There's no single "brightest star" in the entire universe, as we can't see everything. However, we can look at the brightest stars that we *can* see from Earth.

Here's a breakdown:

* By apparent magnitude (how bright they appear from Earth): This is what most people mean when they ask about the brightest star. The title belongs to Sirius, a binary star system in the constellation Canis Major. It's about 8.6 light-years away and appears incredibly bright due to its proximity and intrinsic luminosity.

* By absolute magnitude (intrinsic brightness): This measures how bright a star actually is, regardless of its distance. While we can't definitively say which star is the absolute brightest in the universe, R136a1, a massive star in the Tarantula Nebula, is currently considered the most luminous star known. It's estimated to be about 8.7 million times brighter than our Sun!

Important to note:

* Our understanding of the universe is constantly evolving. New discoveries are always being made, and what we consider the "brightest" could change.

* Brightness can be affected by factors like dust clouds, making it difficult to compare stars across vast distances.

So, while Sirius is the brightest star we see from Earth, R136a1 is likely the most luminous star we've found so far.