Chapter 10 Notes:   The Sun

 Solar Atmosphere

·          Gasses, mainly H and He

·          Hot gasses – core millions of degrees, surface (photosphere) 5800K

·          Heat flows from center then radiates into space

 Photosphere:

·          Thin layer of gas from which we receive most of Sun’s light

·          Gasses below more dense and hot

·          Low density gas – 3400 times less dense than air we breathe

·          density low enough to emit light

·          Solar spectrum is absorption (refer to Kirchhoff’s laws)

·          Mottled appearance due to granulation. Convection going on; warm gas rising from interior comes up in center of granule, cools, then sinks down along edge of granule            

·          Typical granule size of Texas, lasts 10-20 minutes

Chromosphere:

·          Above photosphere, irregularly shaped

·          Reddish pink color from ionized H

·          1000 times fainter than photosphere

·          produces emission spectrum (an excited low density gas)

·          atoms are ionized – lost electrons

·          spicules – rising jets of gas, last 5 – 15 minutes

·          temperature of gas in chromosphere increases with height

 Corona:

Can’t see it visually because light from photosphere so bright

• Instruments that block light from photosphere show corona extending 20 solar radii, 10% of way to Earth
• Corona has continuous spectrum
• Temp ~ 1 million K so electrons moving very fast and lots of random Doppler shifts smear absorption lines
• Superimposed are emission lines of highly ionized gasses
• Temp ~ 500,000K right above chromosphere, 1 million K in corona, 2 million K or more in outer corona
• Though hot, very low density so gas doesn’t emit much radiation

 Solar Wind:

• Gas flowing away from sun following magnetic fields pointing outward
• These ionized atoms make up solar wind
• Originates in coronal holes
• Average speed 300 – 800 km/sec
• Sun loses mass through solar wind at rate of 10^-14 solar mass/year

Solar Activity

Shaped by magnetic fields on Sun; features constantly changing

 Sunspots

• Dark spots on photosphere
• Cooler region on photosphere (4240K vs 5800K – small temp difference but big difference in brightness)
• Umbra, dark region; penumbra, lighter region
• Typical size 2 times Earth diameter
• Number of sunspots varies with a period of 11 years – sunspot cycle

 Sunspot Cycle

• spots appear in Sun’s middle latitudes @ 35 deg above and below equator
• As cycle proceeds spots appear at lower latitudes
• By the end of cycle, spots within 5 deg of equator
• Butterfly diagram plots changing latitude of spots over time

 Sun’s Magnetic Cycle

• Sun doesn’t rotate as rigid body, exhibits differential rotation.
• Equatorial regions of photosphere rotate faster than higher latitude regions Equator rotates in ~ 25 days, 45 deg rotates in ~ 28 days
• Deeper layers of gas rotate more slowly than photosphere

 Sun’s magnetic field powered by energy flowing outward from interior.

• Details poorly understood, but gas is ionized so is a good conductor of electricity
• Dynamo Effect – produced when a rapidly rotating conductor is stirred by convection to produce a magnetic field

 Sunspots and magnetic behavior

• Sunspots generally appear in groups of 2 or more
• Like horseshoe magnet with one end N, one S
• At any given time sunspot pairs south of equator have reversed polarity compared with those north of equator
• This lasts through one sunspot cycle then reverses
• Therefore sun’s magnetic cycle 22 years
• Magnetic field gets twisted over time due to sun’s differential rotation
• After 11 years of tangling the magnetic fields at surface start changing; the field rearranges itself in a simple pattern and a new cycle begins

 Nuclear Fusion

Four known forces in nature: gravity, electromagnetic force, weak force (radioactive decay of certain nuclear particles), strong force (binds together atomic nuclei).

Nuclear energy from strong force

Fission – splitting heavy elements into less massive elements releasing energy in process (eg. Nuclear power plants)

Fusion – light nuclei fused together to form heavier nuclei – powers stars

• Most common reaction in stars is hydrogen fusion, where H fuses to He
• Sun fuses H to He in proton—proton chain
• Four protons (H nuclei w/o electrons)  combine to form one He nucleus
• The four H nuclei contain slightly more mass than He nucleus
• Lost mass converted into energy; the amount can be determined by Einstein’s famous equation E=mc²
• Creating one He nucleus only makes a tiny bit of energy
• Sun has 10³⁸ reactions per second, transforming 5 million tons of mass into energy every second, just to balance its own gravity
• Over 10 billion years sun will have converted less than 0.07% of its matter into energy
• Fusion only happens when nuclei of atoms get very close
• Nuclei carry positive charge so they repel each other with electrostatic force called Coulomb force
• This repulsion is called Coulomb barrier
• To overcome barrier takes violent collisions
• gas must be very hot for such violent collisions to occur (an object’s temperature is a measure of the speed at which its particle move)
• Nuclear reactions take place only in the sun’s core where gas is hot and dense – the heat allows nuclei to overcome Coulomb barrier, the density means these collisions occur a lot

 Proton—proton chain is series of three nuclear reactions, where protons are added one at a time

• 2 H nuclei (protons) combine to form a heavy H nucleus, emitting a positron (positively charged electron) and a neutrino
• heavy H absorbs another proton, emits a gamma ray (photon), and becomes a light weight He nucleus
• 2 light He nuclei combine to form a normal He nucleus and 2 H nuclei
• gamma ray photons are absorbed by surrounding gas before they can travel more than a few cm
• this absorption adds to gas heat
• positrons combine with free electrons and both particle vanish, converting their energy into gamma rays
• neutrinos travel near speed of light and almost never interact with other particles
• neutrinos don’t heat gas, but travel relatively unimpeded from star, carrying with them ~2% of energy produced in reaction

 Solar Neutrino Problem

• Neutrinos produced in Sun’s core flow out but almost never interact with matter
• 1012 neutrinos flow through our bodies every second
• neutrinos hard to detect, but possible. Put cleaning fluid (perchloroethylene) in 100,000 gallon tank 1 mile below Earth surface
• Theory says once a day a neutrino should convert a chlorine atom in the tank to radioactive argon, which can be detected later by its radioactive decay
• Scientists were surprised to find fewer neutrinos than were predicted, only one every three days rather than one every day
• Possible solution – neutrinos come in 3 “flavors” – electron, muon and tau. Above experiment can only detect electron neutrinos
• If neutrinos oscillate through the 3 types, then only 1/3 would be detected – exactly what is observed