Neutron Stars. Neutron Stars and Black Holes. The Crab Pulsar. Discovery of Pulsars. The Crab Pulsar. Light curves of the Crab Pulsar.

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1 Chapter 11: Neutron Stars and Black Holes A supernova explosion of an M > 8 M sun star blows away its outer layers. Neutron Stars The central core will collapse into a compact object of ~ a few M sun. Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object. Typical size: R ~ 10 km Mass: M ~ M sun Density: ρ ~ g/cm 3 Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!! Discovery of Pulsars Angular momentum conservation => Collapsing stellar core spins up to periods of ~ a few milliseconds. Magnetic fields are amplified up to B ~ G. (up to times the average magnetic field of the sun) The Crab Pulsar Pulsar wind + jets => Rapidly pulsed (optical and radio) emission from some objects interpreted as spin period of neutron stars Remnant of a supernova observed in A.D The Crab Pulsar Light curves of the Crab Pulsar Visual image X-ray image 1

2 The Lighthouse Model of Pulsars Images of Pulsars and other Neutron Stars A pulsar s magnetic field has a dipole structure, just like Earth. Radiation is emitted mostly along the magnetic poles. The Vela pulsar moving through interstellar space The Crab Nebula and pulsar The Effects of Pulsar Winds Proper Motion of Neutron Stars Some neutron stars are moving rapidly through interstellar space. Pulsars blow off a constant stream (wind) of highenergy particles: pulsar winds This might be a result of anisotropies during the supernova explosion forming the neutron star. Binary Pulsars Neutron Stars in Binary Systems: X-ray binaries Some pulsars form binaries with other neutron stars (or black holes) Radial velocities resulting from the orbital motion lengthen the pulsar period when the pulsar is moving away from Earth Example: Her X-1 2 M sun (F-type) star Star eclipses neutron star and accretion disk periodically and shorten the pulsar period when it is approaching Earth. Neutron star Orbital period = 1.7 days Accretion disk material heats to several million K => X-ray emission 2

3 Compact Objects with Disks and Jets Black holes and neutron stars can be part of a binary system. The X-Ray Burster 4U Several bursting X-ray sources have been observed: Rapid outburst followed by gradual decay Matter gets pulled off from the companion star, forming an accretion disk. => Strong X-ray source! Heats up to a few million K. Optical Ultraviolet Pulsar Planets Some pulsars have planets orbiting around them. Just like in binary pulsars, this can be discovered through variations of the pulsar period. As the planets orbit around the pulsar, they cause it to wobble around, resulting in slight changes of the observed pulsar period. Black Holes Just like white dwarfs (Chandrasekhar limit: 1.4 M sun ), there is a mass limit for neutron stars: Neutron stars can not exist with masses > 3 M sun We know of no mechanism to halt the collapse of a compact object with > 3 M sun. It will collapse into a single point a singularity: => A black hole! Escape Velocity Velocity needed to escape Earth s gravity from the surface: v esc 11.6 km/s. v esc The Schwarzschild Radius => There is a limiting radius where the escape velocity reaches the speed of light, c: Now, gravitational force decreases with distance (~ 1/d 2 ) => Starting out high above the surface => lower escape velocity. If you could compress Earth to a smaller radius => higher escape velocity from the surface. v esc v esc R s = 2GM c 2 G = gravitational constant M = mass R s is called the Schwarzschild radius. V esc = c 3

4 Schwarzschild Radius and Event Horizon No object can travel faster than the speed of light => nothing (not even light) can escape from inside the Schwarzschild radius We have no way of finding out what s happening inside the Schwarzschild radius. Event horizon Black Holes Have No Hair Matter forming a black hole is losing almost all of its properties. black holes are completely determined by 3 quantities: Gravitational Potential The Gravitational Field of a Black Hole mass angular momentum (electric charge) Distance from central mass The gravitational potential (and gravitational attraction force) at the Schwarzschild radius of a black hole becomes infinite. General Relativity Effects Near Black Holes An astronaut descending down towards the event horizon of the black hole will be stretched vertically (tidal effects) and squeezed laterally. General Relativity Effects Near Black Holes (II) Time dilation Clocks starting at 12:00 at each point. After 3 hours (for an observer far away from the black hole): Clocks closer to the black hole run more slowly. Time dilation becomes infinite at the event horizon. Event horizon 4

