The red hypergiant star UY Scuti seen in visible light.
Below is a list of the largest known stars by radius. The unit of measurement used is the radius of the Sun (approximately 695,500 kilometers, or 432,450 miles).
The exact order of this list is not complete, nor is it perfectly defined:
There are sometimes high uncertainties in derived values and sizes;
The distances to most of these stars are uncertain to differing degrees and this uncertainty affects the size measurements;
All the stars in this list have extended atmospheres, many are embedded in mostly opaque dust shells or disks, and most pulsate, such that their radii are not well defined;
There are theoretical reasons for expecting that no stars in our galaxy are larger than approximately 1,500 times the sun, based on evolutionary models and the Hayashi instability zone. The exact limit depends on the metallicity of the star, so for example supergiants in the Magellanic Clouds have slightly different limiting temperature and luminosity. Stars exceeding the limit have been seen to undergo large eruptions and to change their spectral type over just a few months;
A survey of the Magellanic Clouds has catalogued most of the red supergiants and 44 of them are larger than the 700 solar radii cutoff point of this table, with the largest at 1,200-1,300.
NML Cyg is an unusual hypergiant star surrounded by a circumstellar nebula and is polluted by severe dust extinction. The quoted size is the most likely value; the true range is between 1,642 to 2,775.
RW Cep is variable both in brightness (by at least a factor of 3) and spectral type (observed from G8 to M), thus probably also in diameter. Because the spectral type and temperature at maximum luminosity are not known, the quoted size are just estimates.
KY Cygni is located in a region with heavy dust extinction, thus making it hard to determine its size. The quoted size is the value consistent with stellar evolutionary models, the true range may be larger.
^ abcArroyo-Torres, B.; Wittkowski, M.; Marcaide, J. M.; Hauschildt, P. H. (2013). "The atmospheric structure and fundamental parameters of the red supergiants AH Scorpii, UY Scuti, and KW Sagittarii". Astronomy & Astrophysics554: A76. doi:10.1051/0004-6361/201220920.edit
^Wright, N. J.; Wesson, R.; Drew, J. E.; Barentsen, G.; Barlow, M. J.; Walsh, J. R.; Zijlstra, A.; Drake, J. J.; Eisloffel, J.; Farnhill, H. J. (16 October 2013). "The ionized nebula surrounding the red supergiant W26 in Westerlund 1". Monthly Notices of the Royal Astronomical Society: Letters437 (1): L1–L5. doi:10.1093/mnrasl/slt127.
^ abcdNicolas Mauron; Eric Josselin (2010). "The mass-loss rates of red supergiants and the de Jager prescription". arXiv:1010.5369 [astro-ph.SR].
^o. Chesneau; a. Meilland; e. Chapellier; f. Millour; a.m. Van Genderen; y. Naze; n. Smith; a. Spang; Et Al. (2014). "The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in the common envelope phase". Astronomy & Astrophysics. doi:10.1051/0004-6361/201322421.edit
^Kusuno, K.; Asaki, Y.; Imai, H.; Oyama, T. (2013). "Distance and Proper Motion Measurement of the Red Supergiant, Pz Cas, in Very Long Baseline Interferometry H2O Maser Astrometry". The Astrophysical Journal774 (2): 107. doi:10.1088/0004-637X/774/2/107.edit