The title of "biggest known star" is a bit tricky, as there are different ways to define "biggest."

Here are the contenders, depending on how you measure size:

* By radius: The current record holder for largest radius is UY Scuti. It's a red supergiant estimated to be around 1,700 times the radius of our Sun. This means if it were placed in our Solar System, it would extend out past Jupiter's orbit.

* By mass: While UY Scuti is the largest in radius, it's not the most massive star. That title likely belongs to R136a1, a blue hypergiant estimated to be about 265 times the mass of our Sun. This makes it incredibly dense and hot, burning through its fuel very quickly.

Important note: These measurements are all estimates, and the exact size of these stars is difficult to determine with certainty. The universe is vast, and there might be even larger stars out there that we haven't discovered yet!