Ch. 0: Prologue

0.1 -- Astronomy?

In this course you will learn about astronomy. Astronomy is the study of what lies beyond the Earth -- other planets, stars, comets, meteors, dust, gas, galaxies, novae, black holes, and lots of other stuff. Once this stuff has been identified, classified, and named, we want to understand how it behaves, how it came to be the way it is:

This leads to a second way to define astronomy. Astronomy is how we organize what we learn about the universe into a history that explains how it began, and evolved to its present form. This is a big task, one which is proceeding at telescopes, space agencies, laboratories, and universities.

0.2 -- Astronomy is a Science

What makes somethng a science? According to Merriam-Webster's, a science is: knowledge or a system of knowledge covering general truths or the operation of general laws especially as obtained and tested through scientific method b : such knowledge or such a system of knowledge concerned with the physical world and its phenomena : NATURAL SCIENCE.

Astronomy is a science, and as such is subject to the most fundamental rule of science: the scientific method. The scientific method can be summarized by the following five steps:

  1. Observe a phenomenon, thing, or process.
  2. Formulate hypotheses to explain what is observed, and is consistent with present knowledge.
  3. Test predictions derived from the hypotheses against experiment. Well designed tests yield a clear answer for the prediction.
  4. Modify or reject the hypotheses that disagree with the tests, and repeat the tests if needed.
In science, nothing is taken on faith alone. All ideas and principles are open to scrutiny (observation and experimentation), and subject to revision if found to disagree with reality.

I will try to emulate the scientific method in this class. I will present some observations, which we can then discuss and develop hypotheses from. From these we will derive predictions, and I will discuss tests of these predictions. Finally we will derive conclusions based on this information.

Of course, not every aspect of learning astronomy lends itself to the above approach, in which case I will resort to more standard techniques of presenting information. I will try to utilize this interactive technique as much as possible.

Your cooperation and participation is required for this technique to be succeed. Before class, you must read through the chapters to be covered in that class. You should have a general idea of the topics being discussed that day, and learn new terms used. A list of questions will be distributed by e-mail or blackboard to provide you with guidance. A reading quiz will be given each class (except during an exam week) that will reward you for spending an hour or two looking over the chapters for that week.

During class, you will need to pay attention to the material presented and then participate in the discussions about this material. We will have in-class questions for which you will turn in your responses for grading. These will come in several possible formats, but the grading will emphasize your participation in learning process.

0.3 -- What are Laws of Nature?

Laws of nature are well tested, broadly applicable, fundamental rules for understanding a class of phenomena. Is there a law of antrue you've already studied? Think about possible laws from biology, chemistry, or physics.

physics: gravitation, speed of light, conservation of momentum, conservation of energy, Newton's laws of motion, maxwell's equations of electromagnetism, ...

chemistry: ideal gas law, conservation of mass, conservation of atoms (detailed balance), ...

biology: cell theory, DNA, evolution, ...

Laws of nature are subject to the same scrutiny of experiment as anything else in science. However, for something to be considered a law of nature, many tests must already have been performed to check its validity. It is likely that, if disagreement is found, it is under certain conditions where the "old" law fails. This means that, rather than throw out the "old" law, it will be kept, but its use now restricted to avoid the conditions where it disagrees with experiment. Examples: Newton's laws and quantum mechanics; universal gravitation and general relativity.

0.4 -- Numbers in Astronomy, and this Course

Discuss power of ten notation (scientific notation) below.

  1. We will deal with very large and very small numbers.
  2. It is helpful to use powers-of-ten notation or scientific notation when dealing with such numbers.
  3. An explanation of scientific notation.
  4. We will use the SI or metric system of units and some special astronomical units in this course.

0.5 -- Light Years

We will use the SI or metric system of units, and some special astronomical units in this course. The first example of a special astronomical unit is the light year, abbreviated LY, and defined as the distance light travels in one year. This unit is related to a law of nature -- the speed of light is constant, the same for every observer -- which is a cornerstone of Einstein's theory of special relativity (discussed in more detail later in the course).

Light travels at 3.0×105km/s (300,000 km/s) or a total of 9.5×1012km in one year (nine and a half trillion kilometers). This distance is one light-year (1LY). The nearst star is 4.3LY from the Sun, and most of the stars visible to the naked eye are hundreds or thousands of light-years away.

While the word "year" is a measure of time, be careful not to confuse the distance of a light-year with a period of time.

0.6 -- Consequences of Light Travel Time

Light itself travels at the speed of light (duh!). So, if you look at a star that is 100LY from Earth, when did the light you are seeing now leave the star? (answer: 100 years ago) Jupiter is about 20 light-minutes from Earth (where the concept of a light-minute is the obvious modification of a light-year). To send a command via radio waves to a space probe orbiting Jupiter and receive the response will take about 40 minutes -- 20 minutes for the command to reach the probe and another 20 for the response to return to Earth.

An interesting consequence is to consider a star that is a billion LYs from Earth. The light we see now must have left the star a billion years ago! That light not only tells us about the star, but can tell us something about the universe a billion years ago when the star emitted the light! Distant objects are like a virtual time machine.

0.7 -- A Tour of the Universe

Now let's take a quick tour of the universe to get an overview of what we'll be studying in this course.

