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The history of the Galaxy got a little muddled, for a number of reasons: partly because those who are trying to keep track of it have got a little muddled, but also because some very muddling things have been happening anyway. From “Mostly Harmless”, by Douglas Adams HW – for substitute grade only. Due Dec 9 Ch. 22 RQ 6, P 1 Ch. 20 DQ 6 What is creationism (in 1-4 sentences)? What is the theory of evolution (in 1-4 sentences)? What is the evidence for evolution? X-ray image of Cygnus X-1 from NASA’s Marshall Flight Center. Escape Velocity Let’s re-think Newton’s experiment. If you launch something from Earth with a high enough velocity, it goes into orbit. If the velocity is increased further it can escape. The escape velocity depends of the mass and radius of Earth. Schwartzchild Radius What if vesc = c? (c=300,000 km/s) This happens at a distance from mass M: R = 2GM/c2, known as the Schwartzschild radius. Both matter and light within this distance to a black hole (inside the Schwartzschild radius) can not escape. Black Holes in Binary Systems The most straightforward way to search for a black hole is to Kepler’s third law. The best place to apply this technique is an x-ray binary. In these systems one of the stars is seen in visible light and the other is a copious source of x-rays. The x-rays show the position of the (possible) black hole. How do x-rays escape from a black hole? They don’t. The x-rays are emitted by matter from the visible star that falls into the black hole accelerating to velocities near the speed of light as it falls. If we can determine the orbital period of the binary system, we can then use Kepler’s 3rd law to calculate the mass. If the mass of the unseen companion is large, this and the presence of x-rays suggest that it is a black hole. Currently, the best candidate is Cygnus X-1. Super Massive Black Holes An x-ray image of the center of the Milky Way The center of our galaxy is also a copious source of x-rays and appears to be extremely massive. Stars in the Milky Way orbit around an unseen central object. Analysis of the orbital velocities of the stars about the center of the galaxy (using Kepler’s 3rd law) imply a mass of 2.6106 solar masses inside a volume 0.03 light years in diameter. It is impossible to pack stars together that tightly – they would collide, destroying each other very quickly. It is likely that the object at the center of our galaxy is a super massive black hole. The same is believed to be true of many other galaxies. Summary • Stars die by expelling catastrophically the outer layers. The inner layers contract to a very dense amber. • Massive stars (8 x Ms) explode into supernova, while solar-type stars explode as less energetic planetary nebula. • The remnant of the Sun will be a white dwarf, supported by electron degeneracy. • The remnant of a massive star is a neutron star, supported by neutron degeneracy. • A stellar core more than 3 Ms has enough gravity to overwhelm the neutron degeneracy pressure. No known force can support gravity and collapse continues. The result: a black hole. • The Schwarzschild radius is the distance from a black hole within which even light can not escape. Cosmology There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened. Douglas Adams Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. Albert Einstein The Large-Scale Structure of the Universe is Dominated by Galaxies A Spiral Galaxy An Elliptical Galaxy The Milky Way • • • • The Milky Way is a spiral galaxy. The Milky Way is flat and appears as a bright band in the night sky. It contains ~100 billion (1011) stars. There are ~100 billion galaxies in the universe. Olbers’s Paradox • Why is the sky dark at night? • If the universe were infinite and the density of galaxies was the same everywhere in the universe, then there should be a star in every single direction: the sky should be bright at night, but it is not. Resolution • Distant stars are dim, so we receive little light from them. – Wrong. Though we receive less light from distant stars there are also more distant stars and this makes up for the dimming with distance. • There’s invisible dust between us and the distant stars – Wrong. The dust would eventually heat up and emit its own radiation. We don’t see this. • The universe has a finite age and size. – Right. These two concepts are closely related. When we look to great distances in the universe we are looking far back in time. If we look to distances so great that we are seeing times before the formation of stars, then the sky in that direction is dark. The force of gravity is always attractive. Why then doesn’t the universe collapse under it’s own gravitational attraction? Is the Universe Stable? • The force of gravity is always attractive. Why then doesn’t the universe collapse under it’s own gravitational attraction? • Newton said it was an act of God (Give him credit though, no one else realized that there was a problem). • Einstein realized the same problem occurred with his theory of gravity and, in what he called the biggest blunder of his life, fudged the equations so that the universe would be stable. • Despite these deep-seated prejudices from mankind’s two greatest luminaries, the answer is simple: the universe is not stable, it is expanding. Doppler Shifts A stationary (with respect to an observer) light source emits radiation with a wavelength (the green wave). If that same source is moving towards the observer, the waves get compressed and the wavelength is less than (the blue wave). If the source is moving away from the observer, the waves are stretched out and the wavelength is greater than (the red wave). Light from approaching objects is called blue shifted and light from receding objects is called red shifted, because blue and are on the shortwavelength and long-wavelength ends of the visible spectrum. The light is not necessarily blue or red. We can tell if light is blue or red shifted by examining the characteristic spectral lines of the elements. In the spectrum shown above the entire pattern of spectral lines is shifted to either the blue or red. Because we can recognize the pattern, we can identify the spectral lines and therefore know what the un-shifted wavelengths should be. Hubble, Galaxies and Red Shifts In 1925, Hubble accumulated radial velocities for 40 galaxies. Shown to the right are images of galaxies and their spectra. The distance of the galaxies from Earth in millions of light years (Mly) is listed under the images and the velocity deduced from the Doppler shift is listed under the spectra. The bright bands above and below the spectra are used for calibration. The two dark bands in central stripe are used to measure the Doppler shift. The size of the Doppler shift is indicated by the red arrow. The more distant galaxies show larger Doppler shifts. 