© Sierra College Astronomy Department The Sun Our Star

© Sierra College Astronomy Department

The SunThe SunOur StarOur Star

Summary for Final Last week of classes 12/10 to 12/14 First hour (Monday):

Planetarium Sky Quiz (30 pts) Final, Part 1 (Solar System Object Quiz, 20 pts)

Second hour (Wednesday): Final, Part 3: SCANTRON Test (Form 882, #2 pencils),

70 questions from Review questions, cumulative, includes question on SGA and Planispheres (70 pts)

During your Third Hour Final, Part 2: Group effort on questions relating to 3rd

hour (20 pts) All extra credit due by 12/14 at NOON Review session: TBA

© Sierra College Astronomy Department3

The Sun - Our StarThe Sun - Our Star

Solar DataSolar Data

As viewed from the Earth, the Sun As viewed from the Earth, the Sun has an average angular diameter of has an average angular diameter of 3131’’ 59 59”” and is at an average distance and is at an average distance from the Earth of from the Earth of 1.50 X 101.50 X 1088 km km..

The diameter of the Sun can therefore The diameter of the Sun can therefore be calculated as be calculated as 1.39 X 101.39 X 1066 km km, or , or about about 110 times110 times Earth’s diameter and Earth’s diameter and about about 10 times10 times Jupiter’s. Jupiter’s.

Sun movie

Sun




The mass of the Sun is The mass of the Sun is 1.99 X 101.99 X 103030 kg kg, , more than more than 300,000300,000 times Earth’s mass. times Earth’s mass.

The Sun’s average density is The Sun’s average density is 1.41 g/cm1.41 g/cm33, , about the same as the density of Jupiter.about the same as the density of Jupiter.

The escape velocity of the Sun is The escape velocity of the Sun is 620 km/s620 km/s.. The Sun rotates in The Sun rotates in 25.4 days25.4 days at its equator at its equator

and and 36 days36 days near its poles. near its poles.

Sun in OpticalSun in UV

Solar

http://umbra.nascom.nasa.gov/images/latest.html

Solar 10-27-03676.7 nm28.4 nm

Solar (19.5)






The Sun emits energy is all portions of the The Sun emits energy is all portions of the electromagnetic spectrum with its peak in the electromagnetic spectrum with its peak in the visible portion.visible portion.

LuminosityLuminosity is the total power output of the Sun, is the total power output of the Sun, which is equal to 3.8 X 10which is equal to 3.8 X 102626 watts. watts.

BrightnessBrightness: Solar energy strikes the Earth at the : Solar energy strikes the Earth at the rate of rate of 1,380 watts/m1,380 watts/m22 ( (Solar ConstantSolar Constant).).

We can examine the surface of the sun by looking We can examine the surface of the sun by looking at its spectra (e.g., the at its spectra (e.g., the Fraunhofer linesFraunhofer lines).).

The Sun is a The Sun is a G2VG2V star with an absolute star with an absolute magnitude of magnitude of 4.84.8..



Basic Solar PhysicsBasic Solar Physics

To estimate the internal parameters of To estimate the internal parameters of the Sun, astronomers must solve a set the Sun, astronomers must solve a set of equations that describe how pressure, of equations that describe how pressure, temperature, mass, and luminosity temperature, mass, and luminosity change.change.

We need to know the chemical We need to know the chemical composition, rate of energy production composition, rate of energy production and surface conditions to solve these and surface conditions to solve these equations.equations.



Solar Energy SourceSolar Energy Source

Must produce Must produce 3.9 x 103.9 x 1026 26 WattsWatts CombustionCombustion

Hydrogen + Oxygen produces energy but would last only Hydrogen + Oxygen produces energy but would last only 1000 years1000 years

Meteorite ImpactsMeteorite Impacts The infall of material can be converted into heat and lightThe infall of material can be converted into heat and light About About 1/10 Earth masses1/10 Earth masses of debris would have to fall into the of debris would have to fall into the

Sun each yearSun each year Gravitational contractionGravitational contraction

The infall of material produces energyThe infall of material produces energy Could sustain sun for about Could sustain sun for about 500 million years500 million years Still short of geologic age of Earth Still short of geologic age of Earth (~ 4.5 billion yr(~ 4.5 billion yr))

Demo




Mass/Energy ConversionMass/Energy Conversion Einstein proposed in 1905 that mass and Einstein proposed in 1905 that mass and

energy are interconvertible - derives famous energy are interconvertible - derives famous E = mcE = mc22 equation. equation.

