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Slides for Week 12 4/14/17 Evolution of High-Mass Stars: Evolution of High-Mass Stars Red Supergiants • Very massive stars burn up their H fuel quickly. A massive star is mostly unfused Hydrogen & • They also expand Helium dramatically. – 1000 times larger than In its core, Helium is fusing into Carbon & the Sun! Oxygen Now called red Around the core a shell of Hydrogen can fuse to supergiants. Helium. Betelgeuse, a red supergiant star Death of Massive Stars Core-Collapse Supernovae Close-up of Core • Massive stars can fuse • Once fusion stops, Helium into Carbon and Oxygen after their Hydrogen the core begins to fuel has run out. collapse quickly • They can also create heavier • Outer core layers and heavier elements: rebound off the iron Neon, Magnesium, … and center even Iron. • Rebound causes an • However this process stops enormous explosion, with Iron. Fe Si O C He H called a supernova – Fusing Iron will not produce additional energy. The Crab Supernova Remnant • In 1054 AD observers in China, Japan, and Korea recorded a “Guest Star” • Today in that same part of the sky we find the Crab Nebula • It has a neutron star inside. 1 4/14/17 What’s left after a Low-mass Stars: supernova? 0.4 MSun or less • Only fuse Hydrogen • Remain a star (no planetary nebula) • Depends on the mass of the original star! • Stay on Main Sequence • A star massive enough to supernova usually is too massive to leave behind a white dwarf. • 8-20 MSun: Core collapses to a neutron star – Spinning & Magnetic field? -> Pulsar • More than 20 MSun: Black Hole! Medium-mass Stars: 0.4 M - 8 M High-mass Stars: Sun Sun more than 8 M • Fuse H & He, some Carbon Sun • Fuse heavy elements up to Iron • Become cool red giants • Become supergiants • Expel outer layers -> planetary nebulae & White Dwarf • End as Supernova & Neutron Star or BH Mass Transfer Different types of supernovae: • White Dwarf: white dwarfs in binaries gaining mass & exploding – Type Ia, no H lines – Important for measuring large distances • Core-collapse: deaths of massive stars – Type II, Hydrogen lines present Fig. 10-12, p. 212 2 4/14/17 Observing Supernovae Stellar Evolution Lecture- • We observe supernovae in other galaxies. • A few supernovae per century, per galaxy Tutorial: Page 133-134 • 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. Neutron Stars • Form from a 8-20 MSun star • Leftover 1.4-3 MSun core after supernova • Neutron Stars consist entirely Neutron Star (tennis ball) of neutrons (no protons) -> and Washington D.C. held up by neutron degeneracy pressure (similar to electron Chapter 14: Neutron Stars and degeneracy pressure) Black Holes Pulsars: Stellar Beacons The Lighthouse Model of Pulsars • Pulsars are spinning neutron stars A pulsar’s beam is like a lighthouse • Their strong magnetic fields emit a beam radio waves along the magnetic poles • These are not aligned with the axis of rotation. If the beam shines on Earth, then Model of a Pulsar we see a pulse of energy (radio • The beam of radio waves (a rotating Neutron Star) waves) sweeps through the sky as the neutron star spins. Neutron star’s magnetic field 3 4/14/17 The Crab Pulsar A massive star dies in a Supernova 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 > 3 Msun If the core has more than 3 solar masses… It will collapse completely to single point – The Crab supernova remnant contains a pulsar! => A black hole! Black Holes: Overview Escape Velocity Escape Velocity (vesc) is the speed required vesc to escape gravity’s pull. • A total victory for gravity. On Earth vesc ≈ 11.6 km/s. • Collapsed down to a single point. • This would mean that they have infinite If you launch a spaceship density (but only at one point) at v= 11.6 km/s or faster, it will escape the Earth • Their gravity is so strong, not even light can escape… from inside a certain But vesc depends on the mass distance. of the planet or star… Why Are Black Holes Black? Black Holes & Relativity On planets with more gravity than Earth, • Einstein’s theory of General Relativity says space Vesc would be larger. is curved by mass • So a star like the Sun should bend space, and On a small body like an asteroid, Vesc light traveling past it will get thrown off course would be so small you could jump into • This was confirmed during a solar eclipse in 1919 space. A Black Hole is so massive that Vesc = the speed of light. Not even light can escape it, so it gives off no light! 4 4/14/17 Event Horizon The Schwarzschild Radius Nothing can get out If Vescape > c, then nothing can leave the star, not light, once it’s inside the not matter. event horizon We can calculate the radius of such a star: We have no way of V = c ____2GM esc finding out what’s R = s 2 happening inside! c M = mass G = gravitational constant c = speed of R = Schwarzschild radius light s If something is compressed smaller than Rs 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 gravity than your head. You would experience “tidal forces” pushing & pulling Time is also distorted near a black hole Evidence for Black Holes How do we know they’re real? No light can escape a black hole, so black holes can not be observed directly. • Black holes: However, if a black – Kepler’s Laws, Newton’s Laws hole is part of a binary – Accretion disks star system, we can – Gravitational Waves measure its mass. • Pulsars: – Observe radio jets If its mass > 3 M then – Strong magnetic fields sun it’s most likely a black hole! 5 4/14/17 Evidence for Black Holes: X-rays Evidence for Black Holes Matter falling into a black hole may form an accretion disk. As more matter falls on the disk, it heats up and emits X-rays. • Cygnus X-1 is a source of X rays If X-rays are emitted outside the event horizon we can see • It is a binary star system, with an O type supergiant & a them. compact object The mass of the compact object is more than 20 Msun This is too massive to be a white dwarf or neutron star. This object must be a black hole, Artists’ drawings of located about 6,000 light years accretion disks away. Cygnus X-1: A black hole Supermassive Black Holes (SMBHs) • Stellar black holes come from the collapse of a single star and have masses of several Msun • If a black hole gains mass, its event horizon will expand • Most galaxies (including the Milky Way) have a central SMBH A supermassive black hole devours a star, releasing X-rays 6 .
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