A Star is a luminous body of gas. Mostly hydrogen and helium.star generate light, which makes it possible for use to see them with a telescope or the maked eye. They also release energy from nuclear fission reactions in their core.
Brightest
Not counting the sun, the brightest star as seen from earth is Sirius , know as the dog star , in the constellation of coins major it is a diameter of 149598020 km and is more then 24 times brighter then sun . The star Cygnus OB2 No 12 , discovered in 1992, is so far away that it cannot been seen form earth . It's may be the brightest in the galaxy _ up to six million time as a bright as our sun.
Largest
Our largest star is the M_class supergiant Betelgeuse, or alpha Orionid. It is the top left star in the constellation of Orion, which is 310 light year away. It has a diameter of 700 million km, which is about 500 times greater than that of the sun.