The Accessible Universe

0 --- Introduction

0.1 --- Rationale

This will be a continually updated introductory astronomy course that incorporates interactive content via Javascript, SVG, and the HTML5 Canvas feature. It may not correctly work across all browsers, especially older versions of Internet Explorer on Windows, but I'm writing and testing with the latest builds of the Safari, Chrome, and Firefox browsers. It should also now work on the latest IE 9.

Static figures are generated either with Blender 3D (a free 3D modeling and animation application), Inkscape (a free vector-based illustration package), or with the LaTeX typesetting language and PGF (TiKZ) package. The source files I used to generate the figures are freely available should anyone be interested. Other images, including photographic ones (like the astonishingly beautiful one at left, thanks to Jim Richardson), were shamelessly appropriated. I'll try to keep a page of credits updated with links to the original sources. Screenshots and animations of sky images are likely taken from the (excellent) free software Stellarium.

Clicking on any image should show them at their full resolution. All web page coding and design (including simulation programming in Javascript) is done by me, by hand, with the Vim editor, so I'll accept any due consequences.

This collection of documents grew out of my teaching notes that correspond only roughly to existing textbooks. My motivations for writing this include:

"The Accessible Universe", then, is the title that effectively conveys my feelings about astronomy education --- it ought to be freely accessible by anyone, and the grand ideas themselves should be intellectually accessible to all people.

I have no doubt that there are many confusions, inaccuracies and outright mistakes in what follows. As this set of documents is under continual revision, please let me know what you like and/or don't like, and I'd welcome any ideas for improvement. Yes, even from the grammar nitpickers.

0.2 --- Goals and Assumptions

In my mind, there are 3 high-level goals you should pursue while working through a course such as this.

You should emerge on the other side with some appreciation of your cosmological context --- your relative place in the Universe at large and the real relationships in scale between the cosmos as a whole and its constituent parts. We tend to live our lives ignorant of the realities and relationships of nature on scales very much smaller than we and those much larger (and not just spatial scales, but timescales too; some things happen very much more quickly or slowly than is easily observed --- more on this in a moment). You will hopefully develop an appreciation for these frameworks that bind all objects and processes together into one intimately woven history and future. To paraphrase Neil deGrasse Tyson, we are a small part of, and reside in, the larger Universe; but it's important to understand that the Universe and the products of 14 billion years of cosmic evolution are also within us.

An understanding of the physical scales involved should also be paired with an understanding of the evolution of the Universe in time. A crucial point is that, as a consequence of the natural laws we've discovered, the Universe must change over time. It is not the same today as it was yesterday, and is certainly different from a few billion years ago. If it really evolves, shouldn't these changes be observable somehow? Yes! Read on...

Lastly, the engine that drives our predictions and theories about how the Universe behaves is the process of science. You will (hopefully) develop an appreciation of this human enterprise while keeping in mind not only the chains of logic and inference we use to make sense of the cosmos around us, but the assumptions we tacitly use to learn about the Universe.

For example, to do science, we typically make the following assumptions about the Universe at large:

  1. The Universe does not operate via "magic". There is a consistent set of underlying rules and principles that governs the behavior of objects or phenomena that we observe.
  2. Humans can understand these principles. This is not as obvious as it sounds. Naively we might think that we can understand any set of rules of behavior, but let's be cautious. We'll learn, for example, that we've largely solved the mystery of how the moon orbits the earth --- it is, in fact, the same force that causes apples to fall out of trees. We think we understand that. But just try explaining that to a dog. It's not just that they're not interested; they're not. But more fundamentally, the dog's brain is made of the same stuff as ours is, as far as we know, but it seems safe to say that dogs are somehow limited in their capacity to understand the Universe compared to a human. Perhaps the number of neurons and connections in our brains is sufficient to appreciate abstract concepts that might escape the dog. This works all right for our current model of gravity, but you could imagine some natural process operating via a set of rules that are far too abstract or complex for us to understand. There may be creatures elsewhere with fantastically more complex brains that can understand these new rules, but we might forever find them incomprehensible. Perhaps quantum mechanics, the description of the rules followed by very small particles, is already fundamentally beyond our conceptual understanding. In any case, all I'm saying is that just because we assume everything operates according to rules doesn't mean that we can understand those rules. In order to do science, though, we assume that we can.
  3. The same rules apply everywhere. This breakthrough is commonly first attributed to Galileo and Newton. This is tremendously important since it allows us to perform experiments here on Earth and then assume that the same rules and processes apply in the most distant galaxy.

