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Chapter 11: Neutron Stars and Black Holes Neutron Stars Neutron Stars What's Holding It Up? Pulsars: Stellar Beacons the ___

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Chapter 11: Neutron Stars and Black Holes Neutron Stars Neutron Stars What's Holding It Up? Pulsars: Stellar Beacons the ___ Neutron Stars • Form from a 8-20 MSun star • Leftover 1.4 - 3 MSun core after supernova Neutron Star (tennis ball) and Chapter 11: Neutron Stars • Neutron Stars consist entirely Washington D.C. and Black Holes of neutrons (no protons) Neutron Stars What’s holding it up? • About the size of a large city White dwarfs and neutron stars are held up by (5-10 miles), Several times the Degeneracy pressure mass of the Sun • So they are incredibly dense! White Dwarfs and Neutron Stars are made of degenerate • One teaspoon of a neutron matter. star would weigh 100 million Neutron Star (tennis ball) and tons! Washington D.C. Degenerate matter cannot be compressed….the neutrons are already as close as •Held up by degeneracy pressure: the energy Electron possible. neutrons don’t like to be squished close together! Pulsars: Stellar Beacons The _________ Model of Pulsars • Rotating neutron stars A pulsar is a ______ • Strong magnetic field emits a beam neutron star. radio waves along the magnetic poles • These are not aligned with the axis of rotation. • So the beam of radio waves A pulsar’s beam is like a lighthouse sweeps through the sky as the Neutron Star spins. Model of a Pulsar (a rotating Neutron Star) If the beam shines on Earth, then we see a ______ of energy (radio waves) Neutron star’s magnetic field 1 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 > ___ Msun 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 Black Holes: Overview If a White Dwarf gets too heavy it will collapse… into a Neutron Star (this triggers a •A total victory for _______. second type of Supernova explosion) •Collapsed down to a single point. White dwarfs cannot be more •This would mean that they have ______ massive than ____ Msun density Similarly, Neutron stars cannot •Their gravity is so strong, not even ____ be larger than about ___ M Sun can escape! They will collapse completely and turn into a _____ ____! Escape Velocity Why Are Black Holes Black? Escape Velocity (vesc) is the speed required vesc On planets with more gravity than Earth, to escape _______’s pull. Vesc would be _______. On Earth vesc ≈ 11.6 km/s. On a small body like an asteroid, Vesc would be so small you could ____ into If you launch a spaceship space. at v= 11.6 km/s or faster, it will escape the Earth A Black Hole is so massive that Vesc = the _____ __ _____. But vesc depends on the _____ of the planet or star… Not even light can escape it, so it gives off no light! 2 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 The Schwarzschild Radius ________ can get If Vescape > c, then nothing can leave the star, not ____, out once it’s inside not _______. the 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 _________ 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 ______ than your head. You would experience “tidal forces” pushing & pulling ____ is also distorted near a black hole 3 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 • Pulsars: measure its _____. – Observe radio jets – Strong magnetic fields If its mass > __ Msun then it’s a black hole! Evidence for Black Holes: X-rays Evidence for Black Holes Matter falling into a black hole may form an accretion disk. • Cygnus X-1 is a source of X rays As more matter falls on the disk, it heats up and emits ______. • It is a binary star system, with an O type supergiant & a If X-rays are emitted outside the event horizon we can see “_______ ______” them. The mass of the compact object is more than ___ Msun This is too massive to be a white dwarf or neutron star. This object must be a black hole. Artists’ drawings of Cygnus X-1: A black hole accretion disks Supermassive Black Holes Life Cycles of Stars • Low-mass stars: Fade out, stay on Main Sequence • Stellar black holes come • Sun-like stars: White dwarf & planetary nebula from the collapse of a star. • High-mass stars: Supernova -> SN remnant & dense core • They have masses of – Core < 1.4 MSun = _____ ______ several Msun – 1.4 MSun < Core < 3 MSun = _______ _____ – Core > 3 MSun = _____ ____ • Bigger mass = bigger BH! Lifetime Mass • This happens in the center of most galaxies. A supermassive black hole devours a star, releasing X-rays 4 The Milky