Below is a list of the largest stars currently known, ordered by radius. The unit of measurement used is the radius of the Sun. The exact order of this list is incomplete, as great uncertainties remain, 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 within a large range of values. Values for stellar radii vary significantly in sources and throughout the literature, mostly as the boundary of the very tenuous atmosphere greatly differs depending on the wavelength of light in which the star is observed. Radii of several stars can be directly obtained by stellar interferometry. Other methods can use lunar occultations or from eclipsing binaries, which can be used to test other indirect methods of finding true stellar size. Only a few useful supergiant stars can be occulted by the Moon, including Antares. Examples of eclipsing binaries are Epsilon Aurigae, VV Cephei, and V766 Centauri.
Caveats
Complex issues exist in determining the true 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:
Largest stars are usually expressed in units of the solar radius, where equals 695,700 kilometres.
Stellar distances, and their errors, for most stars, remain uncertain or poorly determined.
Many supergiant stars have extended atmospheres, and many are embedded within opaque dust shells, making their true effective temperatureshighly uncertain.
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.
Extragalactic large stars
In this list are some examples of more distant extragalactic stars, which may have slightly different properties and natures than the currently largest-known stars in the Milky Way:
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 change their spectral types over just a few months.
A survey of the Magellanic Clouds has catalogued many red supergiants, where many of them exceed. Largest of these is about 1,200-1,300, though a few recent discoveries show stars reaching sizes of >1,500.
