Transcript The Sun*s Energy

The Sun’s Energy
Energy from the Sun supports almost all life on
Earth by photosynthesis (conversion of light
energy into chemical energy)
Almost all of the energy used by humans today owes
its origin to sunlight (nuclear power is an exception)
Life (through photosynthesis) uses less than 0.1% of
the energy that reaches the surface of the Earth
About 1/3 of the Sun’s energy that reaches Earth is
reflected back into space by snow & clouds
Most of the rest goes to heat the land & water
The Earth gets only 2
billionth’s of the Sun’s
energy at any given
time!
What powers the Sun?
Lord Kelvin vs. Darwin
Kelvin
Both wanted to figure out how old the Earth was
-Kelvin thought he could determine the age of
Earth based on how old the Sun was
-Darwin based Earth’s age on the amount of time
necessary for natural selection to take place and
provide the vast diversity of life we see today
Darwin
At the time, the most powerful source of energy people knew about was gravitational
energy
Kelvin thought that the energy source of the sun was gravitation
𝐺𝑚2 𝑠𝑢𝑛
𝐸𝑔𝑟𝑎𝑣𝑖𝑡𝑦 =
≈ 1041 𝑐𝑎𝑙𝑜𝑟𝑖𝑒𝑠
𝑟𝑠𝑢𝑛
Kelvin estimated the age of the Sun by calculating how long the Sun would take to radiate
this amount of energy away
𝐸𝑔𝑟𝑎𝑣𝑖𝑡𝑦
1041 𝑐𝑎𝑙
𝑡𝑠𝑢𝑛 =
≅ 26
≈ 1015 sec ~30 𝑚𝑖𝑙𝑙𝑖𝑜𝑛 𝑦𝑒𝑎𝑟𝑠
𝐿
10 𝑐𝑎𝑙/𝑠𝑒𝑐
What Powers the Sun?
Lord Kelvin vs. Darwin
Kelvin
Both wanted to figure out how old the Earth was
-Kelvin thought he could determine the age of
Earth based on how old the Sun was
-Darwin based Earth’s age on the amount of time
necessary for natural selection to take place and
provide the vast diversity of life we see today
Darwin
In 1859, Charles Darwin, in the 1st edition of On the Origin of the Species by Natural
Selection, made a rough estimate of the age of the Earth by estimating how long it would
take erosion occurring at the current observed rate to wash away the Weald, a great valley
in the south of England
Greater than 300 million yrs
Darwin concluded that it was long
enough for natural selection to have
produced the species that exist on
Earth
What Powers the Sun?
Actual age of Sun: ~ 4.6 billion years
Kelvin: 0.03 billion years
Darwin: greater than 0.3 billion years
Kelvin: “what then are we to think of such geological estimates as
(Darwin’s) 300,000,000 years for the denudation of the Weald?”
Darwin was disturbed by the power of Kelvin’s argument and by his
reputation as a brilliant theoretical physicist. In private
correspondence, Darwin admitted that he had no answer to Kelvin’s
logic
Darwin removed all discussion of time scales in subsequent editions of
the Origin of the Species
We now know that Darwin and the evolutionary biologists &
geologists were right—the Sun and Earth are about 4.6 byo.
Kelvin was wrong….Why??
What Powers the Sun?
Remember, Lord Kelvin thought that the Sun was powered by gravitational
energy
-his estimate of Earth’s age was wrong because of this assumption
-we now know that it’s powered by nuclear fusion
Albert Einstein contributed the 1st
key theoretical insight. In 1905 he
inferred that a little amount of mass
could, in principle, be converted to a
very large amount of energy
E = mc2
The “bucket” was much much bigger than
Lord Kelvin realized
-this conversion of hydrogen to helium
could keep stars burning for as long as
Darwin required
Energy Source of the Sun
Most stars generate their energy via nuclear
fusion
- Fusing 4 protons (hydrogen nuclei) into one helium nucleus
- The mass of the He nucleus is 0.7% smaller than the sum of
the 4 hydrogen nuclei that formed it
- The mass difference that “disappears” is the source of the
energy via Einstein’s E=mc2
4 hydrogen nuclei
Helium nucleus
How much energy are we talking?
