What is the biggest star ever seen or detected?

Answer 1

In the early days of the universe really large stars were quite common, They generally were short-lived and ended as supernova and enriched the interstellar medium with the metals they produced when going supernova, such that stars that were born later inherited the raw materials both for rocky planets and for organic chemistry based on carbon and therefore life to be created.

But these days such giant stars are relatively rarer. For a long time VV Cephei was thought the largest known star.

VV Cephei is an eclipsing binary star system located in the constellation Cepheus. It contains a red supergiant which fills its Roche lobe when closest to its companion blue star, which appears to be on the main sequence; matter flows from the supergiant onto the blue companion. The stars are located about 3000 light years away from Earth.

VV Cephei A

VV Cephei A, the supergiant, is one of the largest stars known. It is of spectral type M2 and is about 1600-1900 times the Sun's diameter. If it replaced the Sun in our solar system, it would extend to the orbit of Saturn. It is 275,000-575,000 times as luminous as the Sun.

VV Cephei B
VV Cephei B, the blue main sequence star, is separated from the larger star by a distance of 25AU on average. And the distance is changing between 17 and 34 AU. It is a B0 star about 10 times the Sun's diameter and about 100,000 times its luminosity. The orbital period of the system is 7430 days (20.3 years). The eclipse lasts about 1300 days, and the midpoint of the last eclipse was in early January 1998.

HOWEVER

VV Cephei has recently been eclipsed (in the other sense of the word) by VY Canis Majoris

VY Canis Majoris (VY CMa) is a red supergiant. This is the largest known star and one of the most luminous stars known.

Roberta M. Humphreys (2006) estimates the radius of VY CMa is between 1800 and 2100 solar radii.

IN COMPARISON

Betelgeuse: 650 Solar Diameters (12th largest)
Eta Carinae: 400 Solar Diameters (26th largest)

TOP THIRTY (with size in solar diameters)

VY Canis Majoris 1950 (1800-2100)
VV Cephei 1750 (1600-1900)
V354 Cephei 1520
KW Sagitarii 1460
KY Cygni 1420

Mu Cephei (Herschel's "Garnet Star") 1420
V509 Cassiopeiae 910
V838 Monocerotis 800
V382 Carinae 747
Rho Cassiopeiae 738

Antares (Alpha Scorpii) 700
Betelgeuse (Alpha Orionis) 650
S Pegasi 580
S Doradus 550
T Cepheii 540

S Orionis 530
W Hydrae 520
Y1 Aurigae 511
119 Tauri 510
R Cassiopeiae 500

Delta Canis Majoris (Wezen) 482
Chi Cygni 470
J Cassiopeiae 460
Alpha Herculis (Ras Algethi) 460
Mira A (Omicron Ceti) 400

Eta Carinae 400
R Doradus 370
HR Carinae 350
R Leonis 350
The Pistol Star 340

Answer 2

I believe that Betelgeuse is the biggest.

Answer 3

Its in the southern sky and it is called, Eta Carinae.

Answer 4

the biggest known star is uy scuti.....check it out on you tube