Nuclear FissionNuclear Fission Large atoms break into two and release energyLarge atoms break into two and release energy Problem: Sun does not contain enough of the big atomsProblem: Sun does not contain enough of the big atoms

Nuclear FusionNuclear Fusion Light atoms combine to produce larger atom and release Light atoms combine to produce larger atom and release

energyenergy Sun full of small atomsSun full of small atoms Turns out to be the right answerTurns out to be the right answer

Fusionfission




In In nuclear fusionnuclear fusion, two nuclei (consisting of protons , two nuclei (consisting of protons and neutrons) combine to form a larger nucleus, and neutrons) combine to form a larger nucleus, releasing energy in the process.releasing energy in the process.

Large temperatures (Large temperatures (15.6 million K15.6 million K) and densities ) and densities ((150,000 kg/m150,000 kg/m33) are needed to overcome ) are needed to overcome electromagnetic repulsion between the protons.electromagnetic repulsion between the protons.

This only occurs in the Sun’s core (This only occurs in the Sun’s core (inner 10% of inner 10% of its radiusits radius).).

In the Sun,In the Sun, 44 hydrogen nuclei are fused to formhydrogen nuclei are fused to form 11 helium nucleus and energy (helium nucleus and energy (proton-proton chainproton-proton chain). ).

ppchain

fusion




To produce the Sun’s energy output nearly To produce the Sun’s energy output nearly 5 million tons of matter5 million tons of matter must be converted must be converted into energy into energy each secondeach second..

This in turn requires 610 billion kg of This in turn requires 610 billion kg of hydrogen be transformed into 606 billion kg hydrogen be transformed into 606 billion kg of helium.of helium.

Solar lifetimeSolar lifetime: ~ 10 billion years: ~ 10 billion years But how do we confirm this from But how do we confirm this from

observations?observations?



Solar NeutrinosSolar Neutrinos NeutrinosNeutrinos are highly non-interacting particles are highly non-interacting particles

which can be used as a direct probe into the which can be used as a direct probe into the Sun (if our nuclear fusion idea is right)Sun (if our nuclear fusion idea is right)

However, they are incredibly hard to detectHowever, they are incredibly hard to detect Huge underground detectors are used to look Huge underground detectors are used to look

for neutrinos from the Sunfor neutrinos from the Sun Until recently there seemed to be too few Until recently there seemed to be too few

coming from the Suncoming from the Sun Solution: neutrinos have a little mass and come in Solution: neutrinos have a little mass and come in

different “flavors” and change identity on the way to different “flavors” and change identity on the way to Earth!Earth!

SDHomestakeNeutrinodetector

SudburyNeutrinodetector

ppchain



Basic Interior StructureBasic Interior Structure

Hydrostatic EquilibriumHydrostatic Equilibrium In a star or a planet, In a star or a planet, hydrostatic equilibriumhydrostatic equilibrium

is the balance between pressure caused by is the balance between pressure caused by the weight of material above and the the weight of material above and the upward pressure exerted by material upward pressure exerted by material below. It is a sort of solar thermostat.below. It is a sort of solar thermostat.

Pressure at the Sun’s center is calculated Pressure at the Sun’s center is calculated to be to be 1.3 X 101.3 X 1099 times that on the surface of times that on the surface of the Earth.the Earth.

HydrostaticEquilibrium

HydrostaticEquilibrium2




HelioseismologyHelioseismology studies the Sun’s interior by studies the Sun’s interior by observing pulsations in the photosphere.observing pulsations in the photosphere.

