Our sun is really huge some 1 391 400 km in diameter, but 4 billion years from now, it will be even larger, 150 times its current size; though compared to the biggest stars in the universe even this will be small. The top 10 biggest stars ever discovered are red hypergiants; below is a list of the largest stars ordered by radius.
PZ Cassiopeiae: 1340 solar radii (1 solar radii = half the diameter of the sun). It has a luminosity of 270,000 that of the sun. Its an extreme luminous red supergiant star located in the Cassiopeia constellation at 9500 light years from Earth.
VY Canis Majoris: 1420 solar radii. It’s a red hypergiant star located in the constellation Canis Major, 3900 light years away from Earth. This monster is set to explode in a powerful hypernova 100000 years from now and turning into a giant black hole.
KY Cygni: 1420 solar radii. It is a red supergiant of spectral class M3 star located in the constellation Cygnus, 5000 light years away from Earth. It has 25 times the mass of the sun and 300 000 times luminosity; It is losing mass at one of the highest rates known for a red supergiant.
VX Sagittarii: 1520 solar radii. It is a pulsating variable star, varying over 1000° during 732 day periods. Its located more than 1.5 kiloparsec (5100 light years) away from the Sun in the constellation of Sagittarius.
Westerlund 1-26: 1530 solar radii. Surrounded by a cloud of ionized hydrogen, this cloud is believed to be the layers of Westerlund being blown off. W26 is a red supergiant or hypergiant within the outskirts of the Westerlund 1 super star cluster, located 11000 light years away from Earth in the constellation Ara. If placed at the center of the Solar System, its photosphere would engulf the orbit of Jupiter.
WOH G64: 1540 solar radii. This star has so such a low density that its outer layers cannot be held and are being shed. Its a red hypergiant star in the Large Magellanic Cloud satellite galaxy in the southern constellation of Dorado, 168000 light years away from Earth. Its 3.65 billion times bigger than the Sun.
NML Cygni: 1650 solar radii. It’s diameter would be able to cover Earth’s distance from the sun over 7 times. It’s a red hypergiant located within the Cygnus constellation 5300 light years away from Earth. It’s a relatively young star, just 8 million years old compared to our sun (4.5 billion years). It contains a volume approximately 1.6 billion times that of the Sun. If placed at the center of the Solar System, its surface would extend beyond the orbit of Jupiter.
The biggest of the biggest stars: UY Scuti
UY Scuti: 1708 solar radii. It is approximately 9,140 light-years (2,300 parsecs) from Earth, a bright red supergiant and pulsating variable star in the constellation Scutum. It has a volume 5 billion times that of the Sun; Although the radius is not known for certain, if placed at the center of the Solar System it may be larger than the orbit of Saturn. UY Scuti is the biggest of the biggest stars ever observed in the universe. Even if UY Scuti is the largest star in the Milky Way, there could be enormous other stars in far away galaxies somewhere within this indescribably vast universe that dwarf those of our own; alas, we will may never know.