Structure of the Sun

The Core is where all the action is. The core is the only place in the Sun where the temperature (10 million K) and density are high enough to support nuclear fusion The core is the only place in the Sun where the temperature (10 million K) and density are high enough to support nuclear fusion (hydrogen bomb needs atomic bomb for ignition!) (hydrogen bomb needs atomic bomb for ignition!) Every second, about 600 million tons of Hydrogen are fused into 596 million tons of helium. Every second, about 600 million tons of Hydrogen are fused into 596 million tons of helium. The remaining mass (4 million tons) is converted to energy in line with Einstein ’ s formula E = mc 2 The remaining mass (4 million tons) is converted to energy in line with Einstein ’ s formula E = mc 2

Nuclear Fusion in the Sun Hydrogen is constantly being transformed into helium in the Sun ’ s core. Hydrogen is constantly being transformed into helium in the Sun ’ s core.

The proton-proton chain Reaction (main branch) - neutrinos;  + - positrons;  - gamma rays.

4 1 H --> 4 He + energy ( E = mc 2 ) mass( 4 He) = 0.993{mass(H)+mass(H)+mass(H)+mass(H)} mass loss is 0.007{mass(4H)} = 5 x kg E=mc 2 =(5 x )(3 x 10 8 ) 2 = 4 x joules How many fusions per second? Solor Luminosity = 4 x joules/sec 4 x joules/sec = fusions/sec = 200 million tons/sec!!! 4 x joules Actually, about 500 million tons/sec are needed!

Balance in the stars Thermal Pressure Gravitational Contraction

Pressure and Temperature of a Gas

This balance between weight and pressure is called hydrostatic equilibrium. The Sun's core, for example, has a temperature of about 16 million K. How does a star hold itself?

Air temperature inside drops below the set point of the thermostat Electrical signal triggers ignition of furnace Furnace burns oil/gas and generates heat Heat is absorbed by water running near furnace causing water temperature to rise Hot water is circulated throughout house Radiators transfer energy from water to air Air warms thermostat until electrical signal is shut off Furnace shuts off My House’s Thermostat

Outward thermal pressure of core is larger than inward gravitational pressure Core expands Expanding core cools Nuclear fusion rate drops dramatically Outward thermal pressure of core drops (and becomes smaller than inward grav. pressure) Core contracts Contracting core heats up Nuclear fusion rate rises dramatically The Solar Thermostat

Balance If the star contracts at all, the temperature goes up, the rate of fusion increases, the pressure increases, and the star re-expands. If the star contracts at all, the temperature goes up, the rate of fusion increases, the pressure increases, and the star re-expands. If the star expands too much, the temperature drops, the rate of fusion drops, the pressure drops, and the star contracts. If the star expands too much, the temperature drops, the rate of fusion drops, the pressure drops, and the star contracts.

How is the energy transported from the center to the surface? Radiative transfer Radiative transfer Photons are continually absorbed and re-emitted (reprocessing of radiation), and when they are re- emitted from more external layers, which are colder, the loose energy (but total energy is conserved, because more photons are emitted) Photons are continually absorbed and re-emitted (reprocessing of radiation), and when they are re- emitted from more external layers, which are colder, the loose energy (but total energy is conserved, because more photons are emitted) Convection Convection Large scale motions of gas. Hot gas moves upward and carries energy there. Cold gas move downward to restore balance Large scale motions of gas. Hot gas moves upward and carries energy there. Cold gas move downward to restore balance

Radiative Transport Photons are absorbed and re-emitted in random walk motion. Photons are absorbed and re-emitted in random walk motion. Each photon mean-free path is very, very short Each photon mean-free path is very, very short It takes each γ photon produced by nuclear fusion million years to reach the surface, and when it does, it has been reprocessed into ~1,800 cooler visible-light photons (energy is consesrved!!) It takes each γ photon produced by nuclear fusion million years to reach the surface, and when it does, it has been reprocessed into ~1,800 cooler visible-light photons (energy is consesrved!!)

Convection Activity

Structure of the Sun

An aside about nuclear fusion as a viable energy source A few key points about the P-P Chain: It is a “ clean ” form of energy production. It is a “ clean ” form of energy production. Put 4 protons (Hydrogen) in and get one Helium and some energy back out. Put 4 protons (Hydrogen) in and get one Helium and some energy back out. Given fuel (in the form of Hydrogen) it is a self- sustaining reaction. Given fuel (in the form of Hydrogen) it is a self- sustaining reaction. It requires *very* high temperatures (10 7 K) It requires *very* high temperatures (10 7 K)

The energy emitted by the Sun is produced in a small region at the very center of the Sun. in a small region at the very center of the Sun. uniformly throughout the entire Sun. uniformly throughout the entire Sun. throughout the entire Sun but more in the center than at the surface. throughout the entire Sun but more in the center than at the surface. from radioactive elements created in the Big Bang. from radioactive elements created in the Big Bang.

1) energy output 2) radius 3) electric fields 4) magnetic fields The bulk of the violent surface activity on the Sun is due to “seasonal” variations in the Sun’s _________.

What is burning in stars? Gasoline Gasoline Nuclear fission Nuclear fission Nuclear fusion Nuclear fusion Natural gas Natural gas

Sun Activity: or Weather on the Sun Two key points to remember here: 1.nearly all “solar weather” is a result of changes in the magnetic fields that penetrate the Photosphere. 2.Magnetic Fields are the result of convection in the Sun (=large scale motions of electrically charged particles generate manetic fields)

Due to change in magnetic field Magnetic field lines Measured by Zeeman effect

Why sunspot is colder

Differential Rotation of the Sun causes magnetic cycles Rotation period: about a month The middle rotates faster than the north or south. Time-lapse (27 days) movie of x-ray emission from the Sun's corona. Source: Yohkoh Public Outreach Project.Yohkoh Public Outreach Project

Babcock Model

The Solar Cycle Solar activity peaks roughly each 11 years.

The Solar Constant The S.C tells us the amount of energy the Sun produces: The S.C tells us the amount of energy the Sun produces: It is 1360 Joules per second per square meter… It is 1360 Joules per second per square meter… …or about the luminosity (=ENERGY PER SECOND) of 60 W bulb every square millimeter. …or about the luminosity (=ENERGY PER SECOND) of 60 W bulb every square millimeter. A stable change of ~1% in the S.C. would change Earth’s temperature by 1-2 degrees A stable change of ~1% in the S.C. would change Earth’s temperature by 1-2 degrees Random variation ~0.1% over days or weeks Random variation ~0.1% over days or weeks Long-term, cyclical variation 0.018% per year (period longer than the 22 years of the sunspots) Long-term, cyclical variation 0.018% per year (period longer than the 22 years of the sunspots) Small random fluctuations do not affect Earth’s climate Small random fluctuations do not affect Earth’s climate