Many patterns or Many patterns or modes of oscillationmodes of oscillation can be can be seen and resemble those from a string on a seen and resemble those from a string on a violin or seismic waves from earthquakes.violin or seismic waves from earthquakes. Typical speeds of up/down motions are Typical speeds of up/down motions are 100 to 300 100 to 300

m/sm/s with periods of with periods of 5 minutes5 minutes.. Short wavelength modes of a few Short wavelength modes of a few 1000 km1000 km

resonate only in outermost parts of Sun. Longer resonate only in outermost parts of Sun. Longer wavelengths penetrate deeper.wavelengths penetrate deeper.

Helioseis.




HelioseismologyHelioseismology has been used to detect: has been used to detect: The bottom of the convection zoneThe bottom of the convection zone The Sun has more helium than previously thoughtThe Sun has more helium than previously thought The interior has surprisingly about the same The interior has surprisingly about the same

rotation period as the surfacerotation period as the surface An extensive gas stream system beneath the An extensive gas stream system beneath the

surface and as much as surface and as much as 30%30% of the way to the of the way to the Sun’s centerSun’s center

Sunspots on the far side of the SunSunspots on the far side of the Sun



Energy TransportEnergy Transport

There are three possible mechanisms There are three possible mechanisms for for energy transportenergy transport from one location from one location to another: conduction, radiation, and to another: conduction, radiation, and convection.convection.

ConductionConduction is the transfer of energy is the transfer of energy within a substance by collisions within a substance by collisions between atoms and/or molecules. This between atoms and/or molecules. This is is notnot a significant factor in transporting a significant factor in transporting energy within the Sun.energy within the Sun.

Energytransport




RadiationRadiation The energy produced by the Sun is carried out by The energy produced by the Sun is carried out by

photons emitted at one spot and absorbed at another photons emitted at one spot and absorbed at another ((radiative diffusionradiative diffusion).).

The speed at which photons get out from the Sun The speed at which photons get out from the Sun depends heavily on its depends heavily on its opacityopacity – the ability of a – the ability of a substance to stop photons (the opposite of substance to stop photons (the opposite of transparencytransparency).).

In the Sun’s core, the photons typically travel In the Sun’s core, the photons typically travel 1010-6 -6 mm before being reabsorbed.before being reabsorbed.

About About 10102525 absorptions and reemissions are needed absorptions and reemissions are needed before the energy reaches the Sun’s surface.before the energy reaches the Sun’s surface.

Radiation




ConvectionConvection Radiative diffusion carries solar energy out to Radiative diffusion carries solar energy out to 70%70% of of

the Sun’s radius.the Sun’s radius. At this distance, the temperature has dropped to At this distance, the temperature has dropped to 1.5 1.5

million Kmillion K and hydrogen atoms start to from, increasing and hydrogen atoms start to from, increasing the opacity.the opacity.

As a result, the rate of temperature decline becomes As a result, the rate of temperature decline becomes steep and steep and convectionconvection takes over the energy transport takes over the energy transport in the outer in the outer 30%30% of the Sun’s radius of the Sun’s radius

Typically, the diffusion of energy from core to surface Typically, the diffusion of energy from core to surface takes on average takes on average 170,000 years170,000 years..

convection



The PhotosphereThe Photosphere PhotospherePhotosphere is the visible “surface” of the Sun.is the visible “surface” of the Sun.

It is the part of the solar atmosphere from which light It is the part of the solar atmosphere from which light (most energy is in optical) is emitted into space.(most energy is in optical) is emitted into space.

The photosphere is a very thin layer - The photosphere is a very thin layer - 200 km200 km thick. thick. The photosphere varies in temperature from about The photosphere varies in temperature from about

8,000 K8,000 K at its deepest to at its deepest to 4,000 K4,000 K near its outer edge. near its outer edge. Overall, the light received from the photosphere is Overall, the light received from the photosphere is

representative of an object about representative of an object about 5,800 Kelvin5,800 Kelvin.. Pressure of the outer photosphere is Pressure of the outer photosphere is 0.010.01 the pressure of the the pressure of the

Earth’s surface. The calculated density of particles is Earth’s surface. The calculated density of particles is 0.0010.001 of of the density of air at sea level.the density of air at sea level.