Mass-energy of 20 nickels = 9 x 10 15 Joules
Combustion energy we can extract from a gallon of oil= 10 8
Joules
Typical American uses 2500 gallons of oil/yr = 2.5 x 10 11 Joules
In other words, enough mass-energy in 20 nickels to power the
driving habits of 36,000 people for a year!!!
Nuclear Energy
E = mc2 means there is a lot of energy associated with small amounts
of matter
Problem:
• It’s not easy to extract mass energy
In practice we can tap into E = mc2 energy via
• Nuclear fusion or
• Nuclear fission
Nuclear fusion: light atomic nuclei combine to form a more massive
atomic nucleus. Energy is released because the massive nucleus is less
massive than the sum of the lighter nuclei
Nuclear fission: A heavy atomic nucleus splits into smaller nuclei.
Energy is released because the massive nucleus is more massive than
the sum of the light nuclei products
https://www.youtube.com/watch?v=-Qliifidcuw
Fusion vs. Fission
Fusion
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Powers star
Used in modern nuclear bombs
Proto-type reactors
Benefits:
• Incredibly efficient
• “cleaner” than fission
• Don’t need to spend $ on rare elements like uranium
Problems:
• Easy to do, hard to contain
• Can still create radioactive matter
Fission
•
Used in modern power plants
Benefits:
• Efficient
• Cleaner than burning fossil fuels
Problems
• Nuclear waste can’t just be thrown out like garbage
• Finite supply of expensive/rare elements like uranium
Stars don’t just cram 4 protons
together to make He
In low-mass stars like the Sun
(stars that are 30% more
massive than the Sun and
smaller) helium-4 is made
through a sequence of proton
reactions that start by fusing 2
protons to form deuterium
• Then the deuterium becomes
helium-3 before combining to
form helium-4
• This “hydrogen burning”
happens out to around 0.3 of
the star’s radius and is
relatively temperature
“insensitive”
High-mass stars
In higher mass stars (stars with a
mass 1.3 x greater or more)
“hydrogen burning happens by
relying on nuclei of carbon,
nitrogen and oxygen as stepping
stones to produce a helium
nucleus
• This “CNO cycle” is
temperature sensitive and
only happens at the core
Like us, stars constantly fight for
equilibrium
Once nuclear fusion begins, energy is generated and the pressure is
maintained so that the star does not collapse into itself due to gravity
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•
•
The pressure force generated by the heat balances the inward pull of gravity
This balance between pressure pushing out and gravity pulling in is called
“hydrostatic equilibrium”
The star achieves stability, with a constant temperature & luminosity. The star is
now said to be on the “main sequence”
This is why stars, like our Sun, are not simply
just fusion bombs exploding
-it’s set up so that once the “bomb” does go
off, gravity pushes matter back together and
continues this chain reaction of fusion energy
-most of a star’s life is spent in “main
sequence”
-our own Sun is currently in main sequence
Where does it stop?
Brown Dwarfs
-Stars in formation will begin to have nuclear reactions as long as they have
enough gravitational energy to generate fast-moving, high-pressure cores
where fusion can occur
-If a “pre-star” has a mass less than 8% of the Sun this type of fusion cannot
begin, and the “star” will grow dimmer as it grows older. This is sometimes
called a failed star, or a “brown dwarf”
Life cycle of stars
A star’s life cycle is determined by its mass
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•
•
•
•
•
Larger its mass, the more atoms it has to keep in equilibrium and therefore the
more fuel it burns up. This leads to a shorter life
Once the hydrogen supply in the star’s core begins to run out, and the star is no
longer generating heat by nuclear fusion, the core becomes unstable and contracts
(pulled in by gravity due to lack of pressure pushing out)
The outer shell of the star, which is still mostly hydrogen, starts to fuse hydrogen
into helium and produces a great amount of energy
This causes the luminosity of the star to be much greater and the outer shell
actually begins to expand outward and become cooler since it is spreading its
energy over a greater amount of surface area
This cooling of the outer shell results in a reddish-orange color
It is at this point when a star becomes a red giant
This outward expansion is the result of
fusion still occurring in the outer shell
-the core is now this degenerate core of
helium