The Pistol Star appears as the bright white dot in the center of the image shown left.The largest star measured in solar diameters is either VV Cephei or VY Canis Majoris (right image)
"What's the Biggest Star in the Universe"? Generally students ask this question.
I may give an easy answer, the Universe is a big place, and there's no way we can possibly know what the biggest star is. That didn't go over so well.
However the question may be refined:- What's the biggest star that we know of ?
Pistol Star: The largest known star, in terms of mass and brightness is called the Pistol Star. It is believed to be 100 times as massive as our Sun, and 10,000,000 times as bright! In 1990, a star named the Pistol Star was known to lie at the center of the Pistol Nebula in the Milky Way Galaxy. Observations from the Hubble Space Telescope in 1997 confirmed the relationship between the star and the nebula.
Eta Carinae: One huge, famous star in our galaxy is the monster Eta Carinae, located approximately 7,500 light years away, and weighing in at 100 solar masses. It's 4 million times as bright as the Sun. Most stars blow with a solar wind, losing mass over time. But Eta Carinae is so large that it casts off 500 times the mass of the Earth every year. With so much mass lost, it's very difficult for astronomers to accurately measure where the star ends, and its stellar wind begins.
So the best answer astronomers have right now is that Eta Carinae's radius is 400 times the size of the Sun. Eta Carinae should explode pretty soon as one of the most spectacular supernovae humans have ever seen.
VY Canis Majoris: The largest known star is VY Canis Majoris; a red hypergiant star in the constellation Canis Major, located about 5,000 light-years from Earth. University of Minnesota professor Roberta Humphreys recently calculated its upper size at more than 2,100 times the size of the Sun. Placed in our Solar System, its surface would extend out past the orbit of Saturn. Light takes more than 8 hours to cross its circumference!
Some astronomers disagree, and think that VY Canis Majoris might be smaller; merely 600 times the size of the Sun, extending past the orbit of Mars.
That's the biggest star that we know of, but the Milky way probably has dozens of stars that are even larger, obscured by gas and dust so we can't see them.
But let's see if we can work out the original question, what's the biggest star in the Universe?
Obviously, it's impossible for us to actually find it - the Universe is a big place, and there's no way we can peer into every corner. The largest stars are the coolest. So even though Eta Carinae is the most luminous star we know of, it's extremely hot - 25,000 Kelvin - and so only a mere 400 solar radii.
Whatsoever,
Lots of half-truths and old information are being recycled partly because people don't read the question. The largest star measured in solar diameters is either VV Cephei or VY Canis Majoris,
As the table below makes clear,
LIST OF THE LARGEST KNOWN STARS
Star name / solar diameter (Sun = 1)
1st VY Canis Majoris (Humphreys Model) 1800-2100
2nd VV Cephei 1600-1900
3rd V354 Cephei 1520
4th KW Sagitarii 1460
5th KY Cygni 1420
6th Mu Cephei (Herschel's "Garnet Star") 1420
7th V509 Cassiopeiae 910
8th V838 Monocerotis 800
9th V382 Carinae 747
10th Antares (Alpha Scorpii) 700
11th Betelgeuse (Alpha Orionis) 650
or 11th VY Canis Majoris (Massey-Levesque-Plez Model) 600
(in which case the rest are all one place higher)
Other candidates mentioned by other contributors are:
24th Eta Carinae 400
28th The Pistol Star 340
30th Deneb 220
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