Neutron Stars Form from a 8-20 M Sun star Chapter 11: Neutron Stars and Black Holes Leftover 1.4-3 M Sun core after supernova Neutron Stars consist entirely of neutrons (no protons) Neutron Star (tennis ball) and Washington D.C. Neutron Stars About the size of a large city (5-10 miles), Several times the mass of the Sun So they are incredibly dense! One teaspoon of a neutron star would weigh 100 million 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 Degeneracy pressure Electron energy White Dwarfs and Neutron Stars are made of degenerate matter. Degenerate matter cannot be compressed.the neutrons are already as close as possible. Rotating neutron stars Pulsars: Stellar Beacons Strong magnetic 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) 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 > 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! 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 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 3
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 4
The Milky Way Milky Way : A band of and a The band of light we see is really 100 billion stars Milky Way probably looks like Andromeda. Milky Way Composite Photo Milky Way 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 Harlow Shapely studied. He theorized that they must orbit the true of the galaxy Finding the Center Shapely plotted the of the globular star clusters. He found that they are are not centered on the Sun. but are centered on a point about light years from the Solar System. 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. Questions: Milky Way Scales Lecture 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. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. How big is the Milky Way? Where are stars forming (or not forming)? How much mass is in the Milky Way? What s going on at the center? 6
Milky Way: A Spiral Galaxy Our galaxy seems to be : it has spiral arms These are dense concentrations of and. Stars orbit the, pass through the spiral arms as they go. Stars and pile up in the spiral arms, like cars in a traffic jam. Star Formation in the Milky Way The Disk contains, so stars are still forming there. (Population I stars) The Halo has very little, and no new stars are forming there. The halo of the galaxy is populated by stars. (Population II stars) Stellar Populations Pop. I: Newer, disk & spiral arm stars, with percentage heavy elements Pop. II: Older, bulge and halo stars, with percentage of heavy elements Heavy elements (metals): anything that isn t H, He, or Li Measuring Distances To map the Milky Way Galaxy, we need to measure to stars. Parallax only works for nearby stars (within about light years) For more distant stars, we use Standard 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: We use them to determine the car s distance Such a star is called a standard candle 7
Cepheid Variables In 1908, astronomer Henrietta Leavitt discovered a new standard candle using stars These stars are called They are named for δ Cephei, the first example of such a star. Measuring Distances with Cepheids Cepheid stars change in brightness. They pulsate in a very regular way. Large, bright Cepheids pulsate, while small, dim Cepheids pulsate. Delta Cephei Henrietta Leavitt If we observe the period of pulsation, we can figure out the absolute magnitude & luminosity. If we compare this to the apparent magnitude, we find the distance! The Structure of the Milky Way By measuring the distances to various parts of the Milky Way Galaxy, we map out its structure Mapping the Milky Way 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 We use the Sun s around the center of the Milky Way The greater the mass inside the orbit, the the Sun has move around the center. This way we can measure the mass of the Milky Way. Mass of the Milky Way The mass of the Milky Way is between billion and M Sun and billion M Sun Stars & Gas we see in the Milky Way can only account for a fraction of the total mass. -What is it? - Why can t we see it? Total mass: about M Sun 8
The Center of the Milky Way The Center of Our Galaxy The of stars in the Galactic Center is much greater than in the Sun s neighborhood. They appear to be orbiting a black hole at the center of the galaxy Its mass is over M Sun! Chapter 13 We now realize that our galaxy is only one of billions of galaxies we can see. These galaxies come in three main types: Galaxies Spiral, & Spiral Galaxies Typically very bright, in color Look like (sometimes with ) M 100 NGC 300 9
Elliptical Galaxies are, not flat like spirals They are typically in color. Less gas and dust than spirals. Irregular Galaxies Lack any distinct shape Are generally than spirals and ellipticals Hubble Tuning Fork Galaxies (S): Classified according to spiral arms (a,b,c) and presence of a bar ( B ) Galaxies (E): Classified according to shape (E0-E9) Galaxies (Irr): Basically anything funkylooking! 10