Stars are immense balls of burning plasma that are held together by their own gravity. The star that we are most familiar with is our Sun. However, compared to many of the other stars in the Universe our central stars is, well, kinda puny. Below is a list of the 10 largest (by diameter) stars known to exist in the Universe.
A few caveats: First, this is only known stars, there could be larger ones out there. Second, some of these stars are variable, meaning that they regularly expand and shrink. And lastly, there is, like virtually all astronomy measurements, an inherent bit of error. But it should be fairly close.
1. VY Canis Majoris
This red hypergiant star is by far the largest known. It has an estimated radius between 1800 and 2100 times the radius of our Sun. With this size it would reach nearly to the orbit of Saturn if placed in our solar system. The star is located roughly 4,900 light-years from Earth in the constellation Canis Majoris.
2. VV Cephei A
Located in the constellation Cepheus, about 2,400 light-years from Earth, this red hypergiant star is estimated to be between 1,600 and 1,900 times the radius of the Sun.
3. Mu Cephei
Also located in the constellation Cepheus, this red supergiant is about 1650 times the radius of our Sun. It is also one of the most luminous stars in the Milky Way galaxy, at more than 38,000 times the Sun's luminosity.
4. V838 Monocerotis
This red variable star located in the constellation Monoceros, is about 20,000 light-years from Earth. It may be larger than either Mu Cephei or VV Cephei A, but because of its distance form the Sun its actual size is difficult to determine. Therefore a range is typically given of between 1170 and 1970 solar radii.
5. WOH G64
Once thought to rival Canis Majoris this red hypergiant located in the constellation Dorado has recently been determined to be about 1,540 times the radius of the Sun. It is actually located outside of the Milky Way Galaxy in the Large Magellanic Cloud, another nearby galaxy.
6. V354 Cephei
Slightly smaller than WOH G64, this red hypergiant is 1520 solar radii. At a relatively close 9,000 light-years from Earth, V354 Cephei is located in the constellation Cepheus.
7. KY Cygni
KY Cygni is at least 1420 times the radius of the Sun, but some estimates would have it topping this list at a whopping 2,850 solar radii. However, the actual value is thought to lie closer to the lower bound, bringing it down to number 7 on our list. It is located about 5,000 light-years from Earth in the constellation Cygnus.
8. KW Sagittarii
Representing the constellation Sagittarius, this red supergiant is no slouch at 1460 times the radius of our Sun.
9. RW Cephei
Our third entry from the Constellation Cepheus, this star may not seem all that large in its own neighborhood, but there aren't many others in our galaxy or nearby that can rival it. This red supergiant's radius is not well agreed upon, being placed somewhere between 1260 and 1650 solar radii.
Our final entry, and the last star known to have a radius in excess of 1000 times that of our Sun, Betelgeuse is probably the most well known of the red supergiants. This is partially due to the fact that at roughly 640 light-years form Earth, it is very close compared to the other stars on this list. Also, it lies in perhaps the most famous of all the constellations, Orion. This 1,180 solar radii star is expected to go supernova any time, which would usher in an unprecidenced opportunity to witness such an event so closely.