The Sun’s Energy

1. 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!

2. What powers the Sun? Lord Kelvin vs. Darwin 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 Kelvin 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 Kelvin estimated the age of the Sun by calculating how long the Sun would take to radiate this amount of energy away

3. What Powers the Sun? Lord Kelvin vs. Darwin 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 Kelvin 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

4. 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??

5. 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

6. 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 Helium nucleus 4 hydrogen nuclei

7. 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!!!

8. 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 massivethan 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

9. Fusion vs. Fission • 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 Fusion 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

10. 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”

11. 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

12. 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 • 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

13. 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”

14. Life cycle of stars A star’s life cycle is determined by its mass • 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