What is the name of the brightest star in the night sky?

Sirius (or Alpha Canis Majoris) is the brightest star in the sky and the main star in the constellation "The Big Dog". For that reason it is also known as the Dog Star.

In the evening, when the sun has just set, you can often see Sirius flashing in many colors. This is because the atmosphere is still cooling and the air is therefore unstable. It's not because the star is twinkling.