Question: Why are some stars brighter than the other, such as the star Sirius/Dog Star, which is the 'brightest star'?

Hester Baird answered on 15 Mar 2021:

There are three main reasons that some stars look brighter than others. The first reason is the distance of the star from Earth. When a star emits light, the light travels outwards from it in all directions. The further you are away from the star, the larger the area that the light is spread over, so the light is less concentrated. If you had two identical stars, but one was further away from us than the other, the one that is further away would look dimmer. The second reason why a star might be brighter is because of the star itself – there are different types of stars and some emit more light (are more ‘luminous’) than others. Bigger and hotter stars will be more luminous than smaller, cooler stars. The third reason that a star might appear dimmer is because of dust and gas between us and the star that absorbs some of light.
Sirius is so bright because it is one of the closest stars to us, and it is also pretty luminous (I had to look this up – it is about 25 times more luminous than the Sun).