Published on November 28, 2007
Why is Black Hole Black?: Why is Black Hole Black? Because even light can not escape the black hole! Escape Velocity The minimum velocity required to leave an object’s surface and never returns G: Newton’s gravitational constant M: mass of the object R: radius of the object Escape Velocity: Escape Velocity Light in a Black Hole: Light in a Black Hole Escape velocity increases as Mass is higher or size is smaller No light can escape Black Hole! R(black hole) = 2GM/c2 “Size” of a Black Hole: “Size” of a Black Hole Spacetime is so highly warped around a black hole, even light can not escape. Schwarzchild Radius – the radius where the escape velocity equals the speed of light. R = 2GM/c2 Rs = 3 M (Rs in km; M in M) A sphere of radius Rs around the black hole is called the event horizon. Finding Black Holes: Finding Black Holes We can see the effect that a black hole has on its stellar companion in an binary: Cygnus X-1 was the first good candidate for a black hole - a blue supergiant that is rotating rapidly around an unseen companion Kepler’s 3rd Law gives a mass > 3 M for unseen companion it can not be a neutron star the only thing that massive, yet small enough to be invisible is a black hole We will discuss supermassive BH later in the class Slide7: What have we learned? What is a black hole? A place where gravity has crushed matter into oblivion, creating a true hole in the universe from which nothing can ever escape. What property of a black hole determines its “size”? A black hole’s “size” depends on its mass, because the mass determines the size of the black hole’s event horizon, the boundary of the region from which not even light can escape. What observational evidence is there for the existence of black holes? We cannot see black holes directly, but we can infer their presence by their influence on their surroundings. The most definitive evidence is obtained by measuring the orbit of the companion. If that object is more massive than 3 MSun, it is probably a black hole. Cygnus X-1 is one of the binary systems thought to contain a black hole Lecture 11. Our Galaxy: Lecture 11. Our Galaxy Sep 25, 2007 Chap 19.1, 19.2 The Milky Way Revealed : The Milky Way Revealed Describe the general structure of the Milky Way Galaxy. Where is the Sun located within our Galaxy? What are the basic components of the Milky Way Galaxy? Is there a Black Hole in the Galactic Center Our goals for learning: Regions of the Milky Way Galaxy: Regions of the Milky Way Galaxy diameter of disk = 100,000 l.y. (30,000 pc) radius of disk = 50,000 l.y. (15,000 pc) number of stars = 200 billion thickness of disk = 1,000 l.y. (300 pc) Sun is in disk, 28,000 l.y. out from center Distance: 1 parsec (pc) = 3.3 light year(ly) Regions of the Milky Way Galaxy: Regions of the Milky Way Galaxy Disk younger generation of stars contains gas and dust location of the open clusters Where spiral arms are located Bulge mixture of both young and old stars Halo older generation of stars contains no gas or dust location of the globular clusters The matter in our Galaxy emits different kinds of radiation. They come from matter in different phases…: The matter in our Galaxy emits different kinds of radiation. They come from matter in different phases… What is the Milky Way Galaxy made of?: What is the Milky Way Galaxy made of? Stars 200 billion stars Age: from >10 billion years to just formed Many stars are located in star clusters Interstellar Medium Stuff between stars Nebulae, molecular clouds, and mostly empty space in between Where stars are born from, and where diffuse remnants of stars are stored Galactic Center – with a supermassive Black Hole? Dark Matter The total mass of the Milky Way far exceeds the mass with stars and interstellar medium put together Hidden, invisible or missing component Stars: Halo vs. Disk: Stars: Halo vs. Disk Stars in the disk are relatively young. fraction of heavy elements same as or greater than the Sun plenty of high- and low-mass stars, blue and red Stars in the halo are old. fraction of heavy elements much less than the Sun mostly low-mass, red stars Stars in the halo must have formed early in the Milky Way Galaxy’s history. they formed at a time