Magnitude: What is it?
Magnitude is a measurement of an object’s brightness, but it is not a direct measurement like weight, length, or time. When people first started to ask questions about stars, the technology did not exist to measure amounts of light in the way a ruler measures length. As a result, observers developed a system that compares one star’s brightness to another. When you say one person is twice as tall as another, no units are needed. So, when we measure the magnitude of an object in the sky, we don’t attach a unit (like kilograms, meters, or seconds) to the end.
Astronomers have agreed to use the star, Vega, as the comparison (standard) star. To make this possible, Vega is defined to have an apparent magnitude of approximately zero. This is the same way that the temperature at which pure water freezes is the standard for the zero point on the Celsius temperature scale. Astronomers often refer to two types of measured brightness: absolute and apparent magnitudes.
Apparent magnitudes tell us how much brighter or fainter something is relative to a standard star from our perspective on the Earth. But lots of objects in the sky appear faint to us simply because they are far away, so a separate measurement called absolute magnitude is used to compare how bright an object would appear relative to others if they were all the same distance away. Absolute magnitude measurements can be tricky, however, because they require knowing an object’s apparent magnitude and its distance, and it’s often very difficult to know exactly how far away something is in space. So, we’ll ignore absolute magnitudes for the meantime – yet this is a good thing to keep in mind!
For historical reasons, objects in the sky (such as other stars) that appear fainter than the star Vega have positive apparent magnitudes; likewise, objects in the sky (like the full moon) that appear brighter than Vega have negative apparent magnitudes. In the case of the full moon, the apparent magnitude is -13. The Sun has a whopping apparent magnitude of -27! Venus, which you can often see even in the morning sky, is quite bright… with an apparent magnitude of -5. But Pluto, which you couldn’t see without a telescope, has an apparent magnitude of 14.
So, the magnitude scale runs in the opposite direction to what you might expect… the larger the magnitude, the fainter the object! Think of the magnitude scale as a race with the winners coming in first. It is easier to think of the brightest stars in a brightness race in first place and the dimmer objects in 4th, 5th, or even 23rd place.