Now, it's possible that at least one, or all, of the above are not true. We still don't know, for example, if consciousness has a purely mechanical or physical explanation. But to investigate it scientifically, we have to assume that it does. Even if such a model for consciousness is developed, it's important to keep in mind the following caveat regarding scientific theories:

They can certainly be proven false, but never true. The best we can ever say is that all the available evidence supports a given theory. Indeed, most initial theories (models) that ever have been developed have yielded to modification and replacement over time. New models are ever more precise and can better explain new observations gathered over longer times and with better technology. We'll see many examples of this as we go along. People often make mistakes on both sides --- to say that some model is "just a theory" (evolution, gravity) is ignorantly dismissive of the evidence in favor of its wide adoption (a theory in science is much stronger than a "guess" in popular language. A reasonable "guess" in science is basically a hypothesis.) But to assume that a given theory is "true" in some ultimate sense is also naive. Any theory is subject to further testing and refinement, or perhaps replacement.

Finally, I should draw a distinction between "fact" and "theory" since they are frequently confused in popular media. Facts are observable outcomes of experiments --- anywhere you go on the surface of the Earth, a dropped object will fall towards the center of the Earth. Carefully following the path of the Moon around the Earth, it also "falls" towards the center of the Earth. As we accumulate facts, we then may generalize some abstract set of rules that give some underlying explanation of these facts. In this case, that would originally be Newton's gravitational theory. It generalizes these facts into a unifying mathematical framework that is able to reproduce the observations and then further predicts the outcome of some new set of observations. Later we might find that there are facts that contradict our theory, so it will need to be modified --- Einstein's modification of Newton's theory is just such an example, as we'll see. So there are the "facts" of gravity and the "theory" of gravity that knits the known facts together in an explanatory model. In another familiar case, there are the facts of evolution (the genetic code of organisms change from generation to generation, organisms compete with each other for limited resources in a changing environment), and the theory of evolution attempts to explain these facts using concepts such as genetic mutations and natural selection.

0.3 --- Our Place in the Universe


The above images represent a series of increasing scales of size we'll be investigating. On the small end, we'll start with the atom. There are 92 different atoms, or building blocks, out of which all the "ordinary" matter in the Universe is made. They each have different sizes, but a rough average size for an atom is about a ten-billionth of a meter. Lining up a billion atoms will just about span the width of an apple. Ah, but just how big is a billion? It is a number of increasingly common usage --- describing the costs of massive projects and proposals as well as populations of the largest countries. It is written as a 1 followed by 9 zeroes: 1,000,000,000. As a shorthand, we write it also as 109, indicating that it is 10 multiplied by itself 9 times. It is:

So the first step from an atom to the everday scales represented by an apple is an increase in scale by a factor of a billion. If we line up a billion apples, then we're approaching the size of the Sun. (note the size of the Earth compared to the Sun in the image). Really, the Sun is about 10 billion apples across --- truly gigantic compared to any objects humans have been accustomed to dealing with throughout our history. It is remarkable that just in the last 100 years or so we have pretty well figured out the physics of atomic interactions ten billion times smaller than we as well as the inner workings of the Sun and other stars ten billion times larger. But we'll study objects on larger scales still. 200 billion stars more or less like the Sun are gathered in our Milky Way Galaxy. --- a truly immense structure that is about ten billion times larger than the Earth-Sun separation. Just as the size of an atom is to an apple, and that apple is to the Sun, the Sun is to our enveloping galaxy. It is fascinating that we can view the gigantic Milky Way every clear night from Earth; our perspective in trying to fully comprehend this object is in some way similar to an atom's perspective of aggregate objects such as we.