Sun

Lecture 9: The SunLecture 9: The Sun

The Solar AtmosphereThe Solar Atmosphere LimbLimb is the apparent edge of the

Sun (or any object) seen in the sky.

The edge of the Sun appears dimmer than the center. This is known as limb darkening.

Observing the limb of the Sun one sees to a lesser depth because the line of sight is at a grazing angle.

1

From the Big Bear SolarObservatory 10-22-04

2

Limb



The PhotosphereThe Photosphere

GranulationGranulation is the division of the Sun’s surface into is the division of the Sun’s surface into small convection cells.small convection cells. GranulesGranules are areas where hot material (light areas) is are areas where hot material (light areas) is

rising from below and then descending (dark rising from below and then descending (dark surroundings).surroundings).

Each granule is typically Each granule is typically 1000 km1000 km across, separated across, separated by regions by regions 100 K100 K cooler, and last about cooler, and last about 15 minutes15 minutes..

SupergranulationSupergranulation is convective pattern on a much is convective pattern on a much larger scale, typically larger scale, typically 30,000 km30,000 km across, lasting across, lasting about about 1 day1 day, and extending to the bottom of the , and extending to the bottom of the convection zone.convection zone.

Gran

Gran

Gran2 (Movie)



The PhotosphereThe Photosphere

MassMass of the of the photospherephotosphere is is 71%71% hydrogen, hydrogen, with helium comprising most of the with helium comprising most of the remainder and a few percent consisting of remainder and a few percent consisting of several elements found on Earth.several elements found on Earth.

From our knowledge of nuclear fusion, we From our knowledge of nuclear fusion, we know the Sun’s core must hold more know the Sun’s core must hold more helium. Calculations show that the helium. Calculations show that the hydrogen makes up only hydrogen makes up only 34%34% of the of the center.center.



The Lower AtmosphereThe Lower Atmosphere

The The chromospherechromosphere is the region of the is the region of the solar atmosphere some 2,000 to 3,000 km solar atmosphere some 2,000 to 3,000 km thick that lies between the photosphere thick that lies between the photosphere and the and the coronacorona..

It is not usually observable from Earth It is not usually observable from Earth except during a total solar eclipse.except during a total solar eclipse.

When seen, it has a reddish color, which is When seen, it has a reddish color, which is caused by the hydrogen caused by the hydrogen Balmer Balmer emission emission lineline..

Chrom.




The chromosphere is mostly empty with The chromosphere is mostly empty with the exception ofthe exception of spicules spicules, which:, which: Are narrow jets of gas originating at the base of Are narrow jets of gas originating at the base of

the chromosphere, shooting upward with the chromosphere, shooting upward with speeds of about 25 km/sec, and lasting about speeds of about 25 km/sec, and lasting about 5 minutes5 minutes

form a network above the edges of the form a network above the edges of the photospheric supergranulesphotospheric supergranules

Spic.



The Lower AtmosphereThe Lower Atmosphere The The coronacorona is the outermost portion of the Sun’s is the outermost portion of the Sun’s

atmosphere that can only be seen during a total solar atmosphere that can only be seen during a total solar eclipse.eclipse. Much of the light of the corona originates in the photosphere Much of the light of the corona originates in the photosphere

– light which is then scattered by electrons in the corona.– light which is then scattered by electrons in the corona. Some light is also the result of emission from ionized Some light is also the result of emission from ionized

elements.elements. The temperature change from the chromosphere to the The temperature change from the chromosphere to the

corona is quite dramatic rising from about corona is quite dramatic rising from about 4000 K to 1 4000 K to 1 million Kmillion K in a transition zone only in a transition zone only a few thousand km thicka few thousand km thick..

