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J B Lester

AST 201S - Stars and Galaxies University of Toronto Mississauga Chapter 10 Summary Essential Cosmic Evolution, 5 Edition th 1. Basic properties of the Sun • Angular diameter + distance → diameter = 109 ×D Earth 3 • Volume ∝ D Sun= 1.3 million Earths • Sun’s mass keeps Earth in orbit - law of gravity tells us the Sun’s mass = 300,000 × Earth’s mass • Average density = Sun’s Mass/Sun’s Volume = 1.4 g/cm , only half the density of rock and close to water → the Sun’s composition must be mostly the lightest elements, such as hydrogen • Surface temperature (from thermal radiation) = 5800 K → the matter is entirely gas • Detailed composition from the amount of light removed by the spectral ab- sorption lines of each chemical element – by mass: 70% H, 28% He, 2% others – by number: 90% H, 9% He, 1% others • Luminosity – measure total radiation arriving at Earth above the atmosphere in 1 m2 – assume (now verified) that the Sun radiates equally in all directions – total radiation = 3.8 × 106 W Note, 1 second of the Sun’s luminosity would supply all the Earth’s energy for 500,000 years 2. Structure of the Sun • Sun is in balance: pressure outward = gravity inward → central pressure must be huge to support all the weight • For a gas, pressure and temperature are directly related, T ∝ P. Huge central pressure → huge central temperature = 15 million K 3. What is the source of Sun’s central heat? • Burning H gas? - No Not enough oxygen to burn the gas Burning the Sun would have run out of gas billions of years ago • Very slow shrinking to release stored gravitational energy? - No The Sun would have shrunk to a point billions of years ago • Nuclear Fusion - Yes continued on page 2 AST201H5S – Chapter 10 Summary page 2 of 3 4. What is Nuclear Fusion? • Under very high temperature and pressure, hydrogen nuclei (protons) can fuse (= merge) to form helium + energy • stored “strong energy” is released as radiation • mass of 1 helium atom is less than the 4 protons that created it • “missing” mass (∆m) is converted to energy according to Einstein’s equation E = ∆mc , where c is the square of the speed of light = 9 × 10 16 (m/s) 2 • Even is the Sun’s core, the fusion requires a series of individual steps
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