What is Apparent Brightness?

Instructor: Josh Corbat

Josh has taught Earth Science and Physical Science at the High School level and holds a Master of Education degree from UNC-Chapel Hill.

Looks can be deceiving. This old phrase is particularly true when talking about apparent brightness. In this lesson, we will define apparent brightness, discuss how scientists use it when studying stars, and point out a major drawback.

Introduction and Definition of Apparent Brightness

To best understand apparent brightness, let's start with a scenario. Picture two identical candles. Both candles are on a table in front of you, lit. The brightness of these candles will most likely be the same, right? Now, think of the same two candles, but one is now across the room. Which will appear brighter to you? The candle on the table in front of you will obviously appear brighter because it is closer. This is a perfect way to think about apparent brightness.

You can see the candles toward the back are dimmer than those in the front. It is possible that the back candles are actually brighter, but their distance makes them look dimmer regardless.
Apparent brightness in candles

Apparent brightness, also known as apparent magnitude, is the measure of the luminosity (a fancy word for actual brightness) of a star as seen from Earth. That last part is the most important: as seen from Earth. Apparent brightness is a human measurement, and it would change for each star if the measurement were taken from another location. The more precise counterpart of apparent brightness is called absolute brightness (or absolute magnitude) and is the measure of the luminosity of a star, but on a common scale. Each star's mass and temperature are taken into account to standardize brightness measurements on the absolute brightness scale.

What Apparent Brightness is Used For

In ancient times, scientists made enormous lists and catalogs of stars using measurements like apparent brightness. In those days, apparent brightness was an important measurement in distinguishing stars from each other. Today, though, we understand that stars may appear bright or dim, and that apparent brightness does not take everything important into account. Now apparent brightness is primarily used by backyard astronomers to find certain stars as they stargaze. Apparent brightness can still be useful in this way, especially if you are looking for a particularly bright star. For example, Sirius is the brightest star in our sky (it has the highest apparent brightness), so it would be relatively easy to find it.

A photo of Sirius in the night sky. This is obviously zoomed in, but does a great job showing how much brighter it is than all other stars from our perspective.

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