Chapter 9The Sun

Chapter 9The Sun

Sunspots

Our SunOur Sun

• Is the nearest star

• 8 light minutes away

• Next nearest star is 4.3 light-years away (300,000X further than sun)

Figure 9.1The Sun

Table 9.1Some Solar Properties

Solar RotationSolar Rotation

• Differential rotation

• Measure by timing sunspots

• Faster at equator

• Slower at poles

• X-ray and visible light movie of sun

Figure 9.2Solar Structure

Solar StructureSolar Structure• Core

• Radiation zone

• Convection zone

• Photosphere (“surface” we see)

• Chromosphere

• Transition zone

• Corona

• Solar wind

Solar LuminositySolar Luminosity• Solar constant 1400 W/m2 above earth’s

atmosphere

• 1000 W/m2 at earth’s surface

• Solar constant and distance to sun gives luminosity of about 4 X 1026 W

Figure 9.3Solar Luminosity

Stellar balanceStellar balance• Outward pressure of hot gas

• Inward pull of gravity

• Balanced at every point in a star

Figure 9.4Stellar Balance

Standard solar modelStandard solar model• Mathematical and physical model of sun

• Based on observations and physical laws

• Predicts density and temperature

• Helioseismology allows knowledge of interior structure

Figure 9.5Solar Oscillations

Figure 9.6Solar Interior

Sun’s temperature and densitySun’s temperature and density• Core density 150,000 kg/m3 (20X iron)

• Core temperature 15 million K

• Core is a gas (plasma)

• Photosphere is 0.0001X density of earth’s atmosphere

• Photosphere temperature 5780 K

Discovery 9.1SOHO: Eavesdropping on the Sun

SOHOSOHO• Solar and Heliospheric Observatory

• European Space Agency

Solar energy transportSolar energy transport• Near core - very hot

• Gas is completely ionized (plasma)

• No photons captured - transparent to radiation

• Outer edge of radiation zone cool enough for electrons to re-combine with nuclei

• Photons all absorbed

Convection ZoneConvection Zone• Energy transported by rising hot gases

• Cooler gas sinks

• Convection cells vary in size with depth in convection zone

• Tens of thousands of km to a thousand km convection cells

• Photons from photosphere escape into space

Figure 9.7Solar Convection

Evidence for convectionEvidence for convection• Solar granulation of photosphere

• Granules size of a large US state

• Last 5 to 10 minutes

• Bright regions - hot gas rising

• Dark regions - cool gas sinking

• 500 K difference between hot and cool

Figure 9.8Solar Granulation

Solar Granulation movieSolar Granulation movie• Near infrared

• 60 minute sequence sped up

• At http://www.bbso.njit.edu or click here

Doppler shiftDoppler shift• Bright granules move up at about 1 km/s

• Dark granules move down at about 1 km/s

SupergranulationSupergranulation• Larger scale flow beneath solar surface

• 30,000 km across

Figure 9.9Solar Spectrum

Composition of solar atmosphere

Composition of solar atmosphere

• Primarily H and He

• Also O, C, N, Si, Mg, Ne, Fe, S

• Similar to Jovian planets and rest of universe

Table 9.2The Composition of the Sun

Figure 9.10Solar Chromosphere

Solar chromosphereSolar chromosphere• Above photosphere and less dense

• Pinkish hue from H emission

• Expelling jets of hot matter - spicules

• Last minutes

• 100 km/s

Figure 9.11Solar Spicules

CoronaCorona• Corona visible during total solar eclipse

• Emission spectrum visible against blackness of space

• Ionized atoms - high coronal temperatures

Figure 9.12Solar Corona

Transition ZoneTransition Zone• Minimum temperature of 4500 K in

chromosphere

• Temperature climbs through transition zone

• Reaches several million K in corona

Figure 9.13Solar Atmospheric Temperature

Solar windSolar wind• Starts 10 million km above photosphere

• Hot coronal gas escapes sun’s gravity

• Millions of tons of sun ejected each second

• Only lost 0.1% of mass in 4.6 billion years

X-rays in coronaX-rays in corona• Photosphere emits primarily visible light

• Hotter corona emits primarily X-rays

• Coronal holes - visible in X-rays

• Solar wind escapes in coronal holes

• Related to magnetic fields

Figure 9.14Sunspots

Figure 9.15Sunspots, Up

Close

SunspotsSunspots• In photosphere

• Cooler (darker) than surrounding material

• Dark umbra (4500 K)