Heating of the chromosphere and especially the corona is Heating of the chromosphere and especially the corona is not understood and remains one of the unanswered not understood and remains one of the unanswered questions about the Sun (although the heating process questions about the Sun (although the heating process most likely involves energy transport from the convection most likely involves energy transport from the convection zone via the Sun’s time-varying magnetic fields).zone via the Sun’s time-varying magnetic fields).

Corona

Tempchange




Coronal StructureCoronal Structure Best seen in X-ray and ultraviolet imagesBest seen in X-ray and ultraviolet images Bright areas, also referred to as Bright areas, also referred to as active active

regionsregions, are , are closed magnetic field regionsclosed magnetic field regions with hot gas often seen along with hot gas often seen along coronal coronal loopsloops

Darker and cooler areas are Darker and cooler areas are coronal coronal holesholes, which are , which are open magnetic field open magnetic field regionsregions emitting high-speed gas into emitting high-speed gas into interplanetary spaceinterplanetary space

SolarC3-recent

Corona

Coronalloops




ProminencesProminences Dense, relatively cool regions reaching upwards of Dense, relatively cool regions reaching upwards of

50,000 km50,000 km or more into corona and lasting or more into corona and lasting 2-3 months2-3 months Appear as dark Appear as dark filamentsfilaments against brighter disk of Sun. against brighter disk of Sun. Quiescent prominencesQuiescent prominences located away active regions located away active regions

live longer than live longer than active prominencesactive prominences above active above active regions.regions.

When a prominence reaches about 50,000 km it When a prominence reaches about 50,000 km it erupts sending gas into interplanetary space with erupts sending gas into interplanetary space with speeds in excess of speeds in excess of 1000 km/s1000 km/s. These eruptions are . These eruptions are called called coronal mass ejectionscoronal mass ejections..

Promin.

SolarC3-CME

CME




FlaresFlares Solar flaresSolar flares, much more explosive and energetic than , much more explosive and energetic than

prominences, are erupt releases of magnetic energy.prominences, are erupt releases of magnetic energy. Flares take place in active regions where a prominence is Flares take place in active regions where a prominence is

supported against gravity by magnetic field lines and then the supported against gravity by magnetic field lines and then the magnetic field structure changes abruptly.magnetic field structure changes abruptly.

Coronal gas may heat to Coronal gas may heat to 40 million K40 million K and X-rays and and X-rays and ultraviolet light are emitted. The Sun’s brightness may ultraviolet light are emitted. The Sun’s brightness may increase by increase by 1%1% during an unusually bright flare. during an unusually bright flare.

Flares blast out large numbers of very energetic charged Flares blast out large numbers of very energetic charged particles that will take about particles that will take about 3 days3 days to reach Earth. to reach Earth.

Large solar flares cause spectacular auroras and can affect Large solar flares cause spectacular auroras and can affect earthly radio transmissions if the ionosphere is disrupted by earthly radio transmissions if the ionosphere is disrupted by high-energy particles.high-energy particles.

Flare



The Solar Wind and HeliosphereThe Solar Wind and Heliosphere The Solar WindThe Solar Wind

The flow of coronal gas into interplanetary space is called The flow of coronal gas into interplanetary space is called the the solar windsolar wind..

Coronal gas takes about Coronal gas takes about 4 days4 days to reach the Earth. to reach the Earth. Near the Earth the solar wind travels at 450 km/s; density Near the Earth the solar wind travels at 450 km/s; density

is from 2–10 particles/cmis from 2–10 particles/cm33.. Speeds in excess of 1000 km/sec are possible during Speeds in excess of 1000 km/sec are possible during

coronal mass ejections.coronal mass ejections. The HeliosphereThe Heliosphere

The boundary between the region dominated by the The boundary between the region dominated by the solar magnetic field and interstellar space is called the solar magnetic field and interstellar space is called the heliopauseheliopause..

The region interior to the heliopause is the The region interior to the heliopause is the heliosphereheliosphere..

Heliopause

Heliosphere



Sunspots and the Solar Activity CycleSunspots and the Solar Activity Cycle Dark spots on the Sun were first reported by the Dark spots on the Sun were first reported by the

Chinese in the 5Chinese in the 5thth century B.C. century B.C. Galileo and Thomas Harriott were the first Galileo and Thomas Harriott were the first

Europeans to report these Europeans to report these sunspotssunspots in the early in the early 1717thth century. century.

