Post on 20-Dec-2015
PTYS 214 – Spring2011
Homework #3 DUE in class TODAY
Class website: http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214/
Useful Reading: class website “Reading Material”http://en.wikipedia.org/wiki/Sun
http://www.solarviews.com/eng/sun.htm
Announcements
Classification of living organisms by carbon and energy sources
Plants are… Humans are…
Nutritional Type Energy Source Carbon Source
Photo-autotrophs Light CO2
Chemo-autotrophs Inorganic Compounds
(H2, NH3, NO2, H2S)
CO2
Photo-heterotrops Light Organic Compounds
Chemo-heterotrops Organic Compounds Organic Compounds
Life needs energy
Photosynthesis:
6CO2 + 6H2O + h (Energy) → C6H12O6 + 6O2
99.9% of energy at the Earth’s surface is from
Solar Radiation
The Sun
~4.6 billion years old
G2 class star (~8% of stars are G class) based on photospheric temperature
In our galaxy >100 million stars are of the same class
Sun in X-rays NASA/ESA SOHO
The Sun – Basic FactsDistance from Earth 1 AU = 1.5×108 km Mass
333,000 Earth Masses99% mass of the Solar
system
Diameter109 Earth Diameters
Composition (by mass)74% Hydrogen24% Helium<2% other elements
Average Density1410 kg/m3
Sun and Planets to scale
The size of the Sun is due to the overall balance between two main forces:
Sun Size: Hydrostatic Equilibrium
1) Gravity: Tremendous weight of the mass of the Sun that presses inward under the force of gravity
2) Pressure: Enormous pressure inside the Sun (generated by nuclear fusion) that pushes outward
The life of any star is a constant balancing of
Gravity & Pressure
Solar Structure
(7 Million K)
(2 Million K)
Nuclear Fusion
Fusion Reactions
Combine atomic nuclei to form a heavier nucleus (a heavier element)
12C + 1H→13N + γ (Energy)
(C and H combine to form a new element, N)
Very different from Chemical Reactions:
Deal with electrons binding atoms of certain elements into molecules, but the elements (nuclei) do not change:
6CO2 + 6H2O + h (Energy) → C6H12O6 + 6O2
(number of C, H and O atoms does not change)
Nuclear fusion reactions can happen only under very high temperatures and pressures
Energy released in nuclear fusion reactions is ~1,000,000 times larger than in chemical reactions
Hydrogen bomb
The Sun’s Energy Source is NuclearFusion in its Core
Nuclear fusion occurs only at the very high temperatures and pressures at the Sun’s core
Proton-proton chain
Four hydrogen nuclei “fuse” to form a single helium nucleus
It will continue to heat the Sun for another 5 billion years
Proton-Proton Chain
One He nucleus has 99.3% of the weight of four H nuclei
Excess 0.7% mass is converted into energy: E=mc2
Energy Transfer in the Solar Interior
The convection zone is a region right below the visible surface of the Sun
Here, turbulent convective motions occur, similar to a pot of boiling water
These bubbling motions are responsible for the granulation pattern seen on the Sun’s surface (photosphere)
Core: Nuclear Fusion 1.57107 K ~28,000°F
Convection zone:2.2106 K
Solar surface: 5,800 K
(in the photosphere)
They are 100s to1000s
of km wide
Solar Granulation
Photosphere: Visible surface of the Sun
Mean surface temperature
5,800K (~10,000 oF)
The most obvious features on the surface of the Sun, sunspots have been observded since the 17th century (Galileo)
Sunspots
Sunspots vary greatly in size; typical ones measure a few tens of thousands of kilometers
A Sunspot's life can be as short as an hour or two or as long as several months
They clearly show the Sun’s 27-day rotation!
Regions of low temperature and intense magnetic fields
The Corona Outermost layer of the solar
atmosphere, made of very high-temperature gases at extremely low density
It is extremely hot: temperatures reach up to about 2 million K
What heats the corona remains an open question!
