2010 AST Descriptive Astronomy UNIT 49
1. 2. 3. 4. 5. 6. 7. Introduction Size and Structure The Solar Interior Energy Transport The Solar Atmosphere Pressure Balance Solar Seismology
Claude A Pruneau
Physics and Astronomy Department Wayne State University
Introduction
Intensely hot, self-luminous body of gases (mainly hydrogen and helium) at the center of the SOLAR SYSTEM. The sun is a medium-size main-sequence STAR. Mean distance from the earth is defined as one ASTRONOMICAL UNIT. Pressure of over 1 billion atmospheres Temperature of about 15,000,000 Kelvin. Temperature high enough for the occurrence of nuclear reactions
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Sun Basic Profile
Mass Equatorial radius Mean density Rotational period Escape velocity Luminosity Magnitude Mean Core Temperature Mean surface temperature Age 1.989e+30 kg / 332,830 Earth Mass 695,000 km / 10897 Earth radius 1.410 gm/cm3 2536* days 618.02 km/s 26.8 Vo 3.827e33 ergs/s 15,600,000 K 6,000C 4.5 billion Earth years
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Satellites
Eight main satellites: the planets. It is also surrounded by a vast number of smaller objects.
Asteroids Comets Stellar dust
Planet Distance (000 km) Radius (km) Mass (kg) Discoverer Date
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
57,910 108,200 149,600 227,940 778,330 1,426,940 2,870,990 4,497,070 5,913,520
2439 6052 6378 3397 71492 60268 25559 24764 1160
3.30e23 4.87e24 5.98e24 6.42e23 1.90e27 5.69e26 8.69e25 1.02e26 1.31e22
Herschel Galle Tombaugh
1781 1846 1930
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Rotation of the Sun
The outer layers of the Sun exhibit differential rotation: The Sun is not a solid body like the Earth. Similar effects are seen in the gas planets.
at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days.
The differential rotation extends considerably down into the interior of the Sun but core of the Sun rotates as a solid body.
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Temperature and Energy Output
Sun's core (~ inner 25%)
Temperature: 15.6 million Kelvin Pressure: 250 billion atmospheres. Density: 150 times that of water. 3.86x1033 ergs/second or 386 billion billion megawatts
Energy output:
Produced by nuclear fusion reactions.
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Energy Production Basics
Each second about 700,000,000 tons of hydrogen are converted to: As gamma rays travel out toward the surface, they continuously absorbed and reemitted at lower and lower temperatures
~ 695,000,000 tons of helium ~ 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays.
For the last 20% of the way to the surface the energy is carried more by convection than by radiation.
by the time they reach the surface, they become primarily visible light photons.
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Energy Output vs Time
Output not entirely constant over time. Nor is the amount of sunspot activity. Period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. Coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%.
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Chemical Composition
Same elements as on Earth Different proportions Discovered by Cecilia Payne-Gaposchkin
1 woman to get PhD in AST
st
Elements
Hydrogen Helium Oxygen Carbon Nitrogen Neon Iron Silicon Magnesium Sulfur All others
Fraction
92.1% 7.8% 0.061% 0.030% 0.0084% 0.0076% 0.0037% 0.0031% 0.0024% 0.0015% 0.0015%
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Mostly atoms but some molecules (H20, CO)
Solar Interior/Structure
Sun natural divisions:
Core Radiative Zone Radiative Zone Convection Zone Photosphere Chromosphere Transition Region Corona
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Solar Interior
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Sun Core
Occupies the innermost 10% of the Sun. Where most of the Sun's energy is generated. Generation proceeds by nuclear fusion. Because of gravity/compression core is very hot and dense. Nuclear fusion requires extremely high temperatures and densities. Astronomers estimate the Sun's core is about 16 million K and has a density about 160 times the density of water. Sun's interior nonetheless gaseous all the way to the core because of the extreme temperatures. No molten rock like that found in the interior of the Earth.
