What are magnitudes?

Image courtesy Sky & Telescope

In order to quantify a star’s brightness, astronomers use a “stellar magnitude” system. The star’s “magnitude” or brightness we observe depends both on the star’s intrinsic brightness and its distance from Earth. Based on this, each star is assigned an apparent magnitude number. Because the magnitude scale is a relative scale, there is a “zero point magnitude” to which all other stars we see are compared. For example, the star Vega, found in the constellation Lyra, has an apparent magnitude of zero.

Objects brighter than Vega have negative magnitudes (for example, Sirius has a magnitude of -1.46 and the Sun’s magnitude is -26.74). However, nearly every object in the sky is dimmer than Vega and will have magnitudes greater than zero. The dimmest objects we can see with the naked eye are magnitude 7, and with the aid of telescopes, we can measure up to 25th magnitude.

Remember, the larger the apparent magnitude, the dimmer or fainter the object!

When looking at the sky, the darker the sky, the more faint stars you can see, hence the limiting magnitude is greater. And this indicates less light pollution!

If this confuses you, don’t feel bad! It’s confusing at first for many astronomers too!

Familiarize yourself with the Magnitude Charts for the Globe at Night Constellations.