The variety of galaxies		Galaxies come in great variety. If the Hubble Deep Field is a good guide, there may be as many as 80 billion galaxies in our Universe! The most common types of galaxies are spirals, elliptical galaxies, and irregular galaxies. About 75% of large galaxies are spiral. Most of the very small galaxies are elliptical, but most of the very largest are also elliptical. (In the centers of galaxy superclusters one sometimes finds the very largest elliptical galaxies, called cD galaxies, for central dominant galaxies.) It's thought that the very largest galaxies are elliptical because they form from the merger of spiral galaxies. When I say "elliptical" galaxies, I really mean "ellipsoidal." If you take an ellipse and rotate it abou an axis, you will get the shape of an elliptical galaxy. Elliptical galaxies have less active star formation than spiral galaxies. In that respect, they're more like the bulges of spiral galaxies: old red stars. Intermediate between spirals and ellipticals are lenticular galaxies. These galaxies have a disk-like shape with a bulge, but no spiral arms. They could possibly result from a collision where two spiral galaxies pass through each other, the stars passing through without colliding, but the gas could get separated from the galaxies. Spiral galaxies can be with or without a central bar. It's thought that our Milky Way has a bar. Hubble came up with a classification system for galaxies. It was meant to suggest the way galaxies evolved, but we now know that galaxies don't evolve along the path of his model. The classification system is still used: elliptical galaxies are labelled according to how elliptical they are (how eccentric), from E0 (spherical) to E7 (drawn out.) Spirals are labelled Sa if the bulge is large and the arms wound closely. Sb spirals have smaller bulges relative to arms that spread out more. Finally, Sc spirals have the smallest bulges and most spread out arms. A capital B is put in if the galaxy has a bar. Our Milky Way is likely an SBb galaxy. Astronomy pictures of the day: Elliptical galaxies Spiral galaxies All types of galaxies from the Anglo-Australian Observatory The Andromeda galaxy, below, is the closest galaxy to the Milky Way, at a distance of about 2 million light years. With your naked eyes, you can see it as a smudge in the sky if you're far away from city lights. It's a spiral galaxy, similar in size to our own: Below is an irregular galaxy, the Small Magellanic Cloud. It's in orbit around our own Milky Way: Astronomy pictures of the day of the galaxies in our "local group" of galaxies Here is a diagram showing our "Local Group" of galaxies. The Milky Way and the Andromeda Galaxy are the two main galaxies in the group. Moving even further out, we see the galaxies are in a sense uniform (no special place is apparent), but at all scales there are gaps (voids) and bubble-like structures surrounded by walls and filaments: A map of the galaxies on the largest scales we've measured. This is part of the "2DF" project to map a 2 Degree Field of the sky. Click for more information.
Dark Matter: WIMPS and Machos		There's a puzzle--a mystery--about our galaxy, or any other galaxy, for that matter. Artwork by Lynette Cook. You can find out the speed of gas in orbit in a spiral galaxy by the Doppler effect. The 21-cm hydrogen line can be used to find the motion of neutral gas. Then you can make a plot showing how fast the gas is moving based on how far out it is in the galaxy, you get something like this: And here are examples from several real galaxies: As expected, stars within the bulge move faster the further out they are. This accounts for the curves rising at first. This is because the further out the stars in the bulge are, the more mass there is within the star's orbit. The bulge, filling 3-d space like a sphere, has increasingly more mass (and therefore gravity) within as you look at stars further out. The puzzling thing about galactic rotation curves is that the galaxy's rotation doesn't slow down far out. In the solar system, where most of the mass is concentrated at the center, you see that. In galaxies you don't. The explanation favored by most astronomers is that 90% of the mass of most galaxies is tied up in some form we currently down understand. It's called dark matter, because when we add up all the mass of the stars that cause a galaxy's light, it's not enough to cause the gravity. So there has to be matter that has gravitational pull, but that doesn't give off much light. There are some alternatives to dark matter. Most astronomers accept dark matter, but there's also a theory called "Modified Dynamics" or MOND, that says that our law of gravity somehow breaks down when things move with a very low acceleration. Here is a description of that approach. Scientists are usually reluctant to change the basic laws without a lot of evidence. So, accepting that there really is dark matter, what might it be? There are two main possibilities scientists consider, and probably both exist. The types are called WIMPS and MACHOS. Probably more of the Universe is in WIMPS than machos, however. MACHOs: MACHOs are MAssive Compact Halo Ojects. They can be brown dwarfs (low mass failed stars), renegade planets, burnt out/cooled down white dwarfs, neutron stars, or even black holes. The common elements in MACHOs is that they are all made of normal matter, and that they're spread out in the halos of galaxies. WIMPS: WIMPs are Weakly Interacting Massive Particles. Unlike MACHOs, they are not "ordinary matter." As the name suggests, they interact weakly with normal matter, rarely affecting it. One possibility for a WIMPy universe is that neutrinos have mass. Remember, neutrinos are particles that rarely interact with normal matter, but are produced in nuclear reactions in stars and in supernovas. If neutrinos have a small mass, their gravitational pull could be what's changing the orbits of stars at the edges of galaxies. (If neutrinos had absolutely zero mass, they would have to travel at exactly the speed of light.) Neutrinos would be an example of "hot dark matter"--if they had mass, they would move at nearly the speed of light. Dark matter could have altered the formation of galaxies. The Big Bang started out with some regions of extra high or low density that eventually caused galaxies to be clustered as they are. HOT dark matter, by moving so fast, could equalize out some of the clumps, while COLD dark matter wouldn't do that. Another possibility is that the Universe is filled with a type of particle physicists still haven't discovered. These could be cold dark matter particles. Some people are trying to find WIMPs in experiments here on Earth. Our current physics theories of the matter in the Universe predict there may be new particles that haven't been discovered yet. There's a theory called "supersymmetry" that says that for every boson we know about (like photons that make up light, or gravitons that make up gravity) there is a still-undiscovered corresponding fermion (called a photino or gravitino). Likewise for every fermion (electrons, quarks) there is a supersymmetric particle (selectron, squark) we haven't yet discovered. The dark matter could be the predicted supersymmetric counterparts to the known particles. A third possibility, besides either WIMPs or MACHOs, is the MOND or Modified Dynamics theory, which says that Newton's law F=ma breaks down for very small accelerations a. This theory is consistent with known observations, but most astronomers would rather add a particle than change an entire law of physics.
Dark Matter in Clusters of Galaxies		Dark matter exists not only within galaxies, but within clusters of galaxies. Below is an image combining optical and X-ray light. It shows that surrounding a cluster of galaxies is gas that is so hot that it emits X-rays (the pink blob.) Remember when we talked about escape velocity, that hot gas escapes more easily from the surface of a planet. The gas at the edge has more of a chance of having random thermal velocity that's greater than the escape velocity. A hot atmosphere leaks away faster than a cold atmosphere. The same thing should happen with clusters of galaxies. So that in order to hold in this hot gas, there would have to be a lot of mass in this galaxy. (More mass makes the escape velocity higher--it's higher to break free of the strong gravity.) More mass than is accounted for by the light we see in the form of stars. Click for more information on this Astronomy Picture of the Day Gravitational lenses also show evidence for dark matter in clusters. Below is a Hubble Space Telescope image showing curved lines surrounding a cluster of galaxies. These are caused by galaxies much further away than the cluster that have their light pulled around into arcs by the gravity of the cluster. The arcs depend on the strength of gravity and again--the gravity has to be much more than can be accounted for by the light. Click for more information on this Astronomy Picture of the Day