I. Discovery of Galaxies -- Our galaxy, the Milky Way, can be seen as a swath of white in the sky on very dark nights. This is what it can look like in Argentina, where you can also see the satellite galaxies called the Large and Small Magellanic clouds. There's also a comet visible in this striking photo. -- Little did the early astronomers know, the stars are not evenly spread, but are clumped in galaxies, and there are a lot more than just our own Milky Way! -- Edwin Hubble, in 1923, found the distance to the Andromeda galaxy using the period/luminosity relationship of Cepheid variables. He got a distance in excess of a million light years, which proved it was well outside our galaxy. It must be its own galaxy! -- Many more galaxies were later discovered, and it was found that the Milky Way is one of a small cluster of galaxies that include Andromeda and the Large and Small Magellanic Clouds, as well as the Sagittarius cluster.
II. Spirals and Ellipticals -- There are two basic types, spiral and elliptical. Spirals have a lot of angular momentum, are very flattened, and still have lots of gas and dust and star-forming regions. Ellipticals have little angular momentum because stars are orbiting in both directions, they are not flat, and have very little gas or dust or star formation. -- Spirals have stars of all ages, since some are still forming. Ellipticals have only old stars, since no new stars are being formed. -- Spirals are most common, as seen in the Hubble deep field. Or see pieces of the Hubble deep field survey of distant galaxies in more detail here. -- There are also oddballs called irregular galaxies, often formed when two more normal galaxies interact (we say they "collide", but the stars don't really hit each other, they are too far apart. The interaction is all due to gravity.) An example is the Antennae galaxy . -- You can see a basic summary of galaxy shapes here.
III. Galaxy Formation -- different galaxies can in some cases represent a kind of evolutionary sequence in how galaxies can change over time, but more often they simply represent different ways the galaxies formed. -- spirals form in a manner similar to the way the solar nebula collapsed into our solar system, whereas ellipticals form in a complicated fashion that actually transports angular momentum from the stars to the invisible dark matter. -- collisions between galaxies are frequent, and have dramatic consequences for the structure of the galaxy, although no stars actually collide.
IV. The Distance Scale and the Hubble Expansion -- V. M. Slipher first observed galactic redshifts in 1917, but incorrectly interpreted the overall motion of galaxies as being due to the special motion of the Sun. -- In 1923, Hubble picked up the ball and found that galaxies are generally redshifted in all directions, and he concluded the universe was expanding. -- the expansion law was linear: v = H * d, where H is the Hubble constant. -- note that a linear law means everywhere looks like it is at the center, like ants walking on a balloon that is being inflated. Also, it is the kind of law you'd get in an explosion, if different particles were moving at different speeds. The faster moving particles move farther. -- if you look at the units of 1/H, they can be converted into a time. What is the meaning of that time? It is the time it would take all the galaxies to come together if we ran time backward. So it is the age of the universe!
V. Quasars -- no, these are not TV sets, these are quasi-stars. They look like point sources of light, like stars, but their spectrum is very flat and the atomic lines are all highly redshifted. -- if the redshift is converted to a speed and we refer to the Hubble Law, we find that quasars are extremely far away, so the time it takes their light to get to us can be an appreciable fraction of the age of the universe! -- Quasars are real luminous. I mean, real. Each one can have a luminosity that is a million billion times the Sun. Good thing too-- or we'd never be able to see them halfway across the universe. -- If the light takes that long to get to us, it means we are seeing quasars as they were at an early stage in the universe's life. So it isn't so much that they are far away, it's that they are long ago. They just don't exist anymore. It's useful to think about this by using a space-time graph. -- The peak of the quasar distribution comes at redshifts that correspond to z=5 (where z = the shift in wavelength divided by the normal wavelength). This corresponds to when the universe was only 1/6 of its current age. See a z=5 quasar here. See how red it looks? You're looking at the expanding universe! -- So what are they, and what happened to them? It appears they are entire galaxies, in a stage where they are emitting a tremendous amount of light over a very wide range in wavelengths. As such, they are called "active galaxies". -- What provides the energy? Well, what is our favorite source of energy in the universe? Gravitational energy. Sure, fusion is real good at keeping stars glowing, but when you need real big amounts of energy, like in supernova explosions, you look for lots of mass falling into a real small space. The best way to get that is to have mass fall onto a black hole. So if active galaxies had supermassive black holes at their centers, and if they were sucking in gas at a high rate, this would release an enormous amount of gravitational energy before the mass disappeared into the black hole. Ironically, black holes are thus very good at being the cause of bright lights! -- Where are these galaxies now? Look around! They are just not active any more because the black holes have already sucked in all the gas that wasn't able to be in orbit. We are just left with the high-angular-momentum stars that have managed to avoid the central crunch.
VI. Quasars as Searchlights -- Quasars are so bright that they can be seen at great distance. This makes them extremely useful as a probe of not only what the universe was like a long time ago, but also everything in between us and the quasar. -- there are two very important things that can happen to quasar light before it gets to Earth. One is that it can be scattered by intervening material, especially at particular wavelengths where hydrogen atoms will absorb light. The second is that the light can be bent by gravity, and this can produce an effect called gravitational lensing. This alters the light from a distant quasar, much in the way that lights can look strange coming through the pane of an irregular bathroom window. A famous gravitational lens is called the Einstein cross. -- The strongest absorption line in the universe is the Lyman alpha line of hydrogen, which absorbs light at a wavelength of .00000012 m. This falls in the ultraviolet part of the spectrum if the atom is not moving, but if the hydrogen gas is moving away from us, it can be redshifted into the visible. What results is a "Lyman alpha forest" of absorbtion by clouds of hydrogen gas moving at different speeds in the Hubble Law. -- Gravitational lenses can be used to map out the mass density of clusters of galaxies. If the light from a distant quasar tries to pass near a cluster, it gets bent a little, just like when light passes from air into glass. With the right geometry, this can produce a lens, and give ring-like echoes of the original source.
VII. Quasars as Radio Sources -- quasars were first found by looking with large radio telescopes. Thought at first to be stars, they later turned out to be the centers of very active galaxies. -- many quasars are very bright radio sources, due to the existence of "jets" of material shooting upward and downward from out of the plane of the accretion disk (the disk of material falling into the central black hole). These jets move at speeds close to the speed of light, so are very energetic. Add magnetic fields and you get a good source of radio emission due to the way electrons revolve around magnetic field lines (called "synchrotron" emission, there's a tongue twister!). You can see what the jets, and the lobes of radio emission they create, look like here. -- the jets are so fast that when they point toward us, the radio appears much brighter (it gets "stacked up", due to both the Doppler effect and the way relativity monkeys with space and time). Also, it can look like a blob is moving faster than the speed of light, although this "superluminal" motion is only an optical illusion due to the finite speed of light.
Week 13 notes
