Here are a few handy terms to keep in mind when reading about the appearance of celestial objects.
Luminosity is the intrinsic brightness of a star—compared to the sun—as it would appear if you were there in orbit around it, rather than viewing it from Earth. The sun's luminosity is 1. Sirius has a luminosity of 23 and Betelgeuse has a luminosity of 55,000.
Brightness is the light given off by a celestial object as seen from Earth. Brightness depends on luminosity and the distance from the object.
Magnitude is a logarithmic brightness scale. Magnitude-one objects are 2.512 times brighter than magnitude-two objects, which are 2.512 times brighter than magnitude-three objects, and so on. The difference between magnitude one and magnitude five is one hundred times. The higher the magnitude, the fainter the object. The lower the magnitude, the brighter the object. The brightest stars have negative magnitudes.