Nearly every star that you see with the unaided eye is larger and more luminous than our sun. Astronomers speak of a star’s true brightness as its “luminosity.”
Some stars look bright because they’re near Earth. Others are truly extremely bright members of our Milky Way galaxy. Astronomers call the true, intrinsic brightness of a star its luminosity. The luminosity of any star depends on size and surface temperature. Some extremely large and hot stars blaze away with the luminosity of a million suns!
Read the full explanation at Earthsky.org