The Sun appears to be the brightest star in the sky because it is the closest star to Earth. It is about 150 million kilometers away from our planet, whereas other stars are much farther away. The Sun is also very large, with a diameter of about 1.4 million kilometers, and it emits a tremendous amount of energy in the form of light and heat. This makes it appear much brighter than other stars, which are much smaller and farther away.

In addition, the Sun's light is not scattered or absorbed by the Earth's atmosphere as much as the light from other stars. This is because the Sun is located directly above the Earth, whereas other stars are located at different angles to the Earth's surface. The Earth's atmosphere scatters and absorbs more light from stars that are located at lower angles to the horizon.

As a result of all of these factors, the Sun appears to be the brightest star in the sky.