Stars begin as protostars, which is a large mass formed from a gas cloudAs cloud contracts, it increases in temp (gravitational energy to thermal kinetic energy)Shrinks in radiusSpins as a disk, and grows as it gains more matterWhen it is done with accretion process, it is called Pre-main sequence star (PMS)Stars are powered primarily by nuclear reactions which convert H to He in their cores.They are in hydrostatic equilibrium.They are in energy equilibrium: energy generated in the core equals energy emitted at surface.The nuclear reactions occur only in the core, which is about 1⁄2 the mass of the star.Stars spend most of their lifetime on the main sequence “burning” hydrogenBut when the hydrogen runs out…As hydrogen “burns”, helium “ash” builds up in core, which interferes with hydrogen burningStar’s core contracts to become hotter and denser to continue the “burning”Outer layers of star expand and cools down while core contracts and heats upSo the star appears to become cooler in the HR diagramFundamental principle:The star is in a battle against gravity trying to crush it. The star’s weapon in the war is nuclear “burning” to create energy.It will try everything it can, and changes in the core are mirrored in its physical appearance to us.Evolution of a 1 solar mass star (like our sun)1. H “burning” in the core on the main sequence for 10 billion years.As it reached the end of this the core is contracting and the outer layers expanding.2. Core becomes so filled with helium that fusion stopsHydrogen shell burning – core contracts and becomes hot and dense enoughlayers beyond the core star undergoing nuclear reactionsStar’s outer layers expand and become redderHydrogen shell burning continues and the shell moves outward creating a hotter and denser helium filled core.3. Helium core becomes hot enough to allow the triple alpha reaction to occurHelium in core burns into carbon, which is a new short-lived source of energy for the starStar expands further to become a red giantRed giant is a K or M star with luminosity class III100 times or more the size of sun, due to H-shell burning and helium core burningWhat drives size increase? Think of pot boiling – turn up heat, boils more furiously and froth expands4. For a star like our sun, this is the end of the line for nuclear reactions core becomes hotter and denser but not hot enough for further reactionsSurface is boiling off and being pushed out into spaceAs outer layers are pushed out, hotter layers underneath are revealed and star becomes bluerCan push ½ of mass or more into spaceAs mass is lost, weight bearing down on core decreases, so temp and density decreasesThe core of the star is no longer hot enough to generate energy from fusion but it is still plenty hot5. As layers are removed, hot core is revealed, and contracts to size of earth, called a white dwarfVery hot surface illuminates the material thrown into space to create planetary nebulaWhite Dwarf: The end state of evolution of a star with mass like our Sun. It is about the size of the Earth but can have a mass from 0.3 to 1.4 times the mass of the Sun. Much of the mass of a white dwarf is carbon, oxygen, and helium – the produces of the nuclear reactions during the life of the starPlanetary Nebula: the gaseous nebula illuminated by the bright blue light from the white dwarf. The gases are the envelope of the star thrown out into space during the red supergiant phase.6. White dwarf cools over timeGases of planetary nebula disperse into space and all that is left is cooling white dwarf that cannot generate new energyCools slowly over billions of yearsWhat will happen to the Earth?Sun becomes brighter and redder over the next 5 billion years and the Earth becomes warmerSun expands to orbit of MarsAs sun expands, Earth enters outer atmosphere of Red-Giant Sun and spiral into Sun and vaporizes into atomsSun becomes a white dwarf with deadly high energy radiationThen it will cool off over the next billion years and the Solar System will be a deep-freeze  remaining planets will be colder than PlutoSUMMARY SLIDE WITH HR DIAGRAM WILL BE ON THE FINAL!!!! Where sun is when its dying on diagram  see slide show #19More massive stars:Follow the same path from core hydrogen “burning” to shell hydrogen “burning” to core helium “burning” and shell hydrogen “burning”Since they are bigger, cores can become even hotter and denser and they can “burn” carbonStar creates layer upon layer of nuclear reactions  build up heavier elements deeper in the coreCore eventually becomes filled with iron, which cannot undergo fusion reactions to generate more energyWhy can a White Dwarf cool and not collapse?Star’s life is spent generating energy in the core to create the high temperatures needed to balance against gravity  Hydrostatic Equilibrium of pressure (high density and temperature) against gravityDegenerate Electron Gas – gas density is high enough that electrons are freed from the atomsIn normal gas, the atoms are free to move around.As the gas gets denser, the atoms bounce around more and the electrons in the atoms begin to overlap in their motionThe electrons fight being pushed into smaller volumes as the star tries to shrink as it cools – this is the source of pressure.The pressure does not depend on the temperature.So White Dwarfs can live forever slowly cooling, which is what white dwarfs that are not in close binary systems doSo, stars that are 1 to 8 solar masses go through life and throw off their envelopes of H, and keep locked in their white dwarf core most of the He, C, and O that they produced.If two 1-8 solar mass stars form in a binary, the more massive one has a shorter lifetime so it becomes a white dwarf sooner.As the less massive star becomes a red giant, material begins to flow out from the red giant and some of it falls onto the white dwarf.The material forms a hot accretion disk which funnels material onto the starAs the material accumulates on the surface of the white dwarf, the hydrogen can get hot and dense enough to undergo an explosion which creates a Nova.Novae are explosions just below the surface of white dwarfs in binary systems which throw material out into space and can be 100,000 times as bright as the Sun for a few days.If the mass of the white dwarf in a binary grows with time from the accreted material, gravity can overcome degenerate electron gas pressure.... And cause the