ASTRONOMY 113Laboratory Lab 6: Structure in the Universe, c. 1900 to the Present Introduction Universe - the entire celestial cosmos. Cosmology - a branch of astronomy that deals with the origin, structure and space-time relationships of the universe. (from Webster's Dictionary) In one sense, the definition of "Universe" is unambiguous - the Universe is simply everything. On the other hand, how does one know what constitutes "everything"? Invariably, the more we have searched the more we have found. Every recorded civilization has had a cosmology, or a theory of the nature and origin of the Universe. The progression of these cosmologies has been characterized by a constant increase in the scale of the Universe, usually via intellectual revolutions that result in dramatic changes in worldview in relatively short periods of times. Among the very earliest cultures the Universe did not extend beyond explored territory. For the Greeks the Universe extended to the celestial sphere, which was just above the highest mountain peak. The Copernican revolution not only removed the Sun from the center of the Universe, it also required that the stars be vastly further away to account for their lack of parallax motion. How far away was not known until the first parallaxes were measured in the 1800's. The next - and most recent - revolution occurred less than 100 years ago. Much like the Copernican revolution, it involved a tremendous change in both the scale of the Universe and the location of the Sun within it. A textbook at the turn of the century would notinclude the word "galaxy", and its final chapter on cosmology would discuss only a Universe of stars. By 1936, Edwin Hubble could write a popular text on the new Universe of galaxies. (Although interestingly, the book is entitled "The Realm of the Nebulae" and in it Hubble does not use the word "galaxy".) The Universe about which you learned in grade school has only "existed" for about 70 years! In this lab you will retrace the path through the revolution at the turn of the century. As best you can, discard what you know of galaxies, quasars, and the like; allow your mind to be set back in time. And note: one of the defining properties of a scientific revolution is that the meanings of words become confused, and communication breaks down. You will likely get a sense of that confusion as you do the lab, for the word "Universe" means different things at different stages and indeed sometimes has two meanings at the same time. If this gives you an unsettled feeling - good! For that is the reality of a scientific revolution. AN HISTORICAL ASIDE In the mid-1700's comets were at the cutting edge of astronomical research. In 1759 Halley's Comet returned, as predicted many years earlier by Edmund Halley. Only about fifty comets had been discovered by this time, so the accurate prediction of when one would return to the inner solar system on its long orbit was a great accomplishment. The name of one French comet-hunter of this era - Charles Messier - lives on today, but not for the 21 comets that he discovered. When first seen, a comet appears as a faint, fuzzy patch of light. However other objects in the sky such as gaseous nebulae and distant star clusters looked very much the same as comets in the modest-sized telescopes of the time. The only way they could be distinguished was by laborious observation over many nights - comets moved, nebulae didn't. To keep himself from repeatedly mistaking nebulae and star clusters for his beloved comets, Messier made a list of them, including where they were and what they looked like. Today, the Messier Catalog lives on, for it contains some of the most beautiful and fascinating objects in the northern sky. In all, there are 110 objects in the list, each with its own "Messier number". Thus the Crab Nebula is M1, the Andromeda Galaxy is M31, the Orion Nebula is M42, the Pleiades star cluster is M45, and so on. You will see pictures of these and more in this lab. Before You Come to Class... Read the lab completely. Your time in the lab is best used observing the "sky", not reading this manual. Bring to class this lab manual, your lab book, a pencil or erasable pen, and a scientific calculator. Schedule: This lab is to be completed in two weeks.Section 1 - There is more in the Sky than Meets the Eye In your first exploration of the sky with Voyager you observed the motions of the stars and planets. Prior to the invention of the telescope this was all there was to the Universe. But by the 1700's mankind's inventory of the sky included a wide variety of star clusters and nebulae. Although it was not known at the time, these objects were to be the keys which opened a Universe far more vast than almost anyone could imagine. The first goal of this lab is to introduce you to this world of "deep sky objects". • Double click on the icon "Universe c. 1900", creating a Voyager view of the sky. • Click the "galaxy" button near the lower left hand corner of the screen. This button will display a number of new symbols on the screen. Each shape and color denotes a different type of celestial object, including various kinds of galaxies, star clusters, nebulae, quasars, and X-ray sources. • Turn on Picture Frames (in the Display menu). • Click in any yellow picture frame with a colored symbol in it. This will open a Data Window for the object which tells its name, location, brightness, type, etc. There should be a "picture" box near the upper right corner of the window. (If there isn't one there, it means the cursor selected a close neighbor to the object of interest. Either click closer to the center of the yellow picture frame or try a different one.) The picture box will open an image of the object, taken with a large telescope. • Click the white square near the upper right corner of the window. • Close the Picture Window by clicking the box in its upper left corner. • Look at as many objects as you would like! Once you have the hang of it we will take you on a tour of the sky, choosing a few select targets as examples of each class of object. • Open the Find and Center window (in the Field menu). • Type "M31" and click Search. • Click Center. • Open the Picture Window. M31 is better known as the Andromeda Galaxy, named for the constellation in which it is found. It is the brightest galaxy in the northern sky and visible to the naked eye. As you