This is a work in progress. Of course, due to the nature of astronomy in the 21st century, it will probably always be a work in progress...

It started as notes for an introductory course, but it soon became obvious that my students wanted to look at pretty pictures, and I wanted to use physics to teach them how we understand the astronomical observations we make. I'm not sure at what point I decided to turn my notes into this online text. They are a lot of work, and after writing four previous online texts, I swore there would not be another.

So here we are. I make changes and additions to the text on a regular basis, and anticipate doing so constantly during the 2012-2013 academic year, when I will be using the text in a course for the first time. For those of you contemplating that course, here is your chance to have a serious impact on the text: your questions and comments will in large part help this work in progress to reach a mature state. But I assure you, the text will continue to grow so long as we learn more about our universe.

One important thing that I have added is a portal to the text which allows you to look at the pretty pictures, and when you want to learn more, you can enter the text by simply tapping the "Enter" key. Try it!

The portal also appears embedded in many of the pages of the text. To navigate it, use the up and down arrows.

You will need the Java Runtime Environment (JRE) for the portal, as well as for other applets in the text; if you do not have it, you can download it free.

The text contains a number of parenthetical exhortations to view various video excerpts, many from Carl Sagan's Cosmos. Cosmos is still an excellent and entertaining introduction to the universe, and I highly recommend it.

Also, I have included a number of "portfolio exercises", which are explorations you can do to extend your understanding of the text.

The end of each page has links to the table of contents, references, and the index. There are some links to sites outside of the text scattered throughout it, but most of the external links are on the references page. They are intended to be suggestions about where to go from here to learn more (and find more pictures: I can spend hours contemplating the infinite variety of nebulae). I particularly recommend NASA's Remote Sensing Tutorial: it is a great place for more in-depth discussions of much of what is touched on in this introductory text. The references page also contains some links and suggestions which might aid your own observations, a summary of the equations and constants used in the text, and a brief glossary.

Throughout the text, sources and copyright information for each image are linked as "(source)".

For the images displayed by the portal, the original source (and copyright information) can be accessed for any image by pressing the "Home" key on your keyboard.

I never cease to be amazed at the difficulty of finding mistakes in works of this size. They seem to creep in through the cracks between the keys on the keyboard, and once in place, my tendency to read what I expect and not what I see makes them all but invisible. If you find any, please let me know.

Astronomy is one of the best applications of physics that exists, reason enough for a text of this nature. But beyond that, astronomy is perhaps the ultimate human pursuit which distinguishes us from our evolutionary ancestors. In it, we cast our wonder toward infinity, and attempt to reason out the structure of existence. It wildly and ceaselessly feeds our fascination and our imagination.

Learn and enjoy!

©2012, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

Please send comments or suggestions to the author.