• Grayish penumbra (5500 K)

• Typically 10,000 km across (size of earth)

Sunspot magnetismSunspot magnetism• Magnetic field of photosphere stronger

than earth’s

• Magnetic field in sunspots is 1000X greater than surrounding photosphere

• Field lines perpendicular to surface

• Strong fields interfere with convective flow

• Causes sunspots to be cooler

Sunspot magnetic polaritySunspot magnetic polarity• Sunspots in pairs at same latitude

• Pair members have opposite polarity N&S

• Leading spot in a hemisphere always has same polarity

• Leading spot in other hemisphere has opposite polarity

Figure 9.16Sunspot Magnetism

Magnetic field wrappingMagnetic field wrapping• Differential rotation “wraps” magnetic field

• North-south re-oriented to east-west

• Convection lifts field to surface

• Twisting and tangling results

• Some kinks rise out of photosphere

• Forms sunspot pair

Figure 9.17Solar Rotation

Analogy 9.1A tangled garden hose

Figure 9.18Sunspot Cycle

Maunder minimumMaunder minimum• Cycle varies from 7 to 15 years

• Overall activity varies

• Solar inactivity from 1645-1715

• Maunder minimum caused “Little Ice Age”

Figure 9.19Maunder Minimum

Solar prominencesSolar prominences• Loops or sheets of gas ejected into lower

corona

• Maybe due to magnetic fields near sunspots

• Typically 100,000 km (10X diameter of earth)

Figure 9.20Solar Prominences

Solar prominence movieSolar prominence movie• Big Bear Solar Observatory

• At http://www.bbso.njit.edu or click here

Solar flaresSolar flares• More violent than prominences

• Sweeps across active region in minutes

• Temperature of millions of K

• Material blasted into space

Figure 9.21Solar Flare

Solar flare movieSolar flare movie• Big Bear Solar Observatory

• Go to http://www.bbso.njit.edu or click here

Corona activityCorona activity• Coronal mass ejection

• Several times per day during sunspot maximum

• Can cause communication and power disruption on earth

Figure 9.22Coronal Mass Ejection

Figure 9.23Coronal Hole

Figure 9.24Active Corona

SOHO Coronal mass ejectionSOHO Coronal mass ejection• White ring is size of sun

• Play movie or go to

• http://sohowww.nascom.nasa.gov/data/LATEST/current_c2.mpg

Sun’s energy sourceSun’s energy source

• Nuclear fusion reactions in core

• Two nuclei combine forming 3rd nucleus plus energy

• 3rd nucleus has less mass than sum of two nuclei

• Mass converted to energy E = mc2

• Need high temperature to overcome charge repulsion

Figure 9.25Proton Interactions

Proton-Proton chainProton-Proton chain• 4 protons helium-4 + 2 neutrinos +

energy

• Neutrino is chargeless and virtually massless particle

• Neutrinos easily pass through sun

Figure 9.26Solar Fusion

Energy generatedEnergy generated• 600 million tons of H fused into He every s

• Sun can sustain this another 5 billion years

• Energy produced in core as gamma rays

• Neutrinos also carry off energy

Figure 9.27a Neutrino Telescope in Japan

Figure 9.27bNeutrino Telescope in Ontario, Canada

NeutrinosNeutrinos• Less neutrinos detected at earth than

predicted by standard solar model

• Neutrinos oscillate into new types during journey to earth

• Latest neutrino detectors can find all types

More Precisely 9.2Energy Generation in the Proton–Proton Chain