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Transcript
Conversations with the Earth Tom Burbine [email protected] Quiz on Thursday • Sun • Hertzsprung-Russell Diagram • Death of stars Main Sequence • Is not an evolutionary track – Stars do not evolve on it • Stars stop on the main sequence and spend most of their lives on it Sun ends it time on the main sequence • When the core hydrogen is depleted, nuclear fusion stops • The core pressure can no longer resist the crush of gravity • Core shrinks Why does the star expand? • The core is made of helium • The surrounding layers are made of hydrogen And .. • Gravity shrinks the inert helium core and surrounding shell of hydrogen • The shell of hydrogen becomes hot for fusion • This is called hydrogen-shell burning And … • The shell becomes so hot that its fusion rate is higher than the original core • This energy can not be transported fast enough to surface • Thermal pressure builds up and the star expands And .. • • • • More helium is being created Mass of core increases Increases its gravitational pull Increasing the density and pressure of this region When • When helium core reaches 100 million Kelvin, • Helium can fuse into a Carbon nucleus Helium Flash • The rising temperature in the core causes the helium fusion rate to rocket upward • Creates a lot of new energy However • The core expands • Which pushes the hydrogen-burning shell outwards • Lowering the hydrogen-burning shell’s temperature And • Less energy is produced • Star starts to contract Now • In the core, Helium can fuse to become Carbon (and some Oxygen) • Star contracts • Helium fusion occurs in a shell surrounding the carbon core • Hydrogen shell can fuse above the Helium shell • Inner regions become hotter • Star expands http://upload.wikimedia.org/wikipedia/commons/8/8d/Triple-Alpha_Process.png • Some carbon fuses with He to form Oxygen • 12C + 4He → 16O + gamma ray • Harder to fuse Oxygen with Helium to produce Neon Planetary Nebulae • There is a carbon core and outer layers are ejected into space • The core is still hot and that ionizes the expanding gas Planetary Nebulae White Dwarf • The remaining core becomes a white dwarf • White dwarfs are usually composed of carbon and oxygen • Oxygen-neon-magnesium white dwarfs can also form • Helium white dwarfs can also form High-Mass Stars • The importance of high-mass stars is that they make elements heavier than carbon • You need really hot temperatures which only occur with the weight of a very high-mass star Stages of High-Mass Star’s Life • Similar to low-mass star’s • Except a high-mass star can continue to fuse elements • When the fusion ceases, the star becomes a supernova • Supernova is a huge explosion Fusion • The temperatures of high-mass stars in its latestage of life can reach temperatures above 600 million Kelvin • Can fuse Carbon and heavier elements • Helium Capture can also occur where Helium can be fused into heavy elements “Deaths” of Stars • White Dwarfs • Neutron Stars • Black Holes White Dwarfs • White Dwarfs is the core left over when a star can no longer undergo fusion • Most white dwarfs are composed of carbon and oxygen • Very dense – Some have densities of 3 million grams per cubic centimeter – A teaspoon of a white dwarf would weigh as much as an elephant White Dwarfs • Some white dwarfs have the same mass as the Sun but slightly bigger than the Earth • 200,000 times as dense as the earth White Dwarfs • Collapsing due to gravity • The collapse is stopped by electron degeneracy pressure Electron Degeneracy Pressure • No two electrons can occupy the same quantum state The Sun • Will end up as a White Dwarf Neutron Star • • • • Neutron stars are usually 10 kilometers acroos But more massive than the Sun Made almost entirely of neutrons Electrons and protons have fused together How do you make a neutron star? • Remnant of a Supernova Supernova • A supernova is a stellar explosion. • Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. Type Ia Supernova Type II Supernova This stops with Iron • Fusion of Iron with another element does not release energy • Fission of Iron does not release energy • So you keep on making Iron Initially • Gravity keeps on pulling the core together • The core keeps on shrinking • Electron degeneracy keeps the core together for awhile Then • The iron core becomes too massive and collapses • The iron core becomes neutrons when protons and electrons fuse together Density of neutron star • You could take everybody on Earth and cram them into a volume the size of sugar cube Explosion • The collapse of the core releases a huge amount of energy since the rest of the star collapses and then bounces off the neutron core • 1044-46 Joules • Annual energy generation of Sun is 1034 Joules How do we know there are neutron stars? • The identification of Pulsars • Pulsars give out pulses of radio waves at precise intervals Pulsars • Pulsars were found at the center of supernovae remnants Pulsars • Pulsars were interpreted as rotating neutron stars • Only neutron stars could rotate that fast • Strong magnetic fields can beam radiation out Black Holes • If a collapsing stellar core has a mass greater than 3 solar masses, • It becomes a black hole Black Hole • After a supernova if all the outer mass of the star is not blown off • The mass falls back on the neutron star • The gravity causes the neutron star to keep contracting Black Hole • A black hole is a region where nothing can escape, even light. Event Horizon • Event Horizon is the boundary between the inside and outside of the Black Hole • Within the Event Horizon, the escape velocity is greater than the speed of light • Nothing can escape once it enters the Event Horizon Black Hole Sizes • A Black Hole with the mass of the Earth would have a radius of 0.009 meters • A Black Hole with the mass of the Sun would have a radius of 3 kilometers http://www.astronomynotes.com/evolutn/remnants.gif Can you see a Black Hole? No • Black Holes do not emit any light • So you must see them indirectly • You need to see the effects of their gravity Evidence • The white area is the core of a Galaxy • Inside the core there is a brown spiralshaped disk. • It weighs a hundred thousand times as much as our Sun. http://helios.augustana.edu/~dr/img/ngc4261.jpg Evidence • Because it is rotating we can measure its radii and speed, and hence determine its mass. • This object is about as large as our solar system, but weighs 1,200,000,000 times as much as our sun. • Gravity is about one million times as strong as on the sun. • Almost certainly this object is a black hole. Any Questions?
