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The Sun:The Sun: A Garden -A Garden -

Variety StarVariety Star

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The SunBiggest object in the solar system

diameter 1,392,000 km109 x Earth’s diameter10 x Jupiter’s diameter

Most Massive 333,000 x Earth’s mass1,000 x Jupiter’s massso heavy, everything else orbits around it! so heavy, it makes its own heat and light

Temperature of 15,000,000 K in its corenuclear power!

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The Sun’s Profile

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The Sun’s CompositionThe Sun contains the same elements as the Earth, but not in the same proportionsAbout 73% of the Sun’s mass is comes from hydrogen, and another 25% from helium

Other chemical elements make up the rest 2%

The fact that the Sun are mostly made up of H and He was first shown by Cecilia Payne-Gaposchkin

The 1st woman to get a PhD in astronomy in the U.S.

Elements Fraction

Hydrogen 92.1%

Helium 7.8% 

Oxygen 0.061% 

Carbon 0.030% 

Nitrogen 0.0084% 

Neon 0.0076% 

Iron 0.0037%

Silicon 0.0031% 

Magnesium 0.0024%

Sulfur 0.0015% 

All others 0.0015% 

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The parts of the SunThe parts of the Sun

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The Sun's InteriorFrom inside out:

CoreRadiative zone Convection zone Photosphere ChromosphereTransition region Corona

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The Sun's CoreThe core

is the innermost 10% of the Sun's massgenerates energy from nuclear fusion has the highest temperature and density

temperature 10 million Kdensity = 160 x density of water = 20 x density of ironat this temperature, the core is a gasno molten interior

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How does Heat from the Core Reach Us?

Three ways to transfer heat:Conduction: direct contact

A spoon in a hot cup of coffee gets warm

Convection: moving currents in a fluid

Hot air risesHot current in boiling water

Radiation: electromagnetic waves emitted by a heat source and absorbed by a cooler material

Electric stove

Heat from the Sun reaches us through the EM waves it emits

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Radiative ZoneRadiation transfers heat from the interior of the Sun to its "cooler" outer layersThe core & radiation zone make up 85% of the SunThe temperature drops from 10 million K at the inner side of the radiative zone to 2 million K at its edgeThe energy generated in the core is carried by photons that bounce from particle to particle through the radiative zone

The photons are too energetic to be absorbed by atomsEach photon bounces so many times that it is estimated to take one million years to reach the outer edge of the region

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Convection ZoneMatter at the base of the convection zone is “cool” enough (2 million K) for the atoms to absorb energy and hold on to it Convection occurs in this region The hotter material near the top of the radiation zone (the bottom of the convection zone) rises while the cooler material sinks — heated below like a pot of boiling waterIt takes a week for the hot material to carry its energy to the top of the convection zone

radiative zone

hot

cool

convection zoneconvection zone

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PhotosphereThis is the Sun’s deepest layer that one can see from the outside Photosphere means “light sphere”It is the visible “surface” of the Sun

From this layer, photons can finally escape to spaceThe surface is not something one could land or float on

The photosphere is about 500 km thickThe gas is so dense that you could not see through itThe gas emits a continuous spectrum of light

It features sunspots

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Temperature of Photosphere The photosphere temperature is about 5,800 KThe sunspots appear darker because they are cooler than their surroundings

The center of a typical sunspot has a temperature of 4,000 K

The spectrum and energy output of the radiation emitted from the photosphere obey Wien’s Law and Stefan-Boltzmann law

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Features of PhotosphereSunspots

dark spots, 1500 K, cooler than surroundings glow by themselves

granulestops of convection cells700 to 1000 km diameterlast 10 minutescenters ~ 100 K hotter than edges

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SunspotsDiscovered by Galileo GalileiSun's surface sprinkled with small dark regions - sunspotsSunspots are darker because they are cooler by 1000 to 1500 K than the rest of the photosphere Spots can last a few days or as long as a few months  Galileo used the longer-lasting sunspots to map the rotation patterns of the SunSunspots number varies in a cycle with an average period of 11 years

