# 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. Star Formation and Lifetimes Lecture-Tutorial: Pg Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. Lifetimes of Stars Low-mass stars: High-mass stars: Creating Elements with Nuclear Fusion is the source of energy for all stars. Mass Lifetime 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 Anything heavier than is only made in stars! Examples of Nuclear Fusion The Proton-Proton Chain Used by the Sun to fuse protons (H nuclei) into He 4 H > He CNO Cycle: Another way to fuse H -> He CNO cycle is used in massive stars and involves: Carbon (C) Nitrogen (N) & Oxygen (O) 2

3 Examples of Nuclear Fusion Making Heavy Elements Stellar Recycling Starting with H, He, C, O, or N fusion can create many other elements: The universe starts out with H, He, and a little Li: everything else is formed in stars! Mg, Na, Al, Si, etc. Material gets blown away from dying stars and recycled into nebulae, new stars, and planets An Epic Battle: Gravity vs. Pressure How do the deaths of stars differ based on the stars masses? What happens to a star when fusion stops? How do giants, supergiants, and white dwarfs form? Low-Mass Stars (0.4 Msun or less) HUGE zone! Hydrogen & Helium get throughout star s lifetime T > 100 billion years 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. 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 Longer than the age of the 3

4 Average-Mass Stars 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 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. 4

5 White Dwarfs What s holding it up? A white dwarf is hot, but very. So, they are incredibly. One teaspoon of white dwarf material would weigh 5 tons!!! The Death of Heavier Stars Sirius A (Main Sequence Star) Sirius B (White Dwarf) 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 Degeneracy Pressure: Electron energy White dwarfs are only about as big as the, but have the mass of the! No fusion is happening in a white dwarf So what s stopping it from collapsing? 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 Evolution of Massive Stars Death of Massive Stars Close-up of Core A massive star is mostly unfused & They can also create heavier and heavier elements: Neon, Magnesium, and even Iron. In its core, Helium is fusing into Carbon & Oxygen Around the core a of Hydrogen can fuse to Helium. Massive stars can fuse Helium into Carbon and Oxygen after their Hydrogen fuel has run out. Fe Si O C He H However this process stops with. Fusing Iron will not produce additional. 5

6 Core-Collapse Supernovae Once fusion stops, the core begins to quickly Outer core layers off the iron center Low-mass Stars: 0.4 M Sun or less Only fuse Hydrogen Remain a star (no planetary nebula) Stay on Main Sequence Rebound causes an enormous explosion: A Type II 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. The Cygnus Loop Supernova Remnants The Crab Nebula: Remnant of a supernova observed in a.d

7 How Do Supernovae Explode? Supernovae are very complex and not well understood. Observing Supernovae We observe supernovae in other. supernovae per century per galaxy No supernova in our Galaxy for 400 years. We are overdue! of the explosion must be tested by observations of a real supernova. No supernovae in our galaxy since Kepler s Supernova (1604) 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- : 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 Mass Transfer 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 :! Fig , p

8 Neutron Stars Form from a 8-20 M Sun star Chapter 11: Neutron Stars and Black Holes Leftover - M Sun core after supernova Neutron Stars consist entirely of (no protons) Neutron Star (tennis ball) and Washington D.C. Neutron Stars About the size of a large (5-10 miles), Several times the mass of the So they are incredibly dense! One teaspoon of a neutron star would weigh tons! Neutron Star (tennis ball) and Washington D.C. Held up by degeneracy pressure: the neutrons don t like to be squished close together! What s holding it up? White dwarfs and neutron stars are held up by pressure Electron energy White Dwarfs and Neutron Stars are made of degenerate matter. Degenerate matter cannot be compressed.the are already as close as possible. Pulsars: Stellar Beacons neutron stars Strong field emits a beam radio waves along the magnetic poles The Model of Pulsars A pulsar is a neutron star. These are not aligned with the axis of rotation. So the beam of radio waves sweeps through the sky as the Neutron Star spins. Model of a Pulsar (a rotating Neutron Star) Neutron star s magnetic field A pulsar s beam is like a lighthouse If the beam shines on Earth, then we see a of energy (radio waves) 8

9 The Crab Pulsar A massive star dies in a explosion. Most of the star is blasted into space. The core that remains can be a neutron star. However Neutron stars can not exist with masses M > M sun If the core has more than 3 solar masses It will collapse completely to Inside the Crab Supernova Remnant, a Pulsar has been found => A black hole! Degenerate Matter If a White Dwarf gets too heavy it will collapse into a Neutron Star (this triggers a second type of Supernova explosion) White dwarfs cannot be more massive than M sun Similarly, Neutron stars cannot be larger than about M Sun They will collapse completely and turn into a! Black Holes: Overview A total victory for. Collapsed down to a single point. This would mean that they have density Their gravity is so strong, not even can escape! Escape Velocity Escape Velocity (v esc ) is the speed required to escape s pull. On Earth v esc 11.6 km/s. v esc If you launch a spaceship at v= 11.6 km/s or faster, it will escape the Earth But v esc depends on the of the planet or star Why Are Black Holes Black? On planets with more gravity than Earth, V esc would be. On a small body like an asteroid, V esc would be so small you could into space. A Black Hole is so massive that V esc = the. Not even light can escape it, so it gives off no light! 9

10 Black Holes & Relativity Light Can be Bent by Gravity Einstein s theory of General Relativity says space is by mass So a star like the Sun should space, and light traveling past it will get thrown off course This was confirmed during a solar eclipse in 1919 Event Horizon can get out once it s inside the event horizon We have no way of finding out what s happening inside! The Schwarzschild Radius If V escape > c, then nothing can leave the star, not, not. We can calculate the radius of such a star: M = mass R s = 2GM c 2 G = gravitational constant V esc = c c = speed of R s = Schwarzschild radius light If something is smaller than R s it will turn into a black hole! Black Holes: Don t Jump Into One! If you fall into a Black Hole, you will have a big problem: Your feet will be pulled with more than your head. You would experience tidal forces pushing & pulling is also distorted near a black hole 10

11 How do we know they re real? Black holes: Kepler s Laws, Newton s Laws Accretion disks Pulsars: Observe radio jets Strong magnetic fields Evidence for Black Holes No light can escape a black hole, so black holes can not be observed directly. However, if a black hole is part of a binary star system, we can measure its. If its mass > M sun then it s a black hole! Evidence for Black Holes Cygnus X-1 is a source of X rays It is a binary star system, with an O type supergiant & a Evidence for Black Holes: X-rays Matter falling into a black hole may form an accretion disk. As more matter falls on the disk, it heats up and emits. If X-rays are emitted outside the event horizon we can see them. The mass of the compact object is more than M sun This is too massive to be a white dwarf or neutron star. This object must be a black hole. Cygnus X-1: A black hole Artists drawings of accretion disks Supermassive Black Holes Stellar black holes come from the collapse of a star. They have masses of several M sun Bigger mass = bigger BH! This happens in the center of most galaxies. Life Cycles of Stars Low-mass stars: Fade out, stay on Main Sequence Sun-like stars: White dwarf & planetary nebula High-mass stars: Supernova -> SN remnant & dense core Core < 1.4 M Sun = 1.4 M Sun < Core < 3 M Sun = Core > 3 M Sun = Lifetime Mass A supermassive black hole devours a star, releasing X-rays 11

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