2) How do we know about stars and space anyway? This course will also introduce you to the ways astronomers think. Some of the "facts" we learn may be overthrown in the next decades as new telescopes and spacecraft are built. But the methods and ways of thinking should still be valid.

The scientific method as it applies to Astronomy is a little different than for sciences such as chemistry and biology. In Astronomy, it is rare that we can perform a controlled experiment, and some interesting things we observe are not repeatable in the sense of laboratory work on the Earth. But still we look for explanations and predictions.

The way astronomers find out what's going on has to be very indirect and clever. This class will be partly about figuring things out. Because all astronomers have to go on is the small amount of light they see; they can't go out and manipulate what they study. So it's pretty amazing we've now discovered planets around other stars, especially when you consider that we've never even seen the surface of a star other than the Sun, with an exception or two for close-by, large stars, such as Betelgeuse (pronounced "Beetlejuice"--the inspiration for the movie by that name.) All stars would appear as dots in photographs, except the photos are sometimes overexposed.

I will try not to emphasize memorization much, but instead to emphasize figuring things out. There will be some math, but nothing beyond trigonometry. You should make sure you are familiar with graphing and with scientific notation and units. But the important thing is not to get discouraged, to be able to look at something that's hard to figure out and not turn away too fast.

The syllabus mentions that in addition to 2 exams and 8 homework sets, there will be 2 special projects. In one, you'll keep track of the phases of the Moon for a month, and in the other project, you'll pretend you're writing a proposal to use the Hubble Space Telescope--you'll choose a star or a galaxy or a planet and write up a report of an observation that could be made to help us learn something interesting about the Universe.

Ok, let's talk about (1) now, the actual stuff of our Universe, as opposed to how we've come to know it.

We're going to zoom out--you can zoom in to learn about cells and atoms and particles, but we're not interested in that just now.

You notice you'll need to brush up on your metric units. Most people are under 2 meters tall. Zooming out until the Earth fills our view, we see that the Earth has a radius of 6378 km. The Moon is 390,000 km away, so it takes a couple of 10 times zoomings to see the Moon.

Now the amount of empty space in our Solar System is remarkable. A couple of 10 times zooms and other planets come into view. The Earth-Moon distance is 390,000 km, but the Earth-Sun distance is about 150,000,000 km! The Sun is about 400 times further away than the Moon. They appear the same size in the sky (that's why we can get total solar eclipses, when the Moon exactly covers the Sun) because the Sun is 400 times bigger in radius--109 times bigger than the Earth.

The "jovian" planets, or the gas giants, come into view further out. Jupiter, the largest, has a diameter 11 times that of Earth.

The 9 planets of the solar system, to scale in size (but not distance from the Sun!) The Earth would fit easily within a persistent storm in Jupiter's clouds called "The Great Red Spot"

When we start to consider large distances, we start using units like "astronomical units" and "light years". One astronomical unit or AU is the mean distance from the Earth to the Sun. I say "mean distance" because the Earth's orbit isn't perfectly circular. The Earth is actually about 3% closer to the Sun in our winter! As far as "light years" goes, remember this is a unit of distance, not time! Light doesn't travel instantaneously, but it does travel very fast. It can be slowed down when it passes through matter, but in a vacuum it has a speed of about 300,000 kilometers per second, or 186,000 miles per second. It takes light 8 minutes to go from the Sun to the Earth, but more than 4 years to reach us from the nearest star other than our Sun, alpha Centuri.

The Sun is a rather ordinary star, and in fact one of the stars in the alpha Centuri system could be considered its "twin" because it is so similar.

All stars shine because of nuclear fusion reactions going on in their cores. These nuclear fusion reactions not only cause stars to shine, but they make heavy elements from light ones! The Universe is mostly made of hydrogen and helium. That's why Astronomy is an easier science than Chemistry! Astronomers have most of the Universe covered in those two simple elements. Hydrogen is the lightest of all elements, and helium the second lightest. In these fusion reactions, stars take hydrogen and make helium, and similar reactions can make even heavier elements like Carbon, Oxygen, Nitrogen, Iron, etc., that are needed in our bodies.