5 General Relativity Effects Near Black Holes (III) gravitational redshift All wavelengths of emissions from near the event horizon are stretched (redshifted). Frequencies are lowered. Observing Black Holes No light can escape a black hole => Black holes can not be observed directly. If an invisible compact object is part of a binary, we can estimate its mass from the orbital period and radial velocity. Mass > 3 M sun Event horizon => Black hole! Jets of Energy from Compact Objects Compact object with > 3 M sun must be a black hole! Some X-ray binaries show jets perpendicular to the accretion disk Model of the X-Ray Binary SS 433 Optical spectrum shows spectral lines from material in the jet. Two sets of lines: one blue-shifted, one red-shifted Line systems shift back and forth across each other due to jet precession Gamma-Ray Bursts (GRBs) Short (~ a few s), bright bursts of gamma-rays GRB of May 10, 1999: 1 day after the GRB 2 days after the GRB Later discovered with X-ray and optical afterglows lasting several hours a few days Many have now been associated with host galaxies at large (cosmological) distances. Probably related to the deaths of very massive (> 25 M sun ) stars. 5

6 The Milky Way Chapter 12: The Milky Way Galaxy Almost everything we see in the night sky belongs to the Milky Way. We see most of the Milky Way as a faint band of light across the sky. From outside, our Milky Way might very much look like our cosmic neighbor, the Andromeda Galaxy. First Studies of the Galaxy Strategies to explore the structure of our Milky Way I. Select bright objects that you can see throughout the Milky Way and trace their directions and distances. First attempt to unveil the structure of the galaxy by William Herschel (1785), based on optical observations. The shape of the Milky Way was believed to resemble a grindstone, with the sun close to the center II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalog their directions and distances. III. Trace the orbital velocities of objects in different directions relative to our position. Determining the Structure of the Milky Way Measuring Distances: The Cepheid Method Galactic Plane Galactic Center The structure of our Milky Way is hard to determine because: 1) We are inside. 2) Distance measurements are difficult. 3) Our view towards the center is obscured by gas and dust. Instability Strip The more luminous a Cepheid variable, the longer its pulsation period. Observing the period yields a measure of its luminosity and thus its distance! 1

7 The Cepheid Method Exploring the Galaxy Using Clusters of Stars Two types of clusters of stars: 1) Open clusters = young clusters of recently formed stars; within the disk of the Galaxy Allows us to measure the distances to star clusters throughout the Milky Way Open clusters h and χ Persei Globular Cluster M 19 2) Globular clusters = old, centrally concentrated clusters of stars; mostly in a halo around the galaxy Globular Clusters Locating the Center of the Milky Way Globular Cluster M80 Dense clusters of 50,000 a million stars Old (~ 11 billion years), lower-main-sequence stars Approx. 200 globular clusters in our Milky Way Distribution of globular clusters is not centered on the sun, but on a location which is heavily obscured from direct (visual) observation. The Structure of the Milky Way Sun 75,000 light years Disk Nuclear Bulge Halo Open Clusters, O/B Associations Globular Clusters Observing Neutral Hydrogen: The 21-cm (radio) line (I) Electrons in the ground state of neutral hydrogen have slightly different energies, depending on their spin orientation. Equal magnetic Opposite magnetic fields attract => Magnetic field due to proton spin fields repel => Higher energy Lower energy 21-cm line Magnetic field due to electron spin 2

8 Infrared View of the Milky Way Near-infrared image Nuclear bulge Galactic plane Interstellar dust (absorbing optical light) emits mostly infrared. Orbital Motions in the Milky Way (I) Disk stars: Nearly circular orbits in the disk of the galaxy Infrared emission is not strongly absorbed and provides a clear view throughout the Milky Way Far-infrared image Halo stars: Highly elliptical orbits; randomly oriented Orbital Motions in the Milky Way (II) Differential Rotation Sun orbits around galactic center at 220 km/s 1 orbit takes approx. 240 million years. Stars closer to the galactic center orbit faster. Stars farther out orbit more slowly. Mass determination from orbital velocity: The more mass there is inside the orbit, the faster the sun has to orbit around the galactic center. Combined mass: M = billion sun M sun sun The Mass of the Milky Way If all mass was concentrated in the center, Rotation curve would follow a modified version of Kepler s 3rd law. The Mass of the Milky Way (II) Total mass in the disk of the Milky Way: Approx. 200 billion solar masses Rotation Curve = orbital velocity as function of radius. Additional mass in an extended halo: Total: Approx. 1 trillion solar masses Most of the mass is not emitting any radiation: dark matter! 3

