AST101- Chapter 14.docx

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Astronomy & Astrophysics
Michael Reid

Chapter 14: Our Star  The Sun is virtually the source of all light, heat and energy reaching Earth, and life on Earth’s surface could not survive without it Chapter 14.1 A Closer Look at the Sun Why was the Sun’s energy source such a major mystery?  Ancient thinkers thought of the Sun to be some type of fire (lump of burning coal/wood)  Scientists began to address the question of how the Sun shines around the mid 1800’s, by which time the Sun’s size and distance had been measured with reasonable accuracy o Burning coal/wood idea was quickly ruled out  In the late 1800’s scientists came up with an idea that seemed more plausible o Suggested that the Sun generates energy by slowly contracting in size (gravitational contraction) o Shrinking gas cloud heats up because some of the gravitational potential energy of gas particles far from the cloud centre is converted into thermal energy as the gas moves inward  Major flaw: calculations showed that this process could keep the Sun shining steadily for 25 million years when there is evidence from rock that shows Earth to be far older than 25 million years Why does the Sun Shine?  Einstein’s theory included the famous discovery of , which showed that mass itself contains an enormous amount of potential energy, calculations showed that the Sun’s mass contained more than enough energy to account for billions of years of sunshine o By the end of 1930’s we learned that Sun converts mass into energy through the process of nuclear fusion The Stable Sun:  The conditions for nuclear fusion are found deep in the core (high temperatures and densities)  Gradational Equilibrium: main internal conditions through a natural balance between two competing forces, gravity pulling inward and pressure pushing outward (internal gas pressure); the Sun’s internal pressure precisely balances gravity at every point within it—keeping the Sun stable in size; at each higher level, the overlying weight is less o deep in the Sun’s core, the pressure makes the gas hot and dense enough to sustain nuclear fusion, the energy released in turn, heats the gas and maintains the pressure that keeps the Sun in balance against the inward pull of gravity How Fusion Started  Sun was born about 4 billion years ago from a collapsing cloud of interstellar gas, the contraction of the cloud released gravitational potential energy, most It was radiated as thermal radiation but the rest remained inside causing the inner temperature and pressure to rise o Until it was high enough to sustain nuclear fusion, energy generation in the Sun’s interior came into balance with the energy lost from the surface in the form of radiation which brought into a state of gravitational equilibrium which has lasted until today o Halfway through its 10 billion year lifetime, when the Sun finally exhausts its nuclear fuel, gravitational contraction will occur again where the crush of gravity becomes to overcome a star’s internal sources of pressure What is the Sun’s Structure?  Is a ball of plasma (gas in which many of the atoms are ionized because of the high temperature) o Its positively charged ions/freely moving electrons means it can create/respond to magnetic fields  Differing temperatures/densities of the plasma at different depths gives the Sun its layered structure Basic Properties of the Sun  Spectroscopy tells you that the Sun is made almost entirely of hydrogen/helium, from the Sun’s angular size and distance you can determine that its radius is just under 700,000 kms, sunspots can be larger than Earth  Can measure the Sun’s mass using Newton’s version of Kepler’s Third Law (more than 1,000 times the mass of all the planets in our solar system put together), can observe the Sun’s rotation rate by tracking the motion of sunspots or by measuring Doppler shifts (solar equator completes one rotation in about 25 days and the rotation period increases with latitudes to about 30 days near the solar poles)  sun does not rotate at same rate  Releases an enormous amount of heat of radiative energy, the Sun’s total power output (luminosity) is 3.8 x 10^26 watts  Most of the energy that reaches Earth surface is in the form of visible light but outside of our protective atmosphere, there is significant amounts of dangerous x-ray and ultraviolet radiation The Sun’s Atmosphere  Solar Wind: the stream of charged particles continually blown outward in all directions from the Sun, it helps shape the magnetospheres of planets  Corona: the outermost low-density gas that represents what we usually think of Sun’s