We now wish to begin discussing the properties of stars. The properties which we will discuss include brightness, distance, temperature, and mass. We shall begin with a discussion of distance.

The distance to a star is often the first question asked by someone seeing that star in the sky. For nearby stars, astronomers use a method referred to as parallax. For a simplified example of parallax hold the index finger of each hand up in front of one eye closing the other eye. Your index fingers should be about a foot apart, with the closest finger about a foot away. Now that they are aligned in your open eye, switch eyes. You should note that the nearby finger is no longer aligned with the farther finger. The nearby finger has apparently moved because the observation eye has moved.

In the case of astronomical parallax, the earth at one time of year is one eye and six months later becomes the other eye. Just as the nearby finger seemed to move when switching eyes, so too does the nearby star apparently move with respect to the farther star in that six-month time. By measuring that motion, and by knowing the distance the earth travels from one side of the sun to the other, the astronomer is able to determine the distance to the nearby star.

Referencing the diagram, the angle q is inversely proportional to the distance D. That is to say, as D increases, q decreases. The unit referred to as the parsec finds its origins in parallax. Its name comes from the term "parallax arc second." It is the distance to a star showing a parallax of one arcsecond when observed from Earth. The parsec is approximately 3.26 light years. It should be emphasized that this technique becomes ineffective as we attempt to use it on further stars. Earth based telescopes are capable of measuring parallax to about 100 parsecs. Space based telescopes can measure to almost 1000 parsecs. Methods used for more distant stars shall be discussed later.

Brightness and Temperature

Size and the H-R Diagram

This page was last updated on 06/13/01.