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The Lives of Stars Announcements n n n n Quiz # 5 will take place on Tuesday, November 15th. – The link `Quizzes on the website contains the relevant information, including the Textbook units that are basis for the quiz. From now on, please try to come for all quizzes/ exams. No much time for make-ups. Homework # 5 is due on Tuesday Nov 15th. Homework # 6 starts on Tuesday, Nov 15th. It is due on Tue, Nov 22nd. Assigned Reading n Units 58, 59, 60, 61.1-2-3, 62, 64, 66 Let s recall: Hydrostatic Equilibrium of Stars Thermal Pressure Gravitational Contraction What happens if we increase the mass of the star? n More mass = more gravitational contraction n = need for more balancing pressure = higher temperature at the center (and on the surface) n Higher temperature = more hydrogen fusion = higher energy production = more luminous Thus… n More massive = n Higher Temperature (bluer color) = n More luminous L ~ M3.5 A star 10 times more massive than the Sun is ~3000 times more luminous! Stellar Lifetimes A star s lifetime depends on its mass (the tank of fuel) and its luminosity (the rate at which the fuel is burned). Even though a more massive star starts off with more hydrogen (fuel), it burns it much faster than a less massive star – so it dies earlier. tlife ~ M / L ~ M / M3.5 = 1/ M2.5 A star that is 10 times more massive than our Sun has a lifetime which is about 300 times shorter. - Our Sun: 10 billion years - A 10 Msun star: 30 million years More massive stars live shorter lives! The Role of the Mass in Stars q Mass is everything for a star. q It determines: q Its luminosity (L ~ M3.5) q Its temperature q Its lifetime ( t ~ 1 / M2.5) q … and how it dies! q Low-mass stars like our Sun are far more common than high-mass stars. The Hertzsprung-Russell Diagram The Hertzsprung-Russell Diagram The Hertzsprung-Russell Diagram The Main Sequence - all main sequence stars fuse H into He in their cores - this is the defining characteristic of a main sequence star. - more massive stars are more luminous and hotter: L=4πR2 σT4 The Hertzsprung-Russell Diagram L=4πR2 σT4 Red Giants - Red Giant stars are very large, cool and quite bright. Ex. Betelgeuse is 100,000 times more luminous than the Sun but is only 3,500K on the surface. It s radius is 1,000 times that of the Sun. The Hertzsprung-Russell Diagram The Hertzsprung-Russell Diagram White Dwarfs - White Dwarfs are hot but since they are so small, they are not very luminous. L=4πR2 σT4 The Hertzsprung-Russell Diagram Mass of Star Size of Star The Hertzsprung-Russell Diagram Shorter (more mass) Lifetime of Star Longer (less mass) Measuring Ages with the HR Diagram We can date a star cluster (stars born together) by observing its population of stars. The oldest clusters known have been measured to be ~13.5 billion years old. All these stars in the cluster have burned themselves out! Stages in star Birth 1) A cloud of gas/dust begins to gravitationally collapse. 2) The collapsing gas clouds emit blackbody radiation and are very bright. They are protostars. 3) Eventually, the core of the protostar reaches 10 million Kelvin and nuclear fusion ignites. The star is now an official Main Sequence star. What happens to stars once they reach the Main Sequence? Some live fast and die young. Others plod along for a LONG, LONG time. How they live, how they die, and what is left over depends on the MASS. Mass is everything! High mass => Mass > 8 solar masses Low/Interm. mass => 0.08 < Mass < 8 solar masses Brown Dwarfs => < 0.08 solar masses Brown dwarfs Stars that never start fusion! Stars must have masses at least 8% of the Sun s otherwise fusion never starts (not hot enough) Jupiter is only 0.1% of Sun s mass Between the two are sub stellar brown dwarfs The boundary between stars and planets is observationally hard to define first brown dwarf seen: Gliese 229B How do stars that can start fusion (M> 8% Msun) age? They spend most of their life cycle on the Main Sequence. Main Sequence stars fuse hydrogen into helium in their cores. Main Sequence stars are in hydrostatic equilibrium. What happen when the star runs out of hydrogen in the center? Hydrogen burning core shell Hydrogen fuel Thermal pressure from layers above core compresses the hydrogen layer just above the Helium `ashes’ and causes the hydrogen to initiate fusion in a shell Helium “ash” Core of a star Up the red giant branch Eventually, hydrogen will burn only in the outer parts of the mostly-helium core. The star will swell to enormous size and luminosity, and its temperature will drop, becoming a red giant. Sun in ~5 Gyr Sun today Not to scale! Stars become Red Giants Cool and Large Why are Red Giants much rarer than Main Sequence stars? Main Sequence Star time Protostar Red Giant Survey Question (L=4πR2 σT4) When Hydrogen shell fusion ignites, there is a dramatic increase in the energy production of the core of the star. Follow the energy: What happens to the outer layers of the star? 1) the extra energy causes the outer layers to heat up 2) the extra energy creates thermal pressure which pushes out on the outer layers 3) the extra energy is lost in the long and tortuous journey out of the star 4) more