Is the following sentence grammatically correct?
Sirius, the Dog Star, is the most brightest star in the sky with an absolute magnitude about twenty-three times that of the Sun. 

The correct answer to the question “Is the following sentence grammatically correct?” “Sirius, the Dog Star, is the most brightest star in the sky with an absolute magnitude about twenty-three times that of the Sun.” At first look, of course, some people may say that there is nothing wrong with the question.

However, if you take a careful look, you will notice the words “most brightest”. In English, we are taught about comparative and superlative adjectives. Since the word brightest comes from the word bright, then the rule about comparative adjectives is to ad -er, and add -est on superlatives. Since brightest is already a superlative adjective, then we no longer have to add the word “most” before it. Hope this helps.