Way “Milky Way”: A band of ____ and a _______ The band of light we see is Milky Way probably looks like really 100 billion stars Andromeda. Milky Way Milky Way Composite Photo Before the 1920’s, astronomers used a “__________ model” for the galaxy Tried to estimate our location in the galaxy by counting stars in different __________ • ______ in the center • Dark strip in the middle, from _____ Because some stars are _______ by dust, the true shape of this group of stars was unclear. Finding the Center Finding the Center • Harlow Shapely studied Shapely plotted the ________ of the globular star clusters. _______ ________. He found that they are are not centered on the Sun…. • He theorized that they must …but are centered on a point about _______ light years from orbit the true ______ of the the Solar System. galaxy A Globular Cluster 5 The Milky Way Size: The Milky Way is roughly _______ light years across, and about _____ light years thick. Stars: The Milky Way is comprised of over ___ _______ stars! Almost everything visible with the naked eye is inside the Milky Way Parts of Our Galaxy Parts of Our Galaxy Disk: The ____ Resides in the Disk Nuclear Bulge: The dense _______ region Halo: Spherical region surrounding the disk where the _______ ________ live. Milky Way Scales Lecture Questions: Tutorial: Page 123 • Work with a partner or two • Read directions and answer all questions carefully. Take time to understand it now! • Discuss each question and come to a consensus answer you all agree on before moving on to the next question. • How big is the Milky Way? • If you get stuck, ask another group for help. • Where are stars forming (or not forming)? • If you get really stuck, raise your hand and I will come around. • How much mass is in the Milky Way? • What’s going on at the center? 6 Milky Way: A Spiral Galaxy Star Formation in the Milky Way Our galaxy seems to be _______: it has spiral arms • The Disk contains ___, so stars are still forming • These are dense concentrations of _____ and ____. there. (Population I stars) • Stars orbit the ______ _____, pass through the The Halo has very little spiral arms as they go. ___, and no new stars are forming there. • Stars ____ _____ and pile up in the spiral arms, The halo of the galaxy like cars in a traffic jam. is populated by ___ stars. (Population II stars) Measuring Distances Stellar Populations •To map the Milky Way Galaxy, we need to • Pop. I: Newer, disk & spiral arm stars, measure _______ to stars. with _____ percentage heavy elements • Parallax only works for nearby stars (within • Pop. II: Older, bulge and halo stars, with about ____ light years) _____ percentage of heavy elements • Heavy elements (metals): anything that • For more distant stars, we use Standard isn’t H, He, or Li Candles Standard Candles Standard Candles • We can easily measure how bright a star appears (________ magnitude) •If we knew how bright the star really was (its ________ magnitude) then we could calculate its distance. • We need a star whose absolute magnitude is always the same, wherever we observe it. Car Headlights are standard candles: • Such a star is called a “standard candle” We use them to determine the car’s distance 7 Cepheid Variables Measuring Distances with Cepheids . In 1908, astronomer Henrietta Leavitt discovered a new Cepheid stars change in brightness. standard candle using ______ stars They pulsate in a very regular way. .These stars are called _________ Large, bright Cepheids pulsate .They are named for δ Cephei, the _____, while small, dim Cepheids first example of such a star. pulsate _______. If we observe the period of pulsation, we can figure out the absolute magnitude & luminosity. Henrietta Leavitt If we compare this to the apparent magnitude, we find the distance! Delta Cephei The Structure of the Milky Way Mapping the Milky Way By measuring the distances to various parts of the Milky Way Galaxy, we map out its structure The Sun is about _________ out from the center The Milky Way is a _______ ______ Galaxy It has a straight structure at the center called a Bar A modern map of the Milky Way (computer-generated diagram) Measuring the Mass of the Milky Way Mass of the Milky Way We use the Sun’s ______ • The mass of the Milky Way is between ___ billion around the center of the Milky and MSun and ___ billion MSun Way • Stars & Gas we see in the Milky Way can only The greater the mass inside the orbit, the ______ the Sun account for a fraction of the total mass.
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