Astronomy 1144: Introduction to Stars, Galaxies, and Cosmology

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.