than one of the above happens Survey Question (L=4πR2 σT4) When Hydrogen shell fusion ignites, there is a dramatic increase in the energy production of the core of the star. Follow the energy: What happens to the outer layers of the star? 1) the extra energy causes the outer layers to heat up 2) the extra energy creates thermal pressure which pushes out on the outer layers 3) the extra energy is lost in the long and tortuous journey out of the star 4) more than one of the above happens Survey Question The dramatic changes in the appearance of a star during its red giant phase are primarily due to 1) the changing chemical composition of the outer layers affecting the fusion temperature and rate in the core. 2) the changing temperature and surface area of the outer layers affecting the fusion temperature and rate in the core. 3) the changing fusion temperature and rate in the core affecting the chemical composition of the outer layers 4) The changing fusion temperature and rate in the core affecting the temperature and surface area of the outer layers Stars like the Sun (M< 8 Msun) n n n n We left our Sun-like star as a Hydrogen-shell burning red giant Now the core is hot (100,000,000 K) and `heavy enough to start Helium-core burning The star contracts to a yellow giant. Helium burns and produces an element “crucial” to our existence: – CARBON The Core of a Star changes composition… Core Composition of the Sun 70% 50% 100% 50% 30% H He 5 billion years ago H He today Carbon 5 billion years from now Survey Question Fusion in a Main Sequence star changes the chemical composition of the core. What happens to the material outside the core? 1) Helium becomes more abundant outside the core. 2) The chemical composition outside the core changes very little. 3) The same changes occur outside the core as within the core. 4) Hydrogen becomes more abundant outside the core. After helium fusion gets going in a shell… The Sun will expand and cool again, becoming a red (super) giant. Earth, cooked to a cinder during the red giant phase, will be engulfed and vaporized within the Sun. At the end of this stage, the Sun’s core will consist mostly of carbon, with a little oxygen. The end of the line In its last phase the grossly distended Sun will begin to pulse, becoming unstable. Eventually, the outer parts of the Sun, about half its mass, will break away and the Sun will die. The expanding cloud of gas will resemble a planet in appearance (glowing, roundish) and be called a planetary nebula. The hot core will be a white dwarf. Helix Nebula--125 pc Summary: Life Stages of a low-mass star White dwarfs have typically 1/100 the radius of the Sun (about the size of the Earth), and a mass < 1.4 Msun (they shed a lot of mass during their lifetime!). Life of a Low Mass (< 8 Msun) Star on the HR Diagram: White dwarfs start very hot (they are the nuclei of stars, incredibly dense balls of Helium and Carbon!), and passively cool down to become black dwarfs. What happens in massive stars (>8 Msun) Fusion continues up to the heaviest possible element (iron) The lead-up to disaster in massive stars n n n Iron cores do not immediately collapse (the electrons cannot be `squeezed’). If the density continues to rise, eventually the electrons are forced to combine with the protons – resulting in neutrons. What comes next is … core collapse. Supernova 1987a before/after Massive Star Explosions: Supernovae n The gravitational collapse of the core releases an enormous amount of energy (gravitational potential energy is converted to radiative energy as the material falls in; the layers outside the iron core ignite and cause a run-off fusion). n All the shells ignite, and the stars literally explodes - A neutron star or black hole (core cadaver) is left n 100 times the total amount of energy produced by the Sun in its lifetime is released in a matter of seconds. – For a few days, the star is, sometimes, ~as luminous as a whole galaxy!!! Supernovae are pretty easy to `see Some supernovae have been detected from the deepest past of our Universe! (this is how `dark energy has first been revealed!!!) Supernova Remnant: Crab Nebula The supernova explosion that created the Crab was seen on about July 4, 1054 AD. Summary: Life stages of a high mass star. A massive (e.g., 25 M_sun) star burns: • H in 7 million years • O in 6 months • Si in 1 day • Then… Booom • Core collapse in ~0.001-0.01 sec Life of a High Mass (> 8 Msun) Star on the HR Diagram: Supernova explosion Survey Question The Sun will never supernova because 1) It will become a white dwarf before it has the chance. 2) Its surface temperature is not high enough. 3) It is not large enough. 4) It is not bright enough. 5) It is not massive enough. Stellar Evolution in a Nutshell M < 8 MSun M > 8 MSun Mcore < 3MSun Mass controls the evolution of a star! Mcore > 3MSun End Products of Stars M > 8 Msun à Supernova + neutron star or a black hole n 0.08 Msun < M < 8 Msun à White dwarf n M < 0.08 Msun à Brown dwarf (fusion never starts) n Key points of where elements are produced: Elements Generated in the Big Bang Elements Produced in Low Mass Stars Elements Produced in High Mass Stars Elements Produced in Supernovae We are supernova remnants