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Ryerson University
PCS 181
Dave Kirsh

Life of a Star - Most stars are main sequence stars o Stars that are living - At the start of a star’s life it is at ten million kelvin - a star does not live forever o our sun is middle aged right now - when star is born, key moment is fusion ignition - when star dies, key moment is the end of fusion - sun is powered by nuclear fusion - nuclear fusion: needs high temperature and high pressure - this happens at center of the sun, protons are moving super fast and they fuse - hydrogen fusion: is called the proton-proton chain o hydrogen collide to form deuterium o finally we get helium and there is a mass defect because extra mass is converted entirely into energy - bigger stars burn hotter and brighter but burns out really fast o more massive stars have more gravity  more central temperature • puts out more light o lives much shorter lives why doesn’t the sun explode? - there is soo much pressure at the centre of the sun o however this pressure decreases going outwards  because gravity opposes it!!! - this is called gravitational equilibrium o where pressure balances gravity o we never have to worry about the sun overheating because of the solar thermostat… which uses the gravitational equilibrium structure of sun o sun is ionized plasma  sun’s most outer “layer” is called corona, we cannot see this  the most outer layer of the sun we can see is called photosphere  the core: is where hydrogen fusion happens. Hottest part of the sun 15 million kelvin • high pressure high density • light we see from sun is actually light produced hundreds of thousands years ago Composition of the sun - 70% hydrogen - 28% helium - 2% other - sun is stable in the core - on the surface, a lot of things are happening o we call this solar activity/ solar weather  ex: sunspots, solar flares, solar storms, ejections - sunspots: o darker color because it is cooler in temperature o bigger than the earth o more sunspots = more solar activities and bursts  solar min: least activities , least sunspots  solar max: most activities, most sunspots  takes 11 years to go from solar max – solar min Magnetic field flipping - changes in sun’s magnetic poles - the sun’s magnetic field flips o on sun it flips every 11 years o when it flips, it freaks out and takes awhile to adjust, this produces sunspots how does magnetic field flipping effect earth - UV and X-RAY changes - Tied into earth’s climate - paired sunspots: cooler plasma trapped by a magnetic field loop - solar flares and coronal mass ejections are caused by cool plasma trapped in a magnetic field loop o CME is very dangerous and causes a lot of problems for satellites - theres always CME and solar wind blowing away from the sun o earth protects us with electromagnetic fields the sun does change over time - the sun will die o the sun’s core is shrinking  makes it hotter • rate of fusion goes up o brightness keeps going up  sun getting smaller & hotter Summary: - hydrogen fusion powers the sun - sun is losing mass all the time and turning that directly into energy - solar thermostat: basically means the sun is very stable and always correcting its own temperature - sun spots are connected to solar activity Death of the Sun and Star Brightness - there are 3 different star variation o low mass o intermediate mass o high mass why is our sun gonna die? - star is powered by hydrogen fusion in the core o the core is 70% hydrogen but eventually the sun will run out of hydrogen to burn through and will all eventually be Helium  when core is helium it can’t burn that so fusion slows down, there’s less pressure holding it together and it will collapse within itself  inside the core, even though there is no more fusion, it is still heating up because the core is getting smaller and shrinking.  At 10 million Kelvin, the hydrogen around the core ignites  Now there is a helium core and a hydrogen shell that is getting hotter and hotter (shell burning) • This causes stars to expand, the hydrogen fusion occurring outside of the shell pushes away and makes the outer layers expand and it becomes a red giant  They hydrogen around the helium core will keep burning and the hydrogen will actually make the helium core get so hot, the helium core will keep getting heavier and shrinking more • Eventually the core will reach 100 million kelvin, helium in the core ignites.And helium flash happens o Helium flash: produces carbon and energy  Triple alpha process o Now we have helium fusion and hydrogen fusion going on  At this point the star shrinks back down from a red giant and looks normal again o Death Round 2:  Helium fusion now creates a carbon ash centre  Double shell burning layer of He and H  This time, when it runs out of helium, the core will once again collapse and heat up, the shell burning will again create a red giant….. • Double shell burning giant ( 2 shells and a carbon ash core) • However this time it will never get hot enough to burn the carbon core so it is the end - Death o Most outer layer of the sun will be blown away as stellar wind o Releasing smoke and burning at same time o Corpse of stars = planetary nebulae  Lit from inside, glowing with emission lights - Eventually the planetary nebulae turns into white dwarf o Small and hot for a while but cools off and gets dimmer o White dwarf pretty much live forever High mass stars - high mass star dies and turns into supergiants and into supernovas o remember how our sun dies eventually because it can never get hot enough to ignite carbon. The size of our sun just isn’t big enough to get that high of a temperature. However, high mass stars go through everything our sun goes through.. and more! Onion layers (layer process gets faster and faster) - hydrogen fusion - helium fusion - carbon fusion - oxygen fusion - neon fusion - magnesium fusion - silicon fusion - iron ash: final stage, iron will never burn because it is most stable and cannot release energy o at this stage supergiant  supernova - supernova = huge explosion, huge light - our universe is moving towards the stelliferous era o universe is going from being made up of 70% Hydrogen, into other things, larger atoms, new stars are bo
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