That's why Carl Sagan used to go around saying "We are all made of starstuff." Because the Big Bang started with hydrogen (and some helium was formed in the Big Bang too), the heavier elements that make up the planets and our bodies had to come from the reactions inside stars.

• Planets: Shine only by reflected light, go in orbit about a star
• Stars: Give off their own light from nuclear fusion; our Sun is an ordinary star
• Galaxies: Huge collections of stars. Our own galaxy is called the Milky Way and contains hundreds of billions of stars

(Review on numbers: a million = 1,000,000, a billion is a thousand million or 1,000,000,000, or in scientific notation 10⁹.)

If you go out at night and see the constellation of Orion--one of the easiest to spot this time of year--you are looking into an arm of the Milky Way galaxy. The bright stars in Orion are mostly young, massive stars. The most massive stars actually don't live very long--they burst out real fast. As the poet Edna St. Vincent Millay wrote:

My candle burns at both ends;
It will not last the night;
But ah, my foes, and oh, my friends -
It gives a lovely light.

Betelgeuse, the "shoulder" star of the constellation Orion, was the first star whose surface could be viewed as anything more than a point. The bright stars in Orion are part of a sub-arm of our Milky Way galaxy. Like other bright stars, they will not live long, millions instead of billions of years. More stars are being born in the "sword" of Orion.

Stars are continually being born and dying in our Galaxy. In the constellation of Orion there is a "stellar nursery", often called the "Orion nebula". (A constellation is just a group of stars that appear together in the sky. They don't have to be related to each other at all, and in fact their true 3-d positions may have no relation to each other.)

The Orion Nebula, a cloud of gas and dust in the constellation Orion. Within the nebula, stars are being born

When a star dies, it gives its gas back to space, either gently, as our Sun will 5 billion years from now, or violently, in an explosion that can briefly outshine an entire galaxy.

In a dark summer sky, you can see the main disk ("pancake") of our Galaxy as a white band. It passes through the constellation Cygnus the Swan; you can think of the Swan as flying through the Milky Way.

A spiral galaxy viewed face-on ("from above"). The spiral galaxy here is actually merging with another galaxy, to the left, as gravity pulls it inwards.

The stars in a spiral galaxy are in orbit around the center--but there lies one of the mysteries of astronomy! The stars go faster in their orbits than we can explain based on the gravity of the stars we see. So from the rotation of galaxies, we deduce that there is more matter in the Universe than scientists currently know about--there must be about 10 times as much stuff in the form of mysterious dark matter than there is in normal matter, which makes up the bright stars.

Our current understanding is that the Universe is expanding like the surface of a balloon. It's not expanding into anything, but space itself is expanding.

But we still don't know whether the Universe will keep expanding forever or whether the gravity of galaxies will pull them back together. The latest surprising evidence is that the Universe may be accelerating, that is, expanding faster and faster. This is an exciting time to live in, because we may find out the answers to the cosmological questions in our lifetimes.

So let's have a look at how the Universe has changed over time.

The book does this by talking about a "Cosmic Calendar", compressing to scale the events of all of time into a single year. We'll talk about a "Cosmic Class"--what if the history of the Universe fit into the time period of a single Astronomy class?

Measurements place the Big Bang, the origin of our Universe, at somewhere between 10 and 15 billion years ago. To scale, if the Universe started with the start of today's 75 minute class, then the Earth would have formed about 45 minutes in.

With only 4 minutes left of class (now!) the first large life forms appear on Earth. Only at about a minute from the end do Dinosaurs rule the Earth!

If this class were the entire history of the Universe, the earliest of our humanoid ancestors who evolved away from the other primates would appear only in the last second or two!

Finally, modern human beings would live in only the last 1/60th of a second, a time shorter than a frame appears in a movie, a time so short that the picture on your TV screen or monitor would appear frozen. That's all of human history, even all prehistory, compared with the history of the Universe!

The Cosmic Class

Time	Event
1:15	Start of class! Big Bang
1:21	Milky Way galaxy forms
2:02	Planet Earth forms
2:26	Rise of large organisms
2:28:45	Dinosaurs at their peak
2:29:58	Earliest human ancestors split with other primates
2:29:59.98	Modern human beings evolve
2:30:00.00	End of class! Present day