 113…

•Have a great holiday!!!

Other Types of Stars