9 Stellar Populations Population I: Young stars: metal rich; located in spiral arms and disk Population II: Old stars: metal poor; located in the halo (globular clusters) and nuclear bulge Metal Abundances in the Universe All elements heavier than He are very rare. Linear Scale Logarithmic Scale Metals in Stars Absorption lines almost exclusively from Hydrogen: Population II Many absorption lines also from heavier elements (metals): Population I At the time of formation, the gases forming the Milky Way consisted exclusively of hydrogen and helium. heavier elements ( metals ) were later only produced in stars. => Young stars contain more metals than older stars. The History of the Milky Way The traditional theory: Quasi-spherical gas cloud fragments into smaller pieces, forming the first, metal-poor stars (pop. II); Rotating cloud collapses into a disk-like structure Later populations of stars (pop. I) are restricted to the disk of the galaxy Modifications of the Traditional Theory Ages of stellar population may pose a problem to the traditional theory of the history of the Milky Way. Possible solution: Later accumulation of gas, possibly due to mergers with smaller galaxies. Recently discovered ring of stars around the Milky Way may be the remnant of such a merger. Exploring the structure of the Milky Way with O/B Associations O/B Associations Orion-Cygnus arm Sagittarius arm Sun Perseus arm O/B Associations trace out 3 spiral arms near the sun. Distances to O/B Associations determined using Cepheid variables 4

10 Radio Observations 21-cm radio observations reveal the distribution of neutral hydrogen throughout the galaxy. Sun Distances to hydrogen clouds determined through radial-velocity measurements (Doppler effect!) The Structure of the Milky Way Revealed Distribution of stars and neutral hydrogen Distribution of dust Sun Galactic center Neutral hydrogen concentrated in spiral arms Bar Ring Star Formation in Spiral Arms (I) Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger star formation. Spiral arms are stationary shock waves, initiating star formation. Star Formation in Spiral Arms (II) Spiral arms are basically stationary shock waves. Stars and gas clouds orbit around the galactic center and cross spiral arms. Shocks initiate star formation. Star formation selfsustaining through O/B ionization fronts and supernova shock waves. The Nature of Spiral Arms Chance coincidence of small spiral galaxy in front of a large background galaxy Spiral arms appear bright (newly formed, massive stars!) against the dark sky background, but dark (gas and dust in dense, star-forming clouds) against the bright background of the large galaxy Self-Sustained Star Formation in Spiral Arms Star forming regions get elongated due to differential rotation. Star formation is self-sustaining due to ionization fronts and supernova shocks. 5

11 Grand-Design Spiral Galaxies The Whirlpool Galaxy Grand-design spirals have two dominant spiral arms. Flocculent (woolly) galaxies also have spiral patterns, but no dominant pair of spiral arms. Grand-design galaxy M 51 (Whirlpool Galaxy): M 100 NGC 300 Self-sustaining star forming regions along spiral arm patterns are clearly visible. The Galactic Center (I) Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust: Extinction by 30 magnitudes Only 1 out of optical photons makes its way from the GC towards Earth! Radio View of the Galactic Center Many supernova remnants; shells and filaments Arc galactic center Sgr A Sgr A Sgr A*: The center of our galaxy Wide-angle optical view of the GC region The galactic center contains a supermassive black hole of approx. 2.6 million solar masses. Measuring the Mass of the Black Hole in the Center of the Milky Way By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to be ~ 2.6 million solar masses. X-Ray View of the Galactic Center Galactic center region contains many black-hole and neutron-star X-ray binaries. Supermassive black hole in the galactic center is unusually faint in X rays, compared to those in other galaxies. Chandra X ray image of Sgr A* 6