PTYS 214 – Fall 2019

Turn in extra credit if you haven’t already

Next midterm 10/10!

Amy’s study session availability: any preference on M W?

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Announcements

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Midterm #2

Total Students: 18

Class Average: 61

Low: 10

High: 90

If you have questions see one of us!

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Kortenkamp’s class will meet 9/30 – 10/16

Use your phone to take a photo of the Moon several times during this period

Use consistent settings for all photos!

We will have another opportunity later in the semester

Moon Observing

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Definitions: Energy, Work, Power, Entropy

2nd Law of Thermodynamics

Conversion of Energy for Life:• Photosynthesis• Respiration• etc..

Utilization of Energy in Life: ATP/ADP

Classification of Life by Energy Source

• Photo/Chemo- Auto/Hetero- troph

Last Time

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Life needs energy

Photosynthesis:

6CO2 + 6H2O + Energy → C6H12O6 + 6O2

99.9% of energy at the Earth’s surface is from

Solar Radiation

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Primary sources of energyfor life on Earth

Sun Earth’s Interior

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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 8

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

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Solar Structure

(7 Million K)‏

(2 Million K)‏

Nuclear Fusion

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Photosphere: Visible surface of the Sun

~500 km thick

Mean surface temperature

5,800K (~10,000 oF)‏

The most obvious features on the surface of the Sun, sunspots have been observed since the 17th century (Galileo)‏

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Sunspots

Sunspots vary greatly in size; typically measure a few tens of thousands of kilometers

A Sunspot's life can be as short as an hour or as long as several months

They clearly show the Sun’s 27-day equatorial rotation!

Regions of low temperature and intense magnetic fields

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Chromosphere

~5000 km thick

Temperatures up to 35,000K

Dim reddish color due to Hydrogen emission (Hα‏(

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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 continuously escapes the Sun towards space: Solar wind

Visible light

X-rays 14

The size of the Sun is due to the overall balance between two main forces:

Sun Size: Hydrostatic Equilibrium

Gravity: Tremendous weight of ‏(1the mass of the Sun that presses inward under the force of gravity

Pressure: Enormous pressure ‏(2inside the Sun (generated by nuclear fusion)‏ that pushes outward

The life of any star is a constant balancing of

Gravity & Pressure 15

Fusion Reactions

Combine atomic nuclei to form a heavier nucleus (a heavier element)‏ example:

12C + 1H→13N + γ (Energy)‏

(C and H combine to form a new element, N)‏

Different from Chemical Reactions:

Deal with electrons binding atoms of certain elements into molecules, but the elements (nuclei)‏ do not change:

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

(number of C, H and O atoms does not change)‏

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Nuclear fusion reactions can happen only under very high temperatures and pressures

Energy involved in nuclear fusion reaction is ~1,000,000 times larger than in chemical reactions

Hydrogen bomb

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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

It will continue to heat the Sun for another 5 billion years

Proton-proton chain

Four hydrogen nuclei “fuse” to form a single helium nucleus

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Proton-Proton Chain

One 4He nucleus has 99.3% of the weight of four free 1H nuclei

Excess 0.7% mass is converted into energy: E=mc2

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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 J/s = 3.84 x 1026 W

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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 per second to obtain Solar Luminosity?

(1 Tsar bomb = 2.1×1017 J)

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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 per second to obtain Solar Luminosity?

(1 Tsar bomb = 2.1×1017 J)

~ 1.8 billion Tsar bombs per sec!

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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?

S=L J/s

A m2

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As energy moves away from the Sun, it is spread over a greater (spherical)‏ area

Inverse Square Law

Solar flux at distance d:

S=L( W )

4 πdd 2(m2)

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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

S=LA

=3. 84×1026W

2 . 81×1023m 2 ≃ 1 . 37×103 W/m 2

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Each planet has its own solar constant…

…depending on its distance from the Sun26

SE = 1370 W/m2

dE = 1 AU

We can relate the inverse square law to the Earth’s solar constant:

SE

SdE

d

S=SE d Ed 2

A (sometimes)‏ more convenient form of the inverse square law:

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SMars = ?

SVenus = ?

SJupiter = ?

dMars orbit=1.52 AU

dVenus orbit= 0.72 AU

dJupiter orbit = 5.2 AU

SEarth = 1370 W/m2

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2642 1370 593 51

D= 0.72 1 1.52 5.2 AU

Planetary Solar Constant, W/m2

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The Solar constant is not really constant!

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Why not?

The Solar Luminosity is not really constant!

Solar luminosity varies, e.g. sunspots

Solar flares can release hundreds of millions of megatons of energy in a few minutes

Prominences/coronal loops eject large amounts of charged particles into space in a few minutes

The cause of this variability is an active area of research

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Solar ActivityAny type of variation in the appearance or energy output of the sun

Can solar activity affect the Earth?32

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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

Solar wind

Earth’s magnetic field

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

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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 emits light of a specific wavelength that depends 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

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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

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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 at that time

The “Faint Young Sun” is important for climate - not so critical for photosynthesis

3.843.8410102626 W/m W/m22

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Homework #7 available shortly on the web site

Homework

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