AST101 Chapter 14 Summary

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Astronomy & Astrophysics
Michael Reid

AST101 Chapter 14 Our Star 14.1 A closer look at the sun The sun is a star The suns energy is absolutely vital to human existence Late 19 century: the sun generates energy by slowly contracting in size: gravitational contraction Shrinking gas clouds heat up because some of the gravitational potential energy of gas particles far from the cloud is converted into thermal energy as the gas moves inwards A gradually shrinking Sun would always have some gas moving inward, converting gravitational potential energy into thermal energy this thermal energy would keep the inside of the sun hot Because of its large mass, the sun would need to contract only very slightly each year to maintain its temperature- so slightly that the contraction would have been unnoticeable to 19 century astronomers Einsteins theory : shows that mass itself contains an enormous amount of potential energy Calculations show that the Suns mass contained more than enough energy to account for billions of years of sunshine The sun maintains internal conditions by gravitational equilibrium (aka hydrostatic equilibrium) gravity pulling inward and pressure pushing outward Because the weight of overlying layers is greater as we go deeper into the sun, the pressure must increase with depth Deep in the suns core, the pressure makes the gas hot and dense enough to sustain nuclear fusion The energy released by fusion heats the gas and maintains the pressure that keeps the sun in balance against the inward pull of gravity Steadiness of our suns gravitational equilibrium: gravity pushes inward while the energy released by fusion maintains the pressure that pushes outwards The sun was born ~4 bil yrs ago from a collapsing cloud of interstellar gas contraction of the cloud released gravitational potential energy Much of this energy was radiated away from the clouds surface as thermal radiation Yet much of this remained inside causing the interior temperature and pressure to rise When the central temperature and density grew high enough to sustain nuclear fusion, energy generation in the suns interior came into balance with the energy lost from the surface in the form of radiation This energy balance stabilized the size of the sun, bringing it into a state of gravitational equilibrium that has persisted to this day The sun was born with enough hydrodgen in its core to shine steadily and maintain its gravitational equilibrium for about 10 bil yrs the sun is only ~halfway through its 10-bil-yr lifetime When the sun exhausts its nuclear fuel ~5 bil yrs from now, gravitational contraction of the core will begin once again BASICALLY: - About 4 bil yrs ago, gravitational contraction made the sun hot enough to sustain nuclear fusion in its core - Ever since, energy liberated by fusion has maintained the Suns gravitational equilibrium and kept the sun shining steadily, supplying the light and heat that sustain life on earth The Suns Structure The sun is a giant ball of plasma a gas in which many of the atoms are ionized because of the high temperature The differing temperatures and densities of the plasma at different depths give the sun the layered structure Spectroscopy tells us that the sun is made entirely of hydrogen and helium The suns radius is just under 700,000 km, or 100x the radius of the earth Even sunspots on the suns surface can be larger in diameter than the earth The Suns mass can be measured using Newtons version of Keplers Third Law: it is about 2 x kg (~300,000x the mass of the earth, and ~1000x the mass of all the planets in our solar system put together) The Suns rotation rate can be observed by: o tracking the motion of sunspots or o measuring Doppler shifts on opposite sides of the sun The entire sun does not rotate at the same rate the solar equator completes one rotation in ~25 days the rotation period increases with latitude to about 30 days near the solar poles The sun released an enormous amount of radiative energy into space Power = the rate at which energy is released Standard unit of power is the watt, defined as 1 joule of energy per second; that is 1 watt = 1 joule/s The suns luminosity (total power output) = 3.8 x watts Only a tiny fraction of the Suns total energy output reaches earth The rest of its energy disperses inall directions into space Most of this energy is radiated in the form of visible light after the layer of protection of earths atmosphere, there are other amounts of solar radiation, including dangerous ultraviolet and x rays Basic properties of the sun Radius ( ) 696,000 km (~109x the radius of the earth) Mass ( ) 2 x kg (~300,000x the mass of the earth) Luminosity ( ) 3.8 x watts Composition (by % of mass) 70% hydrogen, 28% helium, 2% heavier elements Rotation rate 25 days (equator) to 30 days (poles) Surface temperature 5800 K (average); 4000 K (sunspots) Core temperature 15 million K The Suns Atmosphere Solar wind: the stream of charged particles continually blown outward in all directions from the sun the solar wind helps shape the magnetospheres of planets and blows back the material that forms the plasma tails of comets Corona: the outermost layer of the suns atmosphere o extends several million km above the visible surface of the sun
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