But the Milky Way galaxy is not the largest structure in the Universe, although 100 years ago we were not aware of anything larger. There are estimated to be around 150 billion galaxies in the observable Universe, which apparently spans the equivalent distance of about a million Milky Ways across and is arrayed in beautiful networks of galaxy clusters and superclusters, as shown here. Only now, for the first time in human history, we are able to map the large-scale structure of the Universe. Many previous cultures wondered and guessed at the nature of the Universe as a whole and what it might look like. We are the first privileged generation to finally address and answer these fundamental questions.


For most of our human history we have imagined our perspective on the Universe as one of central importance; the Universe has been considered to be very local, centered on and rotating around the vast Earth which rested at its center. Now we can finally leave the nest and venture out a little bit into the Universe beyond. For example, we now have a space station orbiting about 200 miles above us (the International Space Station (ISS)), so we have a new perspective on our planet. Most of the time, the ISS takes pictures of the Earth looking straight down. Occasionally, though, the camera turns to capture an image of the limb of the Earth and reveals the tenuously thin atmosphere that blankets and protects us from the inhospitable emptiness surrounding us.

We first were able to capture the entire disk of the Earth when a few adventurous humans set out to explore the Moon. It's a subtle change of perspective --- on the surface of the Earth, we live on an infinite 2-dimensional surface and aren't so aware of the finite nature of our home. This we can clearly see from an outside third dimension. Our world is small and bounded, framed by a cold emptiness that should reinforce our commitment to care for this confined habitat we share with trillions of other creatures that don't share our elevated perspective. This particular image was taken by the astronauts aboard Apollo 17 in 1972 as they were headed towards the Moon.

This famous picture was taken by the crew of Apollo 11 while orbiting the Moon looking at the "Earthrise" above the lunar horizon. It is clear now that the Earth is a world, no different fundamentally from billions of other planets circling billions of other stars, save for the fact that it is the only one we know to harbor life (so far). This was the first view from a completely alien world, looking back on where we came from. Truly a milestone of human achievement that will be marked for as long as we (as a species) survive.

Still farther away, the Martian probe HiRISE captured this picture of the Earth-Moon system as it was on its way to Mars. Here we see that the Earth and Moon really are partners in their annual dance around the Sun.

This remarkable picture was taken by the Cassini orbiting spacecraft as it swung behind Saturn. The view is looking back towards the Sun (Saturn is blocking the Sun here) and there we are!, the tiny dot seen through the rings to the upper left of center. You may need to look at the full-resolution image to see us, just inside the second-farthest ring.

The most humbling picture we've been able to take directly comes from the Voyager 1 mission, originally launched in 1977. Carl Sagan, a popular astronomer about whom we'll learn more later, requested in 1990 that the spacecraft turn around and take a picture of the Earth from a vantage point roughly at the orbit of Pluto. There we are, the tiny blue speck (the glowing beams are artifacts from the camera itself). The Earth is diminished to a speck; but keep in mind that the probe is, on stellar scales, still right in the neighborhood. Even at this distance, it is only 1/5,000 of the way to the nearest star. Sagan writes,
"From this distant vantage point, the Earth might not seem of particular interest. But for us, it's different. Look again at that dot. That's here, that's home, that's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there on a mote of dust suspended in a sunbeam.

The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that, in glory and triumph, they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner, how frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds.

Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.

The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.

It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we've ever known."

Despite our vantage point on this remote speck of a world, we have managed to learn quite a bit about the universe in which we find ourselves embedded. We'll start the story by retracing what we can find out about the sky by observing the repeating patterns that anyone can see. Onward and outward!