More often than not in astronomy stories, you’ll hear us refer to the magnitude of a star or planet. But what exactly does that mean?
Magnitude is a term which indicates an object’s degree of brightness. The lower the figure of magnitude, the brighter the object. The brightest stars in the sky are generally either zero or first magnitude, while the dimmest stars – those at the threshold of our vision without the aid of binoculars or a telescope – are sixth magnitude.
In many astronomy textbooks, magnitude “bins” usually categorize star brightness. Any star that is between magnitudes 2.50 to 3.49 is designated “3rd magnitude.” But in the sky, stars at the extremes of that range can differ in apparent brightness by a factor of 2½.
Conversely, two stars in the sky that are of magnitude 2.49 and 2.50 may appear equally bright, yet one will be classified as 2nd magnitude and the other as 3rd magnitude.
The brightest star in the sky is Sirius, in Canis Major. It shines at a dazzling magnitude of minus 1.44. Its blinding presence overwhelms a neighboring star, Adhara, which shines at magnitude +1.50 and ranks as the twenty-second brightest overall in the sky, yet by virtue of binning, misses by just a shade, the cutoff for first-magnitude classification.
Here’s a dramatic example of how appearances can be deceiving: Adhara is actually 158 times more luminous that Sirius. But because it’s 410 light years away, as compared to just 8.6 for Sirius, Adhara appears only 1/15 as bright.
So the next time you’re wishing upon a star, take a second to think about the number it may be on the magnitude scale.
Joe Rao is an esteemed astronomer who writes for Space.com, Sky & Telescope and Natural History Magazine. Mr. Rao is a regular contributor to the Farmers' Almanac who serves as an associate lecturer for the Hayden Planetarium in New York City.