Our Powerhouse

1. Our Powerhouse

2. Possible sources Chemical Energy: Sun has hydrogen and if it has oxygen, than we can make water. will last 18,000 years. there is not enough oxygen. Gravitational Energy: Gravity compresses the Sun, the gases will be heated will last 30,000,000 years. Falling meteorites will heat the Sun you need Earth’s mass every 100 years will change Earth’s orbit 4 minutes each century. Nuclear Energy: Only option left. So how??

3. Nuclear Reactions Nuclear Fission - produces energy by breaking up massive nuclei Nuclear Fusion - produces energy by fusing together light nuclei Stars are made up of light nuclei: The energy source of the stars is nuclear fusion.

4. Nuclear fusion Fusion requires extremely high temperatures: To fuse hydrogen we need a few million degrees (5-8 million K) To fuse elements with a greater positive charge we need higher speeds (higher temperatures). Helium fuses at 100 million K. Heavier nuclei fuse at even higher temperatures.

5. Einstein?? E = mc2 This formula means that mass can be converted into energy. 10 gr chalk -> 9.1014 Joules. Take water at 20 oC. You can boil 3.5x108 tons of water with this energy. Cube of water with side 70m. In the Sun, Hydrogen (one proton) is converted into Helium (two protons and two neutrons).

6. Sun’s fusion

7. Reactions 1H + 1H  2H + e- +  +  1.4 x 1010 years 2H + 1H  3H +  6 s 3H + 3H  4He + 21H 106 years 1H : 1.007825u 41H : 4.03130u 4He : 4.00268u --------------------- 0.02862u 1 kg H  0.0071 kg = 7.1 g mass defect  6.4 x 1014 Joules Sun’s luminosity : 4 x 1026 joules/sec  6 x 1011 kg/sec  Sun lasts 1010 years

8. Equilibrium • How is this all possible?? • Sun is made up of gas • Heated gases expand • They have a pressure increasing T increases p • Sun is stable • Gravitation should collapse it, but does not • So T in the center should be 15,000,000 K  hydrostatic equilibrium • Temperature does not change • Heat generated = heat lost  thermal equilibrium • Heat is transferred: conduction, convection, radiation

9. Radiation During the fusion reactions two kinds of radiation are generated. 1. Photons (gamma rays): It takes about 1,000,000 years to reach the surface. 2. Neutrinos (massless): It takes about 2 seconds to reach the surface of the Sun. Our climate depends on the photons generated in the Sun as we get almost all of our energy from this radiation.

10. Solar Radiation Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K.

11. Solar Constant • The solar constant is the amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth's atmosphere, in a plane perpendicular to the rays. • The solar constant includes all types of solar radiation, not just the visible light. • It is measured by satellite to be roughly 1366 watts per square meter,[2] though it fluctuates by about 6.9% during a year - from 1412 W/m2 in early January to 1321 W/m2 in early July, due to the earth's varying distance from the sun, and by a few parts per thousand from day to day. • The solar constant is not quite constant over long time periods either. • The Earth receives a total amount of radiation determined by its cross section (π R2), but as the planet rotates this energy is distributed across the entire surface area (4 π R2). Hence, the average incoming solar radiation, taking into account the half of the planet not receiving any solar radiation at all, is one fourth the solar constant or ~342 W/m². • At any given location and time, the amount received at the surface depends on the state of the atmosphere and the latitude.

12. Solar Activity The number of sunspots correlates with the intensity of solar radiation. The variation is small (of the order of 1 W/m² or 0.1% of the total). Sunspots are relatively dark areas on the surface of the Sun where intense magnetic activity inhibits convection and so cools the surface.

13. Sunspot Cycle Solar cycles are cyclic changes in behavior of the Sun. Many possible patterns have been suggested; only the 11 and 22 year cycles are clear in the observations. Various studies have been made using sunspot number (for which records extend over hundreds of years) as a proxy for solar output (for which good records only extend for a few decades).

14. Sunspots and Temperatures The level of solar activity during the past 70 years is exceptional — the last period of similar magnitude occurred over 8,000 years ago. The Sun was at a similarly high level of magnetic activity for only ~10% of the past 11,400 years, and almost all of the earlier high-activity periods were shorter than the present episode.

15. Result: • Solar variations do have correlations with global temperatures. • However, these variations are too small to explain serious climate change issues. • Nevertheless, solar variations can still be the cause of climate change we are observing today. • If so, more research needs to be done to explain how that can be possible as the magnitude of the variations in solar output are very small. • But there are other issues which do not change the solar output, but change the amount of radiation we get from the sun.

16. Milankovitch Cycles The episodic nature of the Earth's glacial and interglacial periods within the present Ice Age (the last couple of million years) have been caused primarily by cyclical changes in the Earth's circumnavigation of the Sun. Variations in the Earth's eccentricity, axial tilt, and precession comprise the three dominant cycles, collectively known as the Milankovitch Cycles. Might be able to explain, but the true nature is unknown.

17. True Nature • What do we mean with “the true nature is unknown”? • Climate change is a very complex area. Certainly the solar energy input plays a very significant role in determining the mechanisms. • For example, just the change in the solar energy input is not sufficient to explain the ice ages. • However, one year, if we get too much snow because of these fluctuations, then the snow may not melt through the summer. Snow cover through the summer makes the summer cooler because of increased reflection. Which in turn reinforces the snow cover. • This is what we call a positive feedback loop. Once you get into such a loop, the more snow makes the earth cooler, and when the earth is cooler, the precipitation comes in the form of snow rather than rain, which cools down the earth even further.