GO IN DEPTH

# Star Light Teacher Sheet

Photo Credit: Clipart.com

### Introduction

In the Motivation, you discussed relative brightness with your students. You will now expand on that concept by having your students study apparent magnitude using some online resources. This sheet provides answers to the questions students are asked about these online resources.

Describe what is meant by the term apparent magnitude.
Apparent magnitude is the observed brightness of stars as expressed in the modern version of the ancient scale to express magnitude.

Describe the scale that is used for apparent magnitude. Do brighter stars have higher or lower magnitudes?
The scale was first developed by a Greek mathematician, Hipparchus, with six categories of brightness. Scientists have expanded that scale. The lower the magnitude, the brighter the object.

Describe the difference between luminosity and apparent magnitude.
Luminosity is the actual light energy produced by a star. Observed brightness is the brightness of a star that we see from Earth.

Why does luminosity not depend on the distance of a star?
Luminosity is the measure of energy emitted from the star, independent of where it is and how far it is from Earth.

Why can a star that is very high in luminosity be of high apparent magnitude?
Although the light energy being emitted by the star may be very high, its observed brightness may be low—or apparent magnitude may be high—due to its distance from Earth.

### The Relation Between Brightness and Luminosity

Describe the mathematical relationship among luminosity, distance, and brightness.
The brightness of a star is proportional to its luminosity divided by its distance from the observer squared.

### How Far is a Light-Year?

How much distance in kilometers and miles does light travel in a single second?
Light travels 300,000 kilometers or 186,000 miles per second.

In one year, approximately how many kilometers and miles does light travel?
Light travels 9.5 trillion kilometers or 5.88 trillion miles in a year.

An astronomical unit is the distance between what two objects in the galaxy?
It is the distance between the Earth and the sun.

An astronomical unit is how many kilometers and miles?
It is 150 million kilometers or 93 million miles.

How long does it take light to travel one astronomical unit?