Below are lists of the largest stars currently known, ordered by radius and separated into categories by galaxy. The unit of measurement used is the radius of the Sun (approximately ).
Although red supergiants are often considered the largest stars, some other star types have been found to temporarily increase significantly in radius, such as during LBV eruptions or luminous red novae. Luminous red novae appear to expand extremely rapidly, reaching thousands to tens of thousands of solar radii within only a few months, significantly larger than the largest red supergiants.
Some studies use models that predict high-accreting Population III or Population I supermassive stars (SMSs) in the very early universe could have evolved "red supergiant protostars". These protostars are thought to have accretion rates larger than the rate of contraction, resulting in lower temperatures but with radii reaching up to many tens of thousands of , comparable to some of the largest known black holes.
The angular diameters of stars can be measured directly using stellar interferometry. Other methods can use lunar occultations or from eclipsing binaries, which can be used to test indirect methods of finding stellar radii. Only a few supergiants can be occulted by the Moon, including Antares and 119 Tauri. Examples of eclipsing binaries are Epsilon Aurigae (Almaaz), VV Cephei, and V766 Centauri (HR 5171). Angular diameter measurements can be inconsistent because the boundary of the very tenuous atmosphere (opacity) differs depending on the wavelength of light in which the star is observed.
Uncertainties remain with the membership and order of the lists, especially when deriving various parameters used in calculations, such as stellar luminosity and effective temperature. Often stellar radii can only be expressed as an average or be within a large range of values. Values for stellar radii vary significantly in different sources and for different observation methods.
All the sizes stated in these lists have inaccuracies and may be disputed. The lists are still a work in progress and parameters are prone to change.
Various issues exist in determining accurate radii of the largest stars, which in many cases do display significant errors. The following lists are generally based on various considerations or assumptions; these include:
The following lists show the largest known stars based on the host galaxy.
Note that this list does not include the candidate JWST dark stars, with estimated radii of up to or quasi-stars, with theoretical models suggesting that they could reach radii of up to .
During some transient events, such as red novae or LBV eruptions the star's radius can increase by a significant amount.
The following list include the largest stars by their apparent size (angular diameter) as seen from Earth. The unit of measurement is the milliarcsecond (mas), equivalent to . Stars with angular diameters larger than 13milliarcseconds are included.