Astronomy 1101: Planets to Cosmos

Todd Thompson
Department of Astronomy
The Ohio State University

Lecture 1: Introduction

How many M & M's fit in a 3000 square foot house?

We will spend the first part of class discussing what this course is about: stars, galaxies, and the universe on its largest scales.

We'll then discuss the course syllabus and mechanics, grading, and the course webpage. Detailed information can be found on the course webpage here.

We'll then get into the course proper:

Three Primary Questions:

When observing an astrophysical object like a galaxy, a star, a cluster of galaxies, a planet, or a large population of objects, or the universe as a whole, we generally ask three questions:

(1) What is it? How do we describe it?

What are its observational properties? What does it look like? How much energy is it radiating? Where is it with respect to other things? What is its composition?

(2) How does it work?

What is the underlying physics ("Astrophysics")? Can we use physical principles to construct testable theories that can then be compared with observational data?

(3) How does it evolve?

How did it form? What other objects do we see that might be future or past versions of the object under consideration? How does it develop over time, what is its future? What is its end-point?

My hope is to provide you with partial answers to these questions (and many more) for stars, galaxies, and the universe.

Characteristics of the Science of Astronomy:

First, we cannot manipulate and experiment with the objects we observe.

We can only make observations from a distance (usually a very large distance).

That is, we deal with vast distances & times.

With a few exceptions, astronomical objects do not change on human timescales. For example,
- The nearest large galaxy (Andromeda) is about 3 x 1019 km away. The light we see from it took about 3 million years to get to us. In this way, our telescopes are time machines: they show us how things were a light-travel-time ago.

- The lifetime of a short-lived star is 5 million years.

- The age of our solar system is about 5 billion years.

- The age of the universe is about 14 billion years.

We must build our understanding of the history of stars, galaxies, and the cosmos from ``snapshots.''

For these reasons, we are often left to make arguments based on an observed large population of objects, rather than an individual.

Example: Human life cycle from an instantaneous snapshot.

Some concepts and numbers: the speed of light, 1 astronomical unit, 1 light year, the size of the solar system, the size of the galaxy, the size of the universe.

Big Numbers

Astronomical Numbers are, well, Astronomical!

Examples:

Average distance of the Earth from the Sun: 149,597,900 kilometers

Mass of the Sun: 1,989,000,000,000,000,000,000,000,000,000 kilograms

Age of the Earth: 4,550,000,000 years (4.55 Billion years)
Other big numbers, while not "astronomical" for comparison:
Number of OREO cookies sold to date: 490,000,000,000 [source: Nabisco]

US National Debt: $17,712,825,868,365.56 (as of 2014 Aug 28). [source: U.S. Treasury Department, Bureau of the Public Debt].

Because the numbers we will encounter in this course range from the very larger to the very small, we need a way of dealing with such numbers sensibly so we don't go crazy counting zero's, risking factor of 10 or greater mistakes at every turn.

Scientific Notation

Scientific Notation is a compact and convenient way of expressing very large and very small numbers using powers of 10. You've all probably encountered scientific notation before. I hope the examples below are reminders for those who haven't used it in a while. If you need a detailed review, please see Section 1-6 of Kaufmann & Freedman.

Examples of Scientific Notation:

The Mass of the Sun:
1,989,000,000,000,000,000,000,000,000,000 kilograms = 1.989x1030 kilograms

The Size of a Hydrogen Atom:
0.0000000000106 meters = 1.06x10-11 meters

In each case, use of scientific notation eliminates most of the zeros which are just place-holders for factors of ten, letting us concentrate on the significant figures. In a field such as astronomy that deals with scales ranging from subatomic particles to the entire universe, this notation is a great simplification!

The Metric System

Astronomers use the Metric System exclusively. The basic units of the metric system are: The metric system is also known as the International System of Units (or "SI" for "Systeme Internationale"). At present, only the United States, Liberia & Myanmar (aka Burma) have not adopted SI units as their primary system of units. For more information on the SI units, see the SI Units page at the US National Institute of Standards & Technology (NIST).

Standard Prefixes

In everyday use, we often add a prefix to the base unit to indicate common powers of ten. A brief listing of some of the more common is given below:

Factor Prefix Examples
103 kilo- kilogram, kilometer, kilobyte
106 mega- megawatt, megayear, megabyte, megaton
109 giga- gigayear, gigaton, gigabyte
1012 tera- terawatt, terabyte
10-2 centi- centimeter
10-3 milli- millimeter, millisecond, milliliter
10-6 micro- microsecond, micron
10-9 nano- nanosecond, nanometer

Common Examples

Length:
1 kilometer = 103 meters (1000 meters)
1 centimeter = 10-2 meters (1/100th of a meter)
1 millimeter = 10-3 meters (1/1000th of a meter)
1 micron = 10-6 meters (short for "micrometer")

Time:
1 nanosecond = 10-9 s (1 billionth of a second)
1 Gigayear = 109 years (1 Billion years)
1 Megayear = 106 years (1 Million years)

Units of Length

The basic unit of length is the meter (m)
Traditional Definition:
1 ten-millionth the distance from the North Pole to the Equator of the Earth.

Modern Definition:
The distance traveled by light in a vacuum in 1/299792458th of a second.
We will most commonly encounter meters and kilometers.

Astronomical Units of Length

Meters and kilograms are fine for most terrestrial applications, but when we start talking about the enormous distances between the planets, or between stars and galaxies, we need to define special units to keep the numbers from getting too big. The most important of these for our purposes in this course are:

The Astronomical Unit (AU):

1 AU is the Mean Distance from the Earth to the Sun:

1 AU = 1.496x108 kilometers
The AU is used for expressing the distances between planets.

In round numbers, you can use "1 AU = 150 Million km" for the purposes of this class.

The Light Year (ly):

1 Light Year (ly) is the Distance Traveled by Light in 1 Year:

1 ly = 9.46x1012 kilometers
The light year is used for expressing the distances between stars.

Space is BIG

For example, what is the distance between the Earth and: As you can see, if you only use kilometers or meters, the numbers would get out of hand very fast.

Units of Time

The basic unit of time is the second (s):
Traditional Definition:
1/86400th of the mean solar day.

Modern Definition:
9,192,631,770 oscillations of a 133Cesium atomic clock.
We will usually measure time in units of seconds, minutes, hours, and years.

Units of Mass

The basic unit of mass is the kilogram (kg):
Traditional Definition:
1 kilogram is the mass of 1 liter of pure water.

Modern Definition:
1 kilogram = mass of the international prototype of the kilogram.
This is a piece of platinum-iridium alloy kept at the International Bureau of Weights & Measures in Sèvres, France.
We will be most often use masses in kilograms.
Updated: 2014 August 28, Todd A. Thompson
Copyright © Richard W. Pogge, All Rights Reserved.