when few heavy elements existed there is no ISM in the halo star formation stopped long ago in the halo when all the gas flattened into the disk Star Clusters: Star Clusters Thousands to millions of stars grouping together Stars in a cluster have the same age, but different masses Most stars were born and once belong to a cluster Two types of star clusters Open cluster: young, sparse, has fewer stars, in disk and spiral arms Globular cluster: old dense, has many stars, in Galactic halo or bulge. Slide17: Open cluster: Globular Cluster The Interstellar Medium (ISM): The Interstellar Medium (ISM) It is the “stuff” between the stars. It is mostly a vacuum (1 atom cm-3). It is composed of 90% gas and 10% dust. gas: individual atoms and molecules dust: large grains made of heavier elements ISM is the place where stars were born; and the place where stars eject most of their matter when dying. The different components of the ISM: The different components of the ISM Ionization Nebulae : Ionization Nebulae Found around high-mass stars (OB associations) O & B stars (T > 25,000K) make enough UV photons to ionize hydrogen in the nebula Gas re-emits Hydrogen line (red) Reflection Nebulae: Reflection Nebulae Light from central star is reflected and scattered by dust Blue light is scattered more easily than red Similar to our blue sky lit up by a yellow Sun The Star–Gas–Star Cycle: The Star–Gas–Star Cycle The Star–Gas–Star Cycle:Pillars of Creation: The Star–Gas–Star Cycle: Pillars of Creation Eagle Nebula’s “Pillars of Creation” Supernova Remnants :Tombs of Massive Stars: Supernova Remnants : Tombs of Massive Stars Crab Nebula: Remnant of a supernova Exploded in 1054 and recorded By ancient Chinese. Supernova 1987A Exploded in 1987 in the Large Magellenic Cloud Observed with HST Center of the Galaxyin Sagittarius: Center of the Galaxy in Sagittarius Visual Infrared Center of the Galaxy: Center of the Galaxy Radio X-ray Although dark in visual light, there are bright radio, IR, and X-ray sources at the center of the Galaxy, known as Sgr A*. Slide27: Gemini 8-m telescope image of the Galactic Center Adaptive Optics:Overcoming the Turbulence of the Atmosphere: Adaptive Optics: Overcoming the Turbulence of the Atmosphere Motion of Stars near the Black Hole: : Motion of Stars near the Black Hole: Center of the Galaxy: Center of the Galaxy We measure the orbits of fast-moving stars near the Galactic center. these measurements must be made in the infrared in particular, this star passed within 1 light-day of Sgr A* using Kepler’s Law, we infer a mass of 2.6 million M for Sgr A* What can be so small, yet be so massive? – Black Hole! This is the best direct evidence for a SUPERMASSIVE black hole Radius of Black Hole ~ 3 x Mass ~ 8 million km Caught in Action: matter falls into Black Hole: Caught in Action: matter falls into Black Hole Slide33: How long does it take the Sun to orbit the galactic center? How do we determine Galactic mass from stellar orbits? What is the significance of a rotation curve that is flat at large distances from the Galactic center? Our goals for learning: Dark Matter in the Milky Way Galaxy Stellar Orbits in the Galaxy: Stellar Orbits in the Galaxy Stars in the disk all orbit the Galactic center: in the same direction in the same plane (like planets do) almost circular orbit they “bobble” up and down this is due to gravitational pull from the disk this gives the disk its thickness Stellar Orbits in the Galaxy: Stellar Orbits in the Galaxy Stars in the bulge and halo all orbit the Galactic center: in different directions at various inclinations to the disk They have highly eccentric orbits Their motions are similar to comets in the solar system Mass of the Galaxy: Mass of the Galaxy We can use Kepler’s Third Law to estimate the mass Sun’s distance from center: 28,000 l.y. = 1.75 x 109 AU Sun’s orbital period: 230 million years (2.3 x 108 yr) P2 = 42/GM a3 mass within Sun’s orbit is 1011 M Total mass of MW Galaxy : 1012 M Total number of stars in MW Galaxy 2 x 1011 Orbital Velocities in the Disk: Orbital Velocities in the Disk rotation curve – a plot of rotational (orbital) speed vs. distance from the center for a merry-go-round (solid object) for our Solar System (Kepler’s Laws) How to use rotation to measure mass?: How to use rotation to measure mass? Orbital Velocities in the Disk: Orbital Velocities in the Disk the rotation curve of the Milky Way galaxy is FLAT: It is small in the center It increases sharply near the center when moving outward Then it becomes flat a flat rotation curve at large radius We can measure the rotation curve of our Milky Way Galaxy Here is what we observe: Mass in the Milky Way Galaxy: Mass in the Milky Way Galaxy the rotation curve of the Milky Way galaxy is FLAT: The total mass of the Milky Way is very large There is a lot of mass at large distance from the Galactic Center But we observe few stars out there… what’s going on? Stars in the Galactic disk should orbit according to Kepler’s Laws Here is how we measure the mass Importance of the Flat Rotation Curve: Importance of the Flat Rotation Curve The flat rotation curve of our Galaxy implies that: its mass in not concentrated in the center its mass extends far out into the halo Do these all make sense? Total mass of MW Galaxy : 1012 M Total number of stars in MW Galaxy 2 x 1011 Most mass in the Milky Way are not in stars, missing!!! We do not “see” this mass we do not detect light from most of this mass so we refer to it as dark matter This is the first time in this class that we are going to see the evidence for dark matter, more later…. Slide42: What have we learned? Describe the general structure of the Milky Way Galaxy. The Milky Way Galaxy consists of a thin disk about 100,000 light-years in diameter with a central bulge. The spherical region surrounding the entire disk is called the halo. Where is the Sun located within our galaxy? The Sun is located in the disk, about 28,000 light years from the galactic center. Can we see through our galaxy’s interstellar medium? The gas and dust that make up the interstellar medium absorb visible light, preventing us from seeing most of the galaxy’s disk in these wavelengths. However, some other wavelengths of light, notably infrared and radio, can pass through the gas and dust, allowing us to study regions of the galaxy whose visible light is blocked. Slide43: What have we learned? How do halo stars differ from disk stars? The halo generally contains only old, low-mass stars, while the disk is home to stars of all ages. In addition, halo stars have a much smaller proportion of heavy elements than stars in the disk. Slide44: What have we learned? How do stellar orbits in the disk differ from those in the halo? Stars in the disk all orbit the galactic center in about the same plane and in the same direction. Halo stars also orbit the center of the galaxy, but with orbits randomly inclined to the disk of the galaxy. How long does it take the Sun to orbit the galactic center? Each orbit takes about 230 million years. How do we determine galactic mass from stellar orbits? By using a star’s orbital speed and distance from the galactic center in the orbital velocity law (Mr = (r v2)/G), we can calculate the mass of the galaxy that lies within the region enclosed by the star’s orbit. Slide45: What have we learned? What is the significance of a rotation curve that is flat at large distances from the galactic center? The Milky Way’s flat rotation curve implies that the matter associated with our galaxy extends to large distances from the center. A rotation curve is a plot of the orbital speed of stars or gas clouds against distance from the center of the galaxy. If most of the galaxy’s mass were concentrated toward the center, orbital speed would decline as distance from the center increased, as in the solar system. Because the rotation curve of the Milky Way is flat, orbital speed in the Milky Way does not decline at great distances. Thus, the Milky Way’s mass is not concentrated toward the center but instead extends far into the halo. Because we do not detect light from all this mass in the halo, we call it dark matter. Slide46: What have we learned? What lies in the center of our galaxy? Motions of stars near the center of our galaxy suggest that it contains a black hole about 2.6 million times more massive than the Sun. The black hole appears to be powering a bright source of radio emission known as Sgr A*.