Why is the Sun Important to Us? LAST TIME - Formation of...
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Lec 14: 12 OCT 11 Chapter 16 - The SUN LAST TIME - Formation of the Solar System TODAY - The “Quiet” Sun
• Why is it important? What can it tell us about other stars, planetary systems, etc.
• Why does it “shine”? • What is it made of? Internal Structure.
How do we know? In Lab This Week - The “Active” Sun
Next: “Debris” in the Solar System Read Chapter 14-9, 14-10, and 15 for next week!
Why is the Sun Important to Us? • contains nearly all the mass
in the solar system; everything in s.s. orbits Sun
• provides energy (heat) through electromagnetic radiation ==>Source of life
• produces particles and magnetic field that interact with planetary magnetospheres, atmospheres, and surfaces
Next semester: “Rosetta Stone” to understand stars
Some Facts About the Sun • ordinary star: middle age, middle size, middle
temperature, middle brightness, etc. One of 100 billion in our galaxy!
• not a binary • huge, nearly constant output of energy; we only
receive a tiny portion of it
4 x 1026 Watts
1365 W/m2 rotation period: 24 days (at equator)
Almost all of the energy used on the Earth comes originally from the Sun
How Does The Sun Generate Energy? How Much Energy Does the Sun Produce? • Sun is in a steady-state and in balance
– not varying (much) – not expanding or contracting (much)
• And it has been for a very long time (4.6 billion years)
Energy Emitted = Energy Produced
Luminosity = 4 x 1026 Watts
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• Remember how solar system formed. Most of material ended up in the center.
• Gravitational collapse -> temperature increases.
• Once begun, the fusion reactions generated energy which provided an outward pressure.
• This pressure perfectly balances the inward force of gravity at all levels
• This balance is called hydrostatic equilibrium
What Keeps It From Blowing Itself Apart? How Does it Do It? • gravitational contraction provided initial heat
source (along with accretion energy) • could only “fuel” present output of Sun for
~100,000 years! • of all known energy sources, only FUSION can
provide this energy at this rate for > 5 billion years
• fusion (of light nuclei into heavier nuclei) requires high temperature and high pressure to begin and to sustain; why?
The Sun’s energy is produced by hydrogen fusion, a sequence of thermonuclear reactions in which four
hydrogen nuclei combine to produce a single helium nucleus. This is known as the proton-proton chain.
The net effect is:���
4H 1He + Energy
• The mass of the 4 Hydrogen nuclei is greater than the mass of the 1 Helium nucleus
• The mass difference is converted to energy.
E = mc2 600 million tons of Hydrogen to Helium per second for 10 billion years
• The Sun’s interior is opaque, we can not see directly into it • We can construct mathematical computer models of it.
• grids of temperature, pressure, & density vs. depth • these values are calculated using known laws of physics • they are tested against the Sun’s observable quantities
• We can indirectly measure sound waves moving through the interior
• these can be used to probe conditions in the interior of the Sun
“Observing” the Solar Interior • Helioseismology is the
study of how the Sun’s “surface” vibrates up and down
• These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun
• There might be another way to see all the way into the core … neutrinos!
Helioseismology
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Neutrinos • produced in fusion reactions • “little neutral” particles;
energetic (fast) • don’t interact with atoms,
molecules, nuclei, very well • therefore fly straight out of
the Sun in all directions • almost impossible to stop, but
a tiny fraction can be caught in a detector
• only about 1/3 as many as expected!
• Core • 1.5 x 107 K 0.25 R
• Radiation Zone • > 2 x 106 K 0.70 R
• Convection Zone – < 2 x 106 K 0.85 R
• Photosphere • 5.8 x 103 K 400 km thick
• Chromosphere • 1– 5 x 104 K 2,500 km thick
• Corona • 2 x 106 K 600,000 km thick
• Solar Wind • > 106 K beyond the Kuiper Belt
Layers of the Sun Temperature Depth
Energy Transfer in Sun • gamma ray photons
produced by fusion in core; neutrinos escape
• radiative diffusion • convection zone • photosphere heated from
below by convection, conduction, radiation
• light - 1 million years to get out then 8 minutes to Earth
• heavy elements made in core but don’t mix
Photosphere The visible “surface” of the Sun. But it’s not a solid surface, just the deepest we can see into the solar atmosphere
Convection in the photosphere produces
granules
Solar Magnetic Activity • Photosphere:
visual sunspots
• Chromosphere: H-alpha & UV plage & spicules
• Corona: X-ray loops & streamers flares
• Solar Wind: charged particles coronal mass ejections
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Sunspots: relatively cool���regions in the photosphere
Sunspots can be used to measure solar rotation
Rotates about once per month, but when it formed it spinned a lot faster!
“Differential Rotation”: equator rotates faster than poles
Sunspots are also regions of intense magnetic field
Strong magnetic field suppresses convection
The Solar Cycle
1992
1996 Minimum Maximum
1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Number of Sunspots
" 11 year cycle of intensity, sunspot number, etc.
" spots emerge at higher latitude at beginning of cycle; at equator near end of cycle
" 22 year cycle of polarity
The solar “dynamo” is controlled by convection and differential rotation���(but we don’t understand it very well)
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Above photosphere, Temperature INCREASES with height!
Where does energy come from?
• magnetic field?
• sound waves?
How does it get deposited into the thin plasma in the outer atmosphere?
We know magnetic field is related to rotation and convection
Chromosphere: Hotter than the Photosphere!
The corona: almost as hot as the core!
Magnetic “Storms” in the Solar Corona
• A solar flare is a brief eruption of hot, ionized gases from a sunspot group
• A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona
Coronal Mass Ejections
Earth Is Shown For Size Comparison
Earth Magnetic Cloud
Coronal Mass
Ejection SUN
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Coronal Holes Effects of Solar Activity on ���Earth’s Environment
power grids pipelines radio communications
spacecraft health astronaut health airline passenger health
aurorae evolution of life climate???
Correlations between activity and climate are highly suggestive, but what is the mechanism?
Some correlations could be spurious:
Number of Sunspots vs. # of Republicans in US Senate