Material (charged particles) from the corona is continuously blown away from the Sun towards space: Solar wind
Visible light
X-rays
Energy from the SunOnce energy reaches the Sun’s surface, it is radiated
outward, into space
Solar Luminosity: Total energy emitted by the Sun per second
L = 3.84 x 1026 W = 3.84 x 1026 J/s
Energy Conversion
The biggest hydrogen bomb ever exploded (Tsar Bomba) was ~50 Megatons of TNT (>3000 times Hiroshima) 1 ton TNT = 4.184×109 J
How many Tsar bombs would be necessary to explode per sec to obtain Solar Luminosity?
1 Tsar bomb = 2.1×1017 J
~ 1.8 billion Tsar bombs per sec!
/s101.8Tsar# 9s
Solar Flux - Luminosity divided by the area over which it is distributed (energy per sec per unit area)
The area over which solar luminosity is distributed increases with distance from the Sun
How?
How does the solar flux change with distance from the Sun?
Why is Earth not destroyed by the energy coming from the Sun?
)A(m
L(J/s)S
2
As energy moves away from the Sun, it is spread over a greater area
Inverse Square Law
Solar constant:
The Solar Constant
1. Determine the area of distribution at the Earth’s orbit
A=4πdE2
1 AU = 1.496×1011 m
2. Divide L by A
23223
26
W/m101.37m102.81
W103.84
A
LS
Each planet has its own solar constant…
…depending on its distance from the Sun
SE ~ 1370 W/m2
dE = 1 AU
We can relate the inverse square law to the Earth’s solar constant:
SE
SdE
d
2
EE d
dSS
A more convenient form of the inverse square law:
SMars = ?
SVenus = ?
SJupiter = ?
dMars orbit=1.52 AU
dVenus orbit= 0.72 AU
dJupiter orbit = 5.2 AU
SEarth = 1370 W/m2
2642 1370 593 51
D= 0.72 1 1.52 5.2 AU
Planetary Solar Constant, W/m2
The Sun is not really constant
Solar luminosity varies, e.g. sunspots
Solar flares can release hundreds of millions megaton of energy in a few minutes
Prominences/coronal loops eject large amounts of charged particles into space in a few minutes
What causes this variability is an active area of research
Solar Activity
Any type of variation in the appearance or energy output of the sun
Sunspots
Flares
Prominences
Can solar activity affect the Earth?
Solar Activity and Earth
Aurora from Space Shuttle
Solar activity releases large amounts of plasma, charged particles (gases) and electromagnetic radiation moving at up to 3 million miles per hour, making up the Solar Wind
Variation in solar activity results in variation of solar wind intensity
This can affect Earth’s atmosphere and climate, and disrupt telecommunications on Earth
Earth’s magnetic field deflects most incoming charged particles toward Earth's north and south poles
Solar wind
Earth’s magnetic field
Polar Auroras
Aurora over Circle, Alaska
Charged particles deflected toward Earth's poles collide with atmospheric molecules causing them to give off light
Each atmospheric gas emit light of a particular wavelength that depend on its electrical state and on the energy of the particle that hits it
The light emitted by atmospheric gases appears as the northern and southern lights, a.k.a. auroras
How does the Sun Influence Earth? Provides energy for life, warms the planet, drives the
dynamic atmosphere and oceans UV light can cause mutations in living organisms
Geomagnetic storms (high solar activity)– Aurora– Power-grid failures (Canada, 1989);
Telecommunications failures
High-energy solar particles (solar wind)– can destroy ozone– dangerous radiation dosages to astronauts and
passengers/pilots on polar air-travel routes
Solar Luminosity Varies in Time
Faint Young Sun
The sun was probably 20-25% cooler about 3.5 Gyr ago
Yet, there is no evidence of a cold (freezing) Earth surface
The “Faint Young Sun” is important for climate - not so critical for photosynthesis
3.843.8410102626 W/m W/m22
Why the Sun gets brighter with time
H fuses to form He in the core
Core slowly loses mass
Core contracts and heats up
Fusion reactions proceed faster
More energy is produced more energy needs to be emitted
warmer!