atmosphere, it extends several million kms above the visible surface of the Sun (1 million K explaining why this region emits most of the Sun’s X rays), very low density  Chromosphere: middle layer of the solar atmosphere where the temperature drops to 10 000 K and is the region that radiates the most ultraviolet light  Photosphere: lowest layer of the atmosphere which is the visible surface of the Sun, it is not a solid surface even though it looks like it, just under 6000 K, has sunspots which are regions of intense magnetic fields The Sun’s Interior  Convection zone: where energy generated in the solar core travels upwards, transported by the rising of hot gas and failing of cool gas, which is the cause of the Sun’s churning appearance  Radiation Zone: has calmer plasma, where energy moves outward primarily in the form of the photons of light, almost 10 million K and immense amounts of X-ray radiation  Core: where nuclear fusion transforms hydrogen into helium, about 15 million K, the density is 100 times that of water and the pressure is 200 billion times that on the surface of Earth 14.2 The Cosmic Crucible  If a nucleus gains/losses protons its atomic number changes and it becomes a different element  Gains/losses neutrons it’s atomic mas number changes and it becomes a different isotope  Nuclear Fission: process of splitting a nucleus into two smaller nuclei  Nuclear Fusion: process of combining nuclei to make a nucleus with a greater number of protons or neutrons How does Nuclear fusion occur in the Sun?  Extreme heat in the core allows bare positively charged atomic nuclei and negatively charged electrons to whiz around at high speeds, which are on high-speed collision courses with each other, in most cases electromagnetic forces deflect the nuclei, but if they collide with enough energy they can stick together to form a heavier nucleus  Sticking positively charged nuclei together is not easy, the strong force, which binds neutrons and protons together in atoms nuclei is the only force in nature that can overcome the electromagnetic repulsion between two positively charged nuclei o Overpowers the electromagnetic force over small distance, therefore the key to nuclear fusion is to push positively charged nuclei close enough for the strong force to outmuscle electromagnetic repulsion  The higher the temperature the harder the collisions, making fusion reaction more likely at higher temperatures (making the core perfect for this) also the high pressure of the overlying layers is necessary because without it, the hot plasma of the soar core would simply explode into space, shutting off nuclear reactions The Proton-Proton Chain  Hydrogen nuclei-individual protons  Helium- most common form consist of 2 protons and 2 neutrons o Overall hydrogen fusion reaction therefore transforms 4 individual protons into a helium nucleus containing two protons and neutrons  It begins with collisions of individual protons, has 3 steps: o Step 1: two protons fuse to form a nucleus consisting of one proton and one neutron (isotope of hydrogen-Deuterium), converts a proton a proton into a neutron, reducing the total nuclear charge from +2 to +1 for the resulting Deuterium nucleus. The lost positive charge is carried off by a positron (anti- electron: has positive charge instead of negative. A neutrino (subatomic particle with a very tiny mass is also produced in this step); this step must occur twice in the overall reaction o Step 2: occurs when one of the deuterium nuclei collides and fused with a proton. The result is a nucleus of helium-3, a rare form of helium with two protons and one neutron, along with the reproduction of a gamma-ray photon, also occurs twice o Step 3: requires the addition of another neutron to the helium-3, thereby making a normal helium-4. Collision of two helium-3 nuclei. The final result is a normal helium-4 nucleus and two protons  Overall reactions combines four protons to make one helium nucleus o This fusion generates energy because a helium nucleus has a mass slightly less than the combined mass of 4 hydrogen nuclei, so when four hydrogen nuclei fuse into a helium, a little bit of mass disappears o About 98% of the energy emerges as kinetic energy of the resulting helium nuclei and radiative energy of the gamma rays which eventually emerges as sunlight The Solar Thermostat FIG 14.8  Sun fuses hydrogen at a steady rate thanks to a natural feedback process that acts as a thermostat for the Sun’s Interior o Rate of nuclear fusion is very sensitive to temperature, a slight increase in the Sun’s core temperature would mean
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