In 1851, Schwabe discovered the sunspot cycle, In 1851, Schwabe discovered the sunspot cycle, which lasts about which lasts about 11 years11 years..

This periodic cycle, however, is not always This periodic cycle, however, is not always present as the present as the Maunder MinimumMaunder Minimum appears to appears to show.show.

Additionally, individual sunspots are temporary Additionally, individual sunspots are temporary phenomena lasting from a few hours to a few phenomena lasting from a few hours to a few months.months.

Sunspots

MM



Sunspots and the Solar Activity CycleSunspots and the Solar Activity Cycle

Magnetic fields can be measured using the Magnetic fields can be measured using the Zeeman effectZeeman effect, which is the splitting of , which is the splitting of spectral lines by a strong magnetic field.spectral lines by a strong magnetic field.

SpectraSpectra

No magnetic FieldNo magnetic Field Magnetic FieldMagnetic Field

Sunspots involve the Sun’s magnetic field -Sunspots involve the Sun’s magnetic field -The magnetic field in a sunspot is about 1,000 The magnetic field in a sunspot is about 1,000 times that of the surrounding photosphere.times that of the surrounding photosphere.

Zeeman



Why are sunspots dark? Sunspots are dark because they are Sunspots are dark because they are coolercooler than there than there

surrounding photosphere.surrounding photosphere. They are They are coolercooler because the magnetic field because the magnetic field

associated with the sunspot disrupts the convection in associated with the sunspot disrupts the convection in the granulethe granule

..

Heat does not flow as efficiently to the photosphereHeat does not flow as efficiently to the photosphere Spot becomes cooler than normal (it is seen as dark, though Spot becomes cooler than normal (it is seen as dark, though

it would be red if seen in isolation)it would be red if seen in isolation) Sunspots, which often appear in pairs aligned in Sunspots, which often appear in pairs aligned in

an east-west direction, have opposite magnetic an east-west direction, have opposite magnetic polarities - one being north and the other south.polarities - one being north and the other south.




Details of the Solar Sunspot CycleDetails of the Solar Sunspot Cycle At a At a sunspot maximumsunspot maximum, most spots occur , most spots occur

about 30° north or south of the equator.about 30° north or south of the equator. As the As the sunspot cyclesunspot cycle progresses, the spots are progresses, the spots are

seen closer and closer to the Sun’s equator.seen closer and closer to the Sun’s equator. When the spots reach the equator, the cycleWhen the spots reach the equator, the cycle is is

at aat a sunspot minimumsunspot minimum and begins again.and begins again. Location and relative number of sunspots can Location and relative number of sunspots can

be plotted on a be plotted on a butterfly diagrambutterfly diagram..

SunspotNumbers2

SunspotNumbers




A Model for the Sunspot CycleA Model for the Sunspot Cycle The current model (The current model (The Babcock modelThe Babcock model) has patterns ) has patterns

of magnetic field lines within the Sun’s interior forming of magnetic field lines within the Sun’s interior forming tubes that gradually become twisted from the Sun’stubes that gradually become twisted from the Sun’s differential rotation.differential rotation.

Forced to the surface, these magnetic tubes become Forced to the surface, these magnetic tubes become visible as sunspots.visible as sunspots.

Breaking the surface weakens the field lines and the Breaking the surface weakens the field lines and the sunspots die out.sunspots die out.

As new lines form deep within the Sun, the magnetic As new lines form deep within the Sun, the magnetic field direction in the emerging tubes will reverse over field direction in the emerging tubes will reverse over time, causing the magnetic field of the Sun to reverse time, causing the magnetic field of the Sun to reverse in direction in a 22-year period. This is called the in direction in a 22-year period. This is called the magnetic cycle of the sunmagnetic cycle of the sun..

Babcock

Documents

© Sierra College Astronomy Department The Sun Our Star