The universe is a big place, and it is home to a lot of big objects. For humans, contemplating even the size of Earth can be a struggle. It can be even more uncomfortable to consider that the planet under our feet is barely a speck when compared to the star it rolls its constant path around: the Sun. It is even more alarming still to think that the Sun is only an average-sized star. The enormity of some of the stellar inhabitants of the universe can be both terrifying and wonderful.
So which is the emperor of the stars, the stellar colossus, the leviathan of the skies? Well, it is called...
Actually, science is rarely as simple as a clear cut answer. There are a few possiblities.
The first candidate is NML Cygni. This star is classified as a red hypergiant (one step up from a supergiant, which is one step up from a giant). A red hypergiant is the next stage in the life of a main sequence (or Sun-like) star, but one that hosts over 40 times the mass of the Sun. As the main sequence star runs out of fuel, the pressure in its core increases until it billows out in a supernova explosion, extending its stellar matter through empty space. In the case of NML Cygni, it has puffed up to a luxurious 1,605 times the radius of the Sun. Were it to settle down where our Sun currently lies, it would extend beyond the orbit of Jupiter.
Then there's the astronomical wild card: UY Scuti. This star is predicted to be 1,708 times the radius of the Sun, even wider than NML Cygni, but with an error of ±192 solar radii. This error is no trivial matter for UY Scuti since the uncertainty is preventing it from cementing its claim as biggest star in the universe.
If UY Scuti is at the smaller end of its error margins (1,516 solar radii instead of 1,708), then there are four or five stars bigger than it (again, accounting for measurement errors). These candidates include the above NML Cygni, a hyperstar called RW Cephei that glows orange, and the memorably named WOH G64 hypergiant star.
Uncertainty in the size of these objects is an inevitable obstacle when squinting out into the unknowable depths of the cosmos. Sometimes stars are hidden behind a mask of stellar dust, obscuring them from view. (There's no way of simply blowing the matter away, like dust off an old book). Another challenge to overcome is that stars become dimmer the further away we look – maybe too dim to even see. And yet another obstacle is the limited sphere of light we can view: We can only see (give or take) 13 billion light-years away. It's possible that even larger monster stars lurk in a region of the universe yet to be discovered.
Fortunately, our quest for discovery isn't over. Telescopes continue to get bigger and better as scientists push the boundaries of technology. Only recently, the scientific community knew of no star larger than VY Canis Majoris. After a mixture of improvements to calculations and wider observations of the sky, it was knocked down the ranks of largest stars. Now, a whole new host of 'big kids' dominate the stellar playground.
Who knows what other stellar titans might dwarf even these stars in the future?