63 Mly 990 Mly 1440 Mly 2740 Mly 3960 Mly The Relationship Between Distance and Velocity Mpc stands for Mega Parsec. 1 Mpc = 3.3 million light years Hubble’s Law Suggests that Galaxies were once much closer together Imagine that all the galaxies were once much closer together and had a spread of velocities – some were moving fast and some slow. After a period of time (billions of years) the fast moving galaxies would be very far away, but the slow moving galaxies not so far away. The faster a galaxy was moving, the further away it would be. This is just what Hubble measured. Thus, it seems that galaxies were once much closer together. Notice that, no matter where you are, everything seems to be moving away. Balloons and Raisin Bread As the bread rises the distance between raisins increases in a more or less uniform manner. It would look the same no matter what raisin you were sitting on. Think of the raisins as galaxies. A better analogy is the expanding balloon. As the balloon expands the dots on the balloon get further apart; however, there is no center to the surface of the balloon and the expansion would look the same no matter where you were. General Relativity & the Big Bang The most beautiful thing that we can experience is the mysterious. It is the source of all true art and science. - Albert Einstein Space is curved Forget forces. Planets travel in orbits, following the curvature of space. Cosmic Microwave Background Arno Penzias and Robert Wilson were trying to make observations of radio emissions from a distant supernova and then hoped to make a map of radio emissions from the Milky Way. They adapted a radio dish previously used for communication satellites. They were startled to find that no matter where they pointed the antenna, they measured the same low-level radio signal. After great efforts to determine that there was nothing wrong with the antenna (e.g. cleaning the antenna of a “thin white dielectric film” left by pigeons), they concluded that the signals were real and a property of the universe. Unknown to Penzias and Wilson, a Russian Astrophysicist, George Gamov, had predicted the existence of these radio signals as a consequence of something we now call the Big Bang. Chance favors only the mind that is prepared - Louis Pasteur COBE Measurements Data & prediction of emission from a 2.726 K body. Why microwave radiation? Why isotropic? Why 2.726 K? Explanation 1 Gas gets hot when it is compressed and to cool when it expands. The same is true for the Universe. The early Universe was a mixture of matter and radiation. Shortly after the Big Bang, the universe was tightly compressed, and thus extremely hot. Its radiation was typical of that for warm bodies (i.e. it obeyed Wein’s law). As the universe expanded both the matter and radiation cooled. In fact, the radiation cooled from unimaginably high temperatures to 2.726 K, the temperature of the universe today. Explanation 2 We get the same answer by considering that when we look very far away we are seeing diffuse radiation from the hot big bang. However, this radiation comes from great distances (10-20 billion light years) and has been Doppler shifted to very long wavelengths. In fact, it has been Doppler shifted all the way from gamma rays to radio waves; the radio waves discovered by Penzias and Wilson. Evidence for the Big Bang • The universe is expanding at a rapid rate and seems to have been doing so since its creation. • The Cosmic Background Radiation can be explained as the afterglow of the Big Bang. • The cosmic abundances of hydrogen, deuterium, and helium are consistent with expectations based on synthesis by nuclear reactions in the Big Bang. The Big Bang solves the problem of the stability of the universe: the force of gravity does, in fact, pull the universe together; however, it is not collapsing because it is still flying apart at high speeds. This raises the question, will the universe continue to fly apart or is gravity strong enough to stop the expansion and pull all the pieces back together in a Big Crunch? Put another way, is the escape velocity of the universe greater or lesser than the rate at which it is expanding? The collision of 2 galaxies. Fate of the Universe 1) The amount of luminous matter in the universe appears to be too small to stop its expansion. This might imply that the universe would continue expanding forever, except that there seems to be a “dark matter” in the Universe. If there is enough dark matter, the universe could be “closed,” i.e. it may someday stop expanding and start contracting. 2) Very careful studies of the velocities of galaxies should be able to reveal if the rate of expansion is slowing down. However, the latest studies found a big surprise: the rate of expansion is increasing! If true, the universe will probably expand forever, but the results are only a couple of years old and deserve more careful scrutiny before firm conclusions are adopted. Evidence for Dark Matter Stars far from the galactic center obey Kepler’s 3rd law, but they indicate the presence of far more mass in the galaxy than expected based on visible light, hence the term “dark matter.” Is the Expansion Speeding Up? Summary • The solution to Olbers’s paradox is that the night sky is dark because the universe is a finite age. • The universe is expanding from a primordial creation event 10-20 billion years ago. • The universe is filled with thermal radiation at a temperature of 3 K that is the modern residue of the primordial fireball. • There is far more mass in the universe than can be seen. The nature of this “dark matter” is unknown. • We’re not sure if the universe will keep expanding forever or if gravity will cause it to contract to a Big Crunch. The latest evidence favors expansion forever.