Astronomers define star brightness in terms of appearance magnitude, this means how how bright the star looks from Earth. From Earth we notice that the night sky is illuminated by these stars but in actuality the distance of the stars is light years; over 36 light years away. Sometimes the stars are illuminated so much it seems like if we had a ladder, we could reach up and stretch on our tippy toes and touch them. Astronomers also measure luminosity that a star emits from its surface. This measurement of luminosity is how Astronomers are able to determine how much energy and the size of the star, a larger star will emit more energy because it is more luminous. The way I remember this is that you will notice an elephant with leopard spots more than an ant with leopard spot because the elephant is much larger in size.
During my post, I found an interesting article at https://astronomynow.com/2017/02/13/see-planet-venus-hit-peak-brightness-in-the-evening-sky/ this article is really cool because it shows how Venus reached its peak brightness. Astronomers have been documenting luminosity data for thousands of years but on February 17th 2013 Venus reached a luminosity of 4.8.
Joliet Junior College - Astronomy 101 class blog 