Cycle starts with minimum and most of them are at around 35° from the solar equator At solar maximum (number peaked), about 5.5 years later, most of the sunspots are within just 5° of the solar equator

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Sunspots and Magnetic FieldSunspots = regions of strong magnetic fieldsFound by observation of Zeeman effect

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Sun RotatesGalileo

discovered sunspotssunspots moved sun rotates

Rotation – speed depends on latitude

equator once/25 days30º N once/26.5 days60º N once/30 daysJupiter also does this

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ChromosphereVisible during solar eclipses as a thin pink layer at the edge of the dark MoonColorful layer – “color sphere”

Color due to hydrogen bright emission lineAlso shows yellow emission due to helium – discovered in 1868 – new element previously not seen on Earth

Helium was found on Earth in 1895

The chromosphere is only 2,000 to 3,000 km thickTemperature rises outward away from the photosphere – from 4,500 K to 10,000 K

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Transition RegionIt’s a thin region (about 10 km thick) in the Sun’s atmosphere where temperature changes from 10,000 K to nearly 1,000,000 K

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Solar WeatherThe Sun has complex and violent weather patternsChromosphere contains jet-like spikes of gas – called spiculesSpicules rise vertically through the chromosphereLast 10 minutesConsist of gas jets, at 30 km/s Rise to heights of 5000 to 20000 kmT ~ chromosphere

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Corona (1)The outermost part of the Sun’s atmosphere is called the corona It is visible during total solar eclipses as a pearly-white glow around the dark Moon The corona has a very high temperature of ~1-2 million KIt is known to be very hot because it contains multiply ionized atoms

At very high temperatures, atoms like iron can have 9 to 13 electrons ejected (the atoms become ionized)9-times ionized iron is only produced at a temperature of 1.3 million K 13-times ionized iron means the temperature gets up to 2.3 million K! 

Total solar eclipse in 1973Total solar eclipse in 1973

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Corona (2)Most of the corona is trapped close to Sun by loops of magnetic field lines

In X-rays, those regions appear bright

Some magnetic field lines do not loop back to the Sun and will appear dark in X-rays

These are called coronal holes

More details visible at short wavelengths

A solar eclipse photographed in the extreme ultraviolet taken by the SOHO spacecraft 

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X-rays from the Corona

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ProminencesBright clouds of gas forming above the sunspotsQuiet prominences

40,000 km above surfaceLast days to several weeks

Eruptive prominences700 km/sRareSurge prominences

Last up to a few hours Shoot gas up to 300,000 kmGas speed ~1300 km/s

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Prominences follow magnetic-field

loops

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Solar FlaresSolar flares are eruptions more powerful than surge prominencesFlares last from a few minutes to a few hoursA lot of ionized material is ejected in a flareUnlike the material in prominences, the solar-flare material moves with enough energy to escape the Sun's gravityWhen such a burst of ions reaches the Earth, it interferes with radio communicationSometimes a solar flare will cause voltage pulses or surges in power and telephone lines

Brownouts or blackouts may result

Humans traveling outside the protection of the Earth's magnetic field will need to have shielding from the powerful ions in a flare

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Solar WindFast-moving charged particles (mostly protons and electrons) can escape the Sun's gravitational attractionThe stream of particles is called the solar windThey move outward at a speed of about 400 km/s

They can reach the farthest reaches of the solar system

Solar-wind particles passing close to a planet with a magnetic field are deflected around the planet

Some are deflected to the planet's magnetic polesAs the particles hit the planet's atmosphere, they cause the molecules in the atmosphere to produce beautiful curtains of light called the auroras

Aurora borealis in the northern hemisphere Aurora australis in the southern hemisphere

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Is the Sun a Variable Star?What is more certain than that the Sun will rise tomorrow?We’ve already seen that sunspots follow an 11-year cycleOn longer time scales, the Sun undergoes changes in overall activityChanges are only about 0.1%!

Yet this is enough to affect our climate

In the mid 1600’s the Sun’s output was particularly low the “Little Ice Age”Other stars are seen to vary by 0.3%, up to 1%