I never heard of R-136
I never heard of R-136
From Earth the Sun is obviously the brightest star. Otherwise Sirius (The Dog Star) is brightest star discovered so far with a magnitude of -1.42. One of the most luminous stars in the Galaxy is called The Pistol Star (but dust hides it from view) Yes, from Earth the Sun is the brightest star. But that's not the question: the brightest star in the universe is certainly not the Sun, nor is it Sirius; Spica for instance, is over 10 times the mass of the Sun, thus having a far greater luminosity. But the light years separating Spica from Earth reduces it to the 15th brightest star in the nighttime sky. And the Pistol Star is only one of the brightest stars in the Milky Way Galaxy... The universe however, houses many stars superior to the Pistol Star. The problem with the question is it refers to such an astronomical entity, the vast universe. And as such, it may never be able to be answered with complete accuracy... That isn't the question either, the brightest star known in the universe is R136a1, a blue hyper-giant which is not only the most massive star known, at 265 solar masses, but the brightest. It shines at 8,700,000 times the luminosity of the sun and is a member of the R136, a super star cluster around the centre of the tarantula nebula in the the magellanic cloud, a satellite galaxy of the Milky way.
The most massive star (The star with the most mass) is a star in a super star cluster called R136 or RMC 136 in the Large Magellanic Cloud (LMC). [See related link for more information] The star, for all its fame has a rather unremarkable name of R136a1. It has the mass of about 265 -> 300 solar masses (256 x the mass of the Sun - See related question). See related link [BBC] for a pictorial size comparison. It is a blue supergiant with a spectral type of O3. Because of its mass and intense luminosity (10 million times that of our own Sun) it, astronomically, will have a very short lifetime in the millions of years rather than billions. At the end of its life, R136a1 will explode as a supernova and because of its relatively close distance to Earth (165,000 light years/ALU [See related question] it might be visible during the day. This question is about the most massive star (mass), not the largest (Radius) [See related question]