Can we measure a star's brightness ?

Yes, a star's brightness is called its magnitude. A star of magnitude 0 is very bright. Stars of magnitude 5 are only just visible. Really bright stars have minus magnitudes. Sirius the brightest star in the sky, has a magnitude of - 1.5.Sirius is about 26 times brighter than the Sun.

The brightness of celestial objects is expressed in magnitudes with each magnitude change equal to the 5th root of 100 or about 2.5119. Also, the brighter an object the smaller the magnitude.

Sun has an extremely bright magnitude of -26.72 and the Moon's magnitude is -12.6.However, this is due to the fact that the Sun is very close to Earth. If all stars were to be seen at the same distance, then the ONLY factor affecting its magnitude would be its intrinsic brightness. Astronomers have chosen the distance of ten parsecs (32.59 Light Years) as the arbitrary point at which all stars would be compared.