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How Are the Lives of Stars with Close Companions Different?

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How Are the Lives of Stars with Close Companions Different? Astronomy 110 Announcements: • Midterm revisions due tomorrow • Reading for tomorrow: pp. 349 - 369 (there will be a quiz) • Homework #4 due Wednesday Life of a 20 MSun star Life of a 1 MSun star The binary star Algol consists of a 3.7 MSun main sequence star and a 0.8 MSun subgiant star. How are the lives of stars with close companions different? What’s strange about this pairing? How did it come about? Stars in Algol are close Star that is now a subgiant enough that matter can was originally more massive flow from subgiant onto main-sequence star As it reached the end of its life and started to grow, it began to transfer mass to its companion Now the companion star is more massive What happens next? White dwarf’s gravity pulls matter off of giant companion, but angular momentum prevents the matter from falling straight in Friction in disk makes it hot, causing it to glow Infalling matter forms an accretion disk around the white Friction also removes angular momentum from inner dwarf regions of disk, allowing them to sink onto white dwarf Thought Question Thought Question What would gas in disk do if there were no friction? What would gas in disk do if there were no friction? A. It would orbit indefinitely. A. It would orbit indefinitely. B. It would eventually fall in. B. It would eventually fall in. C. It would blow away. C. It would blow away. Hydrogen that accretes onto a white dwarf star builds up in a shell on the surface When base of shell gets hot enough, hydrogen fusion suddenly begins leading to a nova Nova explosion generates a burst of light lasting a few weeks and expels much of the accreted gas into space Two Types of Supernova Massive star supernova: Iron core of massive star reaches white dwarf limit and collapses into a neutron star, causing explosion White dwarf supernova: Carbon fusion suddenly begins as white dwarf in close binary system reaches white dwarf limit, causing total explosion One way to tell supernova types apart is with a light curve showing how luminosity changes Supernova Type: Nova or Supernova? Massive Star or White Dwarf? • Supernovae are MUCH MUCH more luminous!!! • Light curves differ (about 10 million times) • Nova: H to He fusion of a layer, white dwarf left • Spectra differ (exploding white dwarfs don’t intact have hydrogen absorption lines) • Supernova: complete explosion of white dwarf, nothing left behind 13.2 Neutron Stars • Our Goals for Learning • What is a neutron star? • How were neutron stars discovered? What is a neutron star? • What can happen to a neutron star in a close binary system? A neutron star is the ball of Electron degeneracy neutrons left pressure goes away behind by a because electrons massive-star combine with protons, supernova making neutrons and neutrinos Degeneracy Neutrons collapse to pressure of the center, forming a neutrons neutron star supports a neutron star against gravity Neutron Star • Original core mass ~ 10 Msun, and size is ~1 REarth. Collapses to only a few kilometers in size! How were neutron stars discovered? The first neutron star was discovered by Bell Burnell in 1967 Using a radio telescope she noticed very regular pulses of radio emission coming from a single part of the sky Pulsars are neutron stars that give off very regular pulses of radiation Pulsar at Pulsars are rotating center of Crab neutron stars that act Nebula pulses like lighthouses 30 times per second Beams of radiation coming from poles look like pulses as they sweep by Earth X-rays Visible light A pulsar’s rotation is not aligned with magnetic poles Why Pulsars must be Neutron Stars Pulsars spin fast because core’s spin Circumference of NS = 2! (radius) ~ 60 km speeds up as it collapses into Spin Rate of Fast Pulsars ~ 1000 cycles per second neutron star Surface Rotation Velocity ~ 60,000 km/s Conservation ~ 20% speed of light of angular ~ escape velocity from NS momentum Anything else would be torn to pieces! Thought Question Thought Question Could there be neutron stars that appear as pulsars to Could there be neutron stars that appear as pulsars to other civilizations but not to us? other civilizations but not to us? A. Yes A. Yes B. No B. No What happens to a neutron star in a close binary system? Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary Accreting matter adds Thought Question angular momentum to a neutron star, According to conservation of angular momentum, increasing its what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell spin onto a neutron star? Episodes of A. The neutron star’s rotation would speed up. fusion on the B. The neutron star’s rotation would slow down. surface lead to C. Nothing, the directions would cancel each other X-ray bursts out. 13.3 Black Holes: Gravity’s Thought Question Ultimate Victory According to conservation of angular momentum, • Our Goals for Learning what would happen if a star orbiting in a • What is a black hole? direction opposite the neutron’s star rotation fell onto a neutron star? • What would it be like to visit a black hole? • Do black holes really exist? A. The neutron star’s rotation would speed up. B. The neutron star’s rotation would slow down. C. Nothing, the directions would cancel each other out. A black hole is an object whose gravity is so What is a black hole? powerful that not even light can escape it. Thought Question Thought Question What happens to the escape velocity from an object What happens to the escape velocity from an object if you shrink it? if you shrink it? A. It increases A. It increases B. It decreases B. It decreases C. It stays the same C. It stays the same Hint: Thought Question Escape Velocity What happens to the escape velocity from an object if you shrink it? Initial Kinetic Final Gravitational = Energy Potential Energy A. It increases B. It decreases (escape velocity)2 G x (mass) = C. It stays the same 2 (radius) Hint: Light would not be able to The “surface” of a black hole is the radius at escape which the escape velocity equals the speed of Earth’s light. surface if you could This spherical surface is known as the event shrink it to horizon. < 1 cm The radius of the event horizon is known as the Schwarzschild radius. A black hole’s No Escape mass strongly warps space and time in vicinity of Nothing can escape from within the event event horizon horizon because nothing can go faster than light. No escape means there is no more contact with something that falls in. It increases the hole mass, changes the spin or charge, but otherwise loses its identity. Neutron Star Limit Beyond the neutron star limit, no known • Quantum mechanics says that neutrons in the force can resist the crush of gravity. same place cannot be in the same state • Neutron degeneracy pressure can no longer As far as we know, gravity crushes all the support a neutron star against gravity if its mass matter into a single point known as a exceeds about 3 Msun singularity. • Some massive star supernovae can make a black hole if enough mass falls onto core Black Hole Thought Question How does the radius of the event horizon change when you add mass to a black hole? 3 MSun Black A. Increases Hole B. Decreases C. Stays the same The event horizon of a 3 MSun black hole is also about as big as a small city Thought Question How does the radius of the event horizon change when you add mass to a black hole? What would it be like to visit a A. Increases B. Decreases black hole? C. Stays the same If the Sun shrank into a black hole, its gravity would be different only near the event horizon Black holes don’t suck! Light waves take extra time to climb out of a deep hole in spacetime leading to a gravitational redshift Thought Question Is it easy or hard to fall into a black hole? A. Easy B. Hard Hint: A black hole with the same mass as the Sun Time passes more slowly near the event horizon wouldn’t be much bigger than a college campus Tidal forces near the event horizon of a 3 MSun black hole would be lethal to humans Tidal forces would be gentler near a supermassive black hole because its radius is much bigger Remember—tidal force is due to the difference in force, which depends on the fractional change in distance!.
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