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 695,700 km; 432,300 mi).
Overview 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 R☉, comparable to some of the largest known black holes.
Angular diameters 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.
Caveats 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:
Stellar radii or diameters are usually derived only approximately using the Stefan–Boltzmann law for the deduced stellar luminosity and effective surface temperature. Stellar distances, and their errors, for most stars, remain uncertain or poorly determined. Many extended supergiant atmospheres also significantly change in size over time, regularly or irregularly pulsating over several months or years as variable stars. This makes adopted luminosities poorly known and may significantly change the quoted radii. Other direct methods for determining stellar radii rely on lunar occultations or from eclipses in binary systems. This is only possible for a very small number of stars. Many distance estimates for red supergiants come from stellar cluster or association membership, because it is difficult to calculate accurate distances for red supergiants that are not part of any cluster or association. In these lists are some examples of extremely distant extragalactic stars, which may have slightly different properties and natures than the currently largest known stars in the Milky Way. For example, some red supergiants in the Magellanic Clouds are suspected to have slightly different limiting temperatures and luminosities. Such stars may exceed accepted limits by undergoing large eruptions or changing their spectral types over just a few months (or potentially years).
Lists
The following lists show the largest known stars based on the host galaxy.
Milky Way
Magellanic Clouds
Andromeda (M31) and Triangulum (M33) galaxies
Other galaxies (within the Local Group)
Outside the Local Group (inside the Virgo supercluster)
Outside the Virgo supercluster Note that this list does not include the candidate JWST dark stars, with estimated radii of up to 61 astronomical units (13,000 R☉) or quasi-stars, with theoretical models suggesting that they could reach radii of up to 40,700 solar radii (189 au).
Transient events During some transient events, such as red novae or LBV eruptions the star's radius can increase by a significant amount.
SN Progenitors
Stars with poorly-defined radii
Largest stars by apparent size 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 10×10−3 arcseconds. Stars with angular diameters larger than 13 milliarcseconds are included.
See also
Notes
References
External links Giant Stars An interactive website comparing the Earth and the Sun to some of the largest known stars Three largest stars identified BBC News What is the Biggest Star in the Universe? Universe Today