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Radiative Zone
Where energy is transported from the extremely hot interior to the colder outer layers by photons. Technically also includes the core. The radiative zone includes the inner approximately 85% of the Sun's radius.
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Convective Zone
Outer layers, roughly the outer 15% of the Sun's radius Energy transported by the bulk motions of gas in a process called convection. At cooler temperatures, more ions are able to block the outward flow of photon radiation more effectively. Convection help the outward transport of energy from the very hot interior to the cold space.
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Photosphere
Deepest layer one can see from the outside. ``photosphere'' means ``light sphere''. Called the ``surface'' of the Sun because at the top of it, the photons are finally able to escape to space. About 500 kilometers thick. Surface is not something one could land or float on. However, the gas is so dense that you could not see through it. The gas, in fact, emits a continuous light spectrum. Features Sun spots
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Photosphere (cont'd)
Photosphere Temperature
Several measurement methods. Temperature of about 5840 K.
Wien's Law
wavelength of the peak emission, peak, in the Sun's continuous spectrum. T(Kelvin) = 2.9 106 nm/ peak.
Light Flux
flux = amount of energy passing through a unit area (e.g., 1 meter2) every second. Inverse Square Law of Light Brightness solar flux at the Earth's distance = the Sun's surface flux (Sun's radius/Earth's = distance)2 1380 Watts/meter2. Sun's photosphere is ~a thermal radiator, the flux of energy at its surface = (the Sun's surface temperature)4, where is the Stefan-Boltzmann constant. T = [(solar flux at Earth)/ ) (Earth distance/Sun's radius)2]1/4.
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Sun Spots
Discovered by Galileo. Sun's surface sprinkled with small dark regions - sunspots. Sunspots 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 Sun. Sunspots 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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Sun Spot and Magnetic Field
Sun Spots = region of strong magnetic field. Found by observation of Zeeman effect.
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Transition Region
Thin region of the atmosphere where temperature changes from 10000 K to nearly 1000000 K. Few 10 km thick.
Temperature in the Solar Atmosphere 1.E+07
Corona
1.E+06 1.E+05 1.E+04 Temperature (K) 1.E+03 0 1000 2000 3000 Height in solar atmosphere (km)
Chromosphere Transition Region
Photosphere
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Energy Transport -Granulation
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Solar Atmosphere - Chromosphere
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Chromosphere
Visible during solar eclipses as a thin pink layer at the edge of the dark Moon. Colorful layer called chromosphere which means "color sphere".
Color arises from hydrogen bright emission line. Also shows yellow emission due to Helium discovered in 1868 new element previously not seen on Earth.
Helium was found on Earth in 1895.
Chromosphere is only 2000 to 3000 km thick. Temperature rises outward away from the photosphere from 4500 K to 10000 K
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Atmosphere - Corona
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Corona
Visible during total solar eclipse as pearly-white corona around the dark Moon. Total solar eclipse (1973) Rarefied upper atmosphere of the Sun. Very high temperature ~ 1-2 million K. Low amount of heat because very tenuous. 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 - ionized. 9-times ionized iron is only produced at temperatures of 1.3 million K 13-times ionized iron means the temperature gets up to 2.3 million K!
During strong solar activity, the temperature can reach 3.6 million K and lines from 14-times ionized calcium are seen.
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Corona (cont'd)
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.
Called ``coronal holes''.
More details visible at short wavelengths.
Solar eclipse photographed in extreme ultraviolet taken by the SOHO spacecraft
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Corona (cont'd)
Why is it so hot ? Powered by Sun's magnetic field.
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Sun Weather
Sun has complex and violent weather patterns. Chromosphere contains jet-like spikes of gas called spicules. Spicules rise vertically through the chromosphere. Last 10 minutes Consist of gas jet, at 30 km/s Rise to heights of 5000 to 20000 km. T ~ chromosphere.
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Solar Activity
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Solar Wind
Fast-moving (+/-) ions can escape the Sun's gravitational attraction. Moving outward at...