How Big are Galaxies?
• How massive are galaxies? Although we can fill journals with details about galaxies, an astronomer cannot really answer these three basic questions with any confidence. The first question raises the “nature vs. nurture” problem. Is the morphological type of a galaxy determined at birth by its initial conditions, or does the environment of the galaxy control its destiny? While the standard picture that different initial conditions cause the difference in morphology still seems plausible (e.g., Sandage, Freeman, and Stokes 1970, Gott and Thuan 1976), recent interest has centered on Toomre’s (1977) suggestion that all galaxies begin as disks and then merge to form elliptical galaxies. The problem with the second question is that a galaxy does not really have an “edge”, so the definition of size is somewhat arbitrary. What makes things worse is that rotation curves in spiral galaxies stay flat as far out as we can observe them, so the mass is still increasing linearly with radius with no
Alongside stars, the galaxy is the most important unit of organization in the entire universe. There are an estimated 100 billion galaxies in the visible universe, each with between about ten million and a trillion stars. The average galaxy includes about 1071 atoms, and ten times more invisible mass in the form of dark matter. Galaxies themselves tend to be arranged into large structures called superclusters, which are in turn arranged into massive filaments separated by immense voids. The largest of these voids, the Eridanus Supervoid, has a diameter of nearly 1 billion light-years, about a 10,000 times wider than the Milky Way Galaxy. Our own galaxy, the Milky Way, has between 200 and 400 billion stars, many of which are very faint due to exhausting their nuclear fuel long ago. To get a size of its scale, if 100 billion human beings could occupy the space around each star, then the entire Galaxy could provide space for roughly 1022 humans, more than the number of grains of sand on a
• How far away? • How are they grouped? • What are galaxies made of? • Why do they look the way they do? • Are galaxies moving? In what way? • How old are galaxies? • What is our galaxy like? • Explain that over the course of this workshop we’ll be able to answer some of these questions. To do so, we’ll use spectral lines. • Exploring Optical Spectral lines: • Use overhead projector, cardboard, and grating to display optical continuum spectra. • Use light meter (if possible) to show that the intensity of light in spectra varies from one color to another. Or, have them estimate the changes in brightness. • Graph the data implied by the light meter or as guessed by the eye. If time, have students make the measurements and graph their result. [NOTE: most light meters measure intensity on a base-2 logarithmic scale so you might have to teach them how to do the converting to a linear scale] Graph should look like: • Use glow tubes and slide-mounted gratings to display spectral lines of H, N
