# Protostars on the HR Diagram. Lifetimes of Stars. Lifetimes of Stars: Example. Pressure-Temperature Thermostat. Hydrostatic Equilibrium

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1 Protostars on the HR Diagram Once a protostar is hot enough to start, it can blow away the surrounding gas Then it is visible: crosses the on the HR diagram The more the cloud, the it will form stars Lifetimes of Stars Estimate a star s lifetime based on how much fuel it has, and how fast it uses up its fuel 1 T = M 2.5 M = Mass of star in (M Sun ) T = Lifetime of star in (T Sun ) Lifetimes of Stars: Example How long will a star with 1/5 of the Sun s mass (0.2 M Sun ) live? 1 T = M 2.5 Hydrostatic Equilibrium is pulling the outer part of a stars toward the center can resist the pull of gravity. Nuclear fusion in the core of a star releases huge amounts of energy. This energy (heat) creates the thermal pressure. So if the Sun s lifetime is about 10 billion years, this star will last years! Hydrostatic Equilibrium occurs when gravity and pressure forces Hydrostatic equilibrium keeps stars stable for billions of years. The inward force of is balanced by the force of pushing out. All stars on the Main Sequence (of the HR Diagram) are and in. Pressure-Temperature Thermostat Main sequence stars are self-regulating systems, small changes get corrected If thermal pressure drops: 1. Star a little 2. Density 3. Temperature 4. Nuclear reactions 5. Thermal pressure again 1

2 Leaving the Main Sequence Stars live most of their lives on the Main Sequence When they run out of Hydrogen Fuel, they leave the and begin to. Lifetimes of Stars Low-mass stars: High-mass stars: Mass Lifetime Creating Elements with Nuclear Fusion is the source of energy for all stars. Hydrogen (H) can be fused into Helium (He) in two ways: Proton-Proton Chain C-N-O Cycle Stars can also fuse He into: Carbon, Nitrogen & Oxygen Examples of Nuclear Fusion The Proton-Proton Chain Used by the Sun to fuse protons (H nuclei) into He 4 H > He Anything heavier than is only made in stars! CNO Cycle: Another way to fuse H -> He CNO cycle is used in massive stars and involves: Carbon (C) Nitrogen (N) & Oxygen (O) Examples of Nuclear Fusion Making Heavy Elements Starting with H, He, C, O, or N fusion can create many other elements: Mg, Na, Al, Si, etc. 2

3 Stellar Recycling How do the deaths of stars differ based on the stars masses? The universe starts out with H, He, and a little Li: everything else is formed in stars! What happens to a star when fusion stops? How do giants, supergiants, and white dwarfs form? Material gets blown away from dying stars and recycled into nebulae, new stars, and planets Low-Mass Stars (0.4 Msun or less) An Epic Battle: Gravity vs. Pressure What happens at the end of a star s life? 3 possible outcomes: wins: the star collapses completely into a black hole wins: the entire star explodes into space : The center of the star contracts, while the outer layers expand. HUGE zone! Hydrogen & Helium get throughout star s lifetime T > 100 billion years Longer than the age of the Average-Mass Stars Average-Mass Stars Stars like the Sun will expand, and turn into They lose their outer layers which expand to become a All that remains is the hot core of the star: a Lifetime of the Sun: About 10 billion years total After H fuel is used up in the core, fusion Core : heats up area around the core H shell fusion around He core Energy from shell fusion forces outer layers to, : the Sun becomes a Red Giant 3

4 Average-Mass Stars The Sun becomes a Giant, leaves the Main Sequence (!) It runs out of fuel, the Thermal Pressure in the core decreases, and gravity will cause the core to shrink If the core can shrink enough, it can start fusion Helium Fusion The Sun s Last Gasp Produces Carbon and Oxygen Is than hydrogen fusion. So the star s outer layers heat up and expand even more.until they are lost to space. These layers form great called: planetary nebulae Planetary Nebulae The last gasps of dying stars (not related to planets!) Helix Nebula (close-up view) Planetary Nebulae Helium burning ends with a pulse that ejects the H and He into space as a planetary nebula The core left behind becomes a White Dwarfs The core of the dying star is left behind. It is very hot: K It is blue or even white, and called a White Dwarf White dwarfs are! The Sun will end its life as a White Dwarf, slowly cooling down. A white dwarf is hot, but very. White dwarfs are only about as big as the, but have the mass of the! So, they are incredibly. One teaspoon of white dwarf material would weigh 5 tons!!! White Dwarfs Sirius B (White Dwarf) Sirius A (Main Sequence Star) 4

5 What s holding it up? No fusion is happening in a white dwarf So what s stopping it from collapsing? Electron energy Degeneracy Pressure: Counteracts gravity in white dwarfs, keeping them stable Death of Massive Stars: Red Supergiants Very massive stars burn up their H fuel. They also expand dramatically. times larger than the Sun! Now called red supergiants. Betelgeuse is a red supergiant in the constellation Orion Death of Massive Stars Close-up of Core Fe Si O C He H The Death of Heavier Stars Heavyweight stars expand and turn into A supergiant runs out of fuel & causes a massive explosion called a. Could become neutron star or a black hole Evolution of Massive Stars A massive star is mostly unfused & In its core, Helium is fusing into Carbon & Oxygen Around the core a of Hydrogen can fuse to Helium. Core-Collapse Supernovae Massive stars can fuse Helium into Carbon and Oxygen after their Hydrogen fuel has run out. Once fusion stops, the core begins to quickly They can also create heavier and heavier elements: Neon, Magnesium, and even Iron. Outer core layers off the iron center However this process stops with. Fusing Iron will not produce additional. Rebound causes an enormous explosion: A Type II 5

6 Low-mass Stars: 0.4 M Sun or less Only fuse Hydrogen Remain a star (no planetary nebula) Stay on Main Sequence Medium-mass Stars: 0.4 M Sun - 8 M Sun Fuse H & He, some Carbon Become cool giants Expel outer layers -> planetary nebulae & WD High-mass Stars: more than 8 M Sun Fuse heavy elements up to Iron Become supergiants End as Supernova & Neutron Star or BH The Crab Supernova Remnant In 1054 AD observers in China, Japan, and Korea recorded a Guest Star Bright enough to be seen during the! Today in that same part of the sky we find the It has a inside. It is also expanding in size. Supernova Remnants The Crab Nebula: Remnant of a supernova observed in a.d How Do Supernovae Explode? Supernovae are very complex and not well understood. of the explosion must be tested by observations of a real supernova. No supernovae in our galaxy since Kepler s Supernova (1604) The Cygnus Loop 6

7 Observing Supernovae We observe supernovae in other. supernovae per century per galaxy No supernova in our Galaxy for 400 years. We are overdue! Supernovae are! Most stars: red dwarfs & white dwarfs Most stars: Blue giants White Dwarfs result from common main sequence stars Supernovae result from common main sequence stars Different types of supernovae: Mass Transfer Mass- : white dwarfs in binaries gaining mass & exploding Type Ia, no H lines Mass- : large star loses outer layers in a binary, core explodes Type Ib, no H lines - : deaths of massive stars Type II, Hydrogen lines present Fig , p. 212 What s left after a supernova? Depends on the of the original star! 8-20 M Sun : Core collapses to a star Spinning? -> More than 20 M Sun :! 7

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