STARS I: INTRODUCTION

Stars (like everything else) have intrinsic properties and observed properties. An intrinsic property is a true property of an object, like height for a person. An observed property is one that depends upon how you look at the object.

Four intrinisc properties of stars are:

Luminosity - the amount of light energy emitted by the star per second

Mass - amount of matter bound in object

Size - the radius of the star

Temperature - temperature of the photosphere

There are also two important observed properties of stars...

Brightness - this is the measured light energy, and it depends upon your distance to the star

Distance - how far away the star is (this is only circumstancial)

For stars, the brightness and the distance can be measured. But, it is the luminosity that matters (because it is intrinic to the star). The luminosity is important because it tells you how much energy the star is consuming and generating. The brightness only tells you how much light you can collect with a telescope (see below). To measure the luminosity of a star you must first know the brightness and the distance to the star. To recap, luminosity can be determined once the brightness distance are measured.

Measuring Brightness

Brightness is measured by adding up all the energy in the lght you collect at a telescope. You must account for how big the telescope is (bigger telescopes collect light more quickly) and for the amount of time you are adding the light (the longer you collect, the more energy you are adding up).

Measuring Distance

Parallax is one method for measuring the distances to stars. It uses trigonometry. To measure a stars distance, one measures the parallax angle. The measurementis as follows: 1. take a photo of a the sky; 2. take a second photo of the sky 6 months later; 3. measure the angle that the star has appeared to move on the sky. This angle is the parallax angle, PA. This angle is measure in arcseconds (or seconds of arc). 1 arcsecond is 1/3600 of a degree; there 360 degrees around a circle; 60 arminutes in 1 degree; and 60 arcseconds in 1 arcminute.

The distance, d, is obtained be computing

d = 1/PA

The distance is measured in Parsecs, 1 Parsec = 3.26 light years.

Computing Luminosity

The brightness of a star decreases with the square of the distance to the star. A star twice the distance away will appear four times dimmer. A star half as far away will appear four times brighter. So, to determine the luminosity, you use the inverse square relationship for light ...

Brightness = Luminosity / (distance)²

This says, that the further away you are from the star (the greater the distance), the smaller the brightness of the star will be for its luminosity. (Think of moving a flashlight further and further away from you into the distance). You solve this equation for Luminosity. In other words, you multiply brightness and the square of the distance.

Measuring Star Masses

The masses of stars are measured in binary systems using the laws of gravity. Stars orbit about a center of mass. The star motions are measured using spectra of the stars and the Doppler shift of their absorption lines. See the Notes again to study this- it is too involved to summarize here.