Part 3B: Magnitude

In the Introduction to Section 3, the term “apparent magnitude” was used.

The light curve we will be constructing is a plot of the change in apparent magnitude of the asteroid versus the time of observation.

This part, Part B of Section 3, explores the meaning of the phrase “magnitude”.

Then, in the next parts, we will connect this term to photometry and the use of “standard stars”, which is the method we will use to find the apparent magnitude. Are you ready?


If you live in a city or if you have low vision, you might not know that some stars are much brighter than others. Up until the late 1890’s, the only way people could assign a brightness value to stars was by comparing one star to another.

Over two thousand years ago, Hipparchus, a Greek astronomer, made a catalog of stars using this method. He assigned a brightness class to each of the stars in his catalog. There were six classes.

The dimmest star visible in the night sky at the time was labeled “sixth class.” The brightest star in the night sky, Vega, was a first-class star.

This scale remains with us today, much to the confusion of beginners who tend to think that brighter stars should have bigger numbers.

To make matters worse, today we refer to the brightness of a star as its magnitude. The word magnitude makes people think of size, which should go up with each number.

To avoid confusion, think of the magnitude scale as a race where the most powerful car comes in first, in the same way that the brightest stars are first class, or first magnitude.


Using your journal, number your answers to each corresponding question.

Imagine three stars: Star A has a magnitude of 2. Star B has a magnitude of 5 and Star C has a magnitude of 9.

1. Which star is the brightest?

2. Which star(s) could have been in the catalog that Hipparchus made?

3. Do you think it would be more or less difficult for a person who had never seen stars to understand the magnitude scale? Why?

4. What do you think it means if the magnitude has a negative sign, for example, -2.5?


Wait! There’s one more complication. When we assign a magnitude to a star, we have no idea how far away it is.

Back when Hipparchus was creating his catalog, he had no idea how stars were distributed in space. He was only thinking about how bright each star was.

Today we know a lot more, so astronomers are very careful when talking about brightness.

Apparent magnitude is a measure of an object’s brightness as it appears to us from Earth.

Just like sounds are fainter at greater distances, brightness is affected by distance, too.

Apparent magnitude does not consider the effect of distance.

Another term, absolute magnitude, does. It will be precisely defined later.


5. Below is a list of four objects and their apparent magnitudes. They are listed in order of their distance from Earth. Answer the questions below concerning these four objects:

  • Full Moon: -12.5
  • Sun: -26.7
  • Neptune: 7.8
  • Rigel (star): 0.13 (a very bright bluish star in the constellation Orion)

5A. Think about what you know about each object. Now make a new list, from most bright to least bright, of how each object would appear if they were the SAME distance away from you. (This is actually called the ABSOLUTE magnitude: how bright the object would appear if it were a standard distance of 10 parsecs from Earth. If you do not know what a parsec is, do a quick search!)

5B. Now look back at the original list of APPARENT magnitudes. Using the numbers given, list in order from most bright to least bright appearing.

5C. Why are the lists from 5A and 5B different?