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Lecture

The Sun

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Department
Astronomy
Course
Astronomy 1021
Professor
Chris Racknor
Semester
Winter

Description
Jan 10 12 The Sun26The Suns total power output is called luminosity and it is huge at about 38 x 10Watts In perspective if we could somehow capture and store 1 seconds worth of the Suns luminosity it would be enough to meet current human energy demands for roughly the next 500000 yearsThe Mystery of the Suns Energy For a long time the source of the suns energy remained a complete mystery The first guess was that it was a chemical reaction but a fire on something that big would not last very long and there is no oxygen to fuel it in space guess two was that is was caused by gravitational contraction and this was a better idea but not correct because calculations showed that this would keep the sun shining steadily for about 25 million years but then geologists found out the earth is much older than that Einsteins Crucial Piece of the Puzzle 2With his Emc formula in 1905 there was some outlook Energy has mass and lots of it and there is plenty stored in atoms if you could convert 1kg of matter completely into energy in about a second you would generate 100 000 trillion watts of energy 1 Helium nucleus is lighter than 4 HydrogensWith the formula it became clear that in principle the sun contains enough mass to produce large amounts of energy for billions of yearsNuclear Fusion as the AnswerThe fine details took a long time to work out but now we know nuclear fusion is how the Sun is powered Nuclear fusion is the general term for processes in which atomic particles join together to make a heavier nucleus hydrogen atoms joining together to make helium for instance Understanding how it works requires a bit of particle physics and really understanding it requires Quantum Mechanics Strong Force There exists a very strong force called the strong force The strong force acts like superglue binding neutrons and protons together in an atomic nucleus but it only works for ridiculously short distances like the typical sizes of atomic nuclei Repulsion Versus Strong Force Since the strong force only acts on very short distances other forces are dominant at larger distances in particular electrostatic repulsion Since protons are positively charged they repel each other and thus never really come close enough for the strong force to glue them together unless they come flying at each other at tremendous velocities At low speeds electromagnetic repulsion prevents the collision of nuclei At high speeds nuclei come close enough for the strong force to bind them together Sticking Nuclei Together So highspeed collisions between nuclei and the strong force explain how we can transform one element into another or how we can make sure protons and neutrons stick together to form elements In the Sun the fusion reaction that provides the energy is hydrogen fusion Basically it just involves such collisions between protonsHydrogen Fusion Hydrogen atoms contain just one proton Regular Helium 4He consists of two protons and two neutrons What the Sun does through a complex process of fusions is take 4 hydrogen atoms and transform them into 1 helium atom plus energy
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