  1. We start with our home, the planet Earth
    1. Spherical planet, 13,000 km in diameter.
    2. Distinguished by the large amount of liquid water.
    3. Many signs of intelligent light -- lights, radio and television signals.
  2. Our nearest neighbor is the Moon
    1. Earth's satellite
    2. 384,000km from the Earth -- light takes 1.3 seconds to travel between
    3. 3476km in diameter
    4. Revolves about the Earth about once per month
  3. The largest object in our solar system is the Sun
    1. Our star, 150 million km or 1 astronomical unit (AU), away.
    2. Light takes 8 min. to travel from the Sun to the Earth.
    3. The Earth revolves around the Sun once per year.
    4. Earth's gravity holds us firmly, so we don't feel that we are moving around the Sun.
    5. 1.5 million km in diameter.
  4. The Sun, nine planets (including Earth), their satellites, and many smaller bodies make up the solar system.
    1. We see the other planets because they reflect light from the Sun.
    2. The nine planets are, in order of their distance from the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
    3. The planet Jupiter
      1. is the largest planet, 143,000 km in diameter.
      2. is five times further from the Sun than Earth is, or 5AU.
    4. The planet Pluto
      1. is the furthest planet from the Sun, about 40AU.
      2. has a significantly non-circular orbit.
  5. The Sun is but one of many stars in the Milky Way Galaxy
    1. Stars and our Sun are similar, our Sun is just alot closer than any other star.
    2. All the stars visible to the naked eye are part of the Milky Way Galaxy.
    3. The Galaxy contains hundreds of billions of stars.
    4. The Milky Way is a wheel-shaped galaxy about 100,000LY in diameter.
    5. Our sun is about 30,000LY from the center of the Galaxy.
    6. In addition to stars, our Galaxy contains lots of gas, dust, and something invisible we call dark matter.
  6. Stars
    1. Sometimes stars are grouped together in twos, threes, or more.
    2. We can learn more from grouped stars than isolated stars.
    3. Though very long lived, stars are not eternal.
    4. At the end of their lives, some stars explode.
    5. Many of the atoms in our bodies came out of star explosions.

0.8 -- The Universe on the Large Scale

  1. The universe is made up of billions of galaxies.
  2. The nearest galaxy is 75,000LY from us.
  3. The Milky Way and about 40 other galaxies form a cluster we call the Local Group.
  4. Our Local Group is part of a supercluster about 60 million LY in diameter.
  5. At very large distances we find quasars, believed to be giant black holes.
  6. Quasars can be seen out to about 10 billion LY, or 10 billion years in the past.
  7. Past the quasars, we see only the remaining glow of the Big Bang.

0.9 -- The Universe of the Very Small

Strangely, it turns out that to understand the largest objects in the universe, including the universe itself, requires understanding the behavior of the smallest objects. To understand the light given off by stars we will learn about the structure of atoms, and how the nuclei of two atoms can be fused together to form a heavier atom and give off eneergy. To understand the conditions under which a dying star will collapse under the force of its own gravity to form a black hole, we will learn about the funny behavior of electrons and neutrons. When we discuss the phenomena of exploding stars called supernovae, we will learn about etheral particles called neutrinos. To understand how the universe evolved from the moments after the big band until now, we will learn about Einstein's theory of general relativity, and a little bit of quantum mechanics. There is now a lively interaction between scientists studying the largest objects of the universe and scientists studying the fundamental particles of the universe.

  1. The universe is mostly empty space.
  2. On the other hand, even solid objects, such as books, are mostly empty space.
    1. Everything is made from molecules, the smallest particle of something that retains its chemical properties.
    2. Molecules are built from atoms, the smallest unit of one of the more than 100 elements in the universe.
    3. Atoms themselves consist of protons, neutrons, and electrons. The protons and neutrons are stuck together in the nucleus, while the electrons move around the nucleus.
    4. The number of protons in an atom determines the type of the atom, that is, which element it is.
    5. There are 4 forces that govern the behavior of all the atoms making up the universe.

0.10 -- A Conclusion and a Beginning

  1. Imagining the lifespan of the universe compressed into a single year we find that:
    1. Life begins on Earth around September 25th.
    2. The extinction of the dinosaurs occurs on December 30th.
    3. The first humans appear in late evening on December 31st.
    4. The alphabet is invented only 9 seconds before midnight on December 31st.
  2. In this perspective, the history of astronomy is a mere blip.

Discussion question:

Everyone will need a sheet of paper to write ideas on and turn in at the end of this exercise. Divide into groups of 2 (3 if there is one person left). I will give you a question which I want you to consider, and discuss with your partner(s) for the next 5 minutes. Then I want you to write down your response(s) to the question, which I will collect for grading. Then we'll discuss the question and some of the answers you came up with.

Are there any laws of nature that you're already familiar with? Try to give a statement of the law in words.

What is the largest number you can think of? I'd like you to (try to) express this number in scientific notation.

What astronomical objects, events, or phenomena have you seen? If you had the possibility, what would you like to see? Please restrict your response to things visible from Earth.

Each group should pick an ancient society (Athenian Greeks, Han dynasty Chinese, 21st dynasty Egyptians, Renaissance Italians, ...), and consider how astronomy entered their lives. Consider where appropriate the influence on agriculture, religiion, politics, art, and daily lives in general. Choose a representative to present your conclusions to the class.

© Robert Harr 2005