Largest Stars

The largest stars in the known universe are mostly cool red supergiants and hypergiants. Their radii are notoriously difficult to measure for several reasons. For one, the distances to these stars are largely uncertain. They are difficult to calculate accurately for red supergiants that are not members of star clusters or stellar associations.

Stephenson 2-18 (Stephenson 2 DFK 1) is currently the largest star known. The red supergiant or hypergiant star has an estimated radius 2,150 times that of the Sun. However, the value is highly uncertain because it is significantly larger than stellar evolution theory predicts. The upper limit on stellar size is around 1,500 solar radii.

Stephenson 2-18 is a member of Stephenson 2 (Red Supergiant Cluster 1), a massive young open star cluster in the constellation Scutum that contains several other exceptionally large stars. However, several studies have cast doubt on the star’s membership in the cluster, making its distance (and therefore size) uncertain.

Size comparison of a hypothetical quasi-star/black hole star (diameter of ~10 billion kilometres or ~7,187 solar diameters, mass of 1000+ solar masses) and several known giant stars: Stephenson 2-18 (~2150 solar diameters), VY Canis Majoris (~1420 solar diameters, ~17 solar masses), Betelgeuse (~887 solar diameters, ~11.6 solar masses), the Pistol Star (~306 solar diameters, ~27.5 solar masses), Rigel (~78.9 solar diameters, ~23 solar masses), and R136a1 (~35.4 solar diamaters, ~265 solar masses). Image credit: Wikimedia Commons/Sauropodomorph (CC0 1.0)

WOH G64 is the largest star known in the Large Magellanic Cloud (LMC), one of the nearest galaxies to the Milky Way. The currently accepted radius of 1,540 solar radii is on the lower end of size estimates for the star. The red supergiant was once believed to have a radius between 2,575 and 3,000 solar radii.

The distances to red supergiants and giants are always being recalculated based on the latest available data. For this reason, the candidates for the biggest star known have changed several times in recent decades.

RSGC1-F01 is another red supergiant that would extend past the orbit of Jupiter if placed at the centre of the solar system. It is the brightest and probably the largest member of RSGC1 (Red Supergiant Cluster 1), a young open cluster that includes several of the largest stars discovered to date. Like Stephenson 2, RSGC1 lies in the constellation Scutum.

UY Scuti (BD-12 5055), a previous record-holder, was thought to have a radius of 1,708 solar radii based on an assumed distance of 9,500 light-years. With this radius, if the red supergiant star replaced the Sun at the center of our solar system, its photosphere would extend past the orbit of Jupiter.

However, more recent studies place the star considerably closer, at 4,900 light-years away, which yields a radius of about 755 solar radii. Based on the latest astrometric data (Gaia Early Data Release 3), astronomers derived a distance of 5,870 light-years, corresponding to a radius of 909 solar radii.

A size comparison of Stephenson 2-18 and UY Scuti, image credit: Wikimedia Commons/THE COLOSSAL GALAXY NAMED IC1101 (CC BY-SA 4.0)

Similarly, NML Cygni, a red supergiant in the constellation Cygnus, was once believed to have a radius of 3,700 solar radii. However, more recent estimates are in the range from 1,183 – 2,770 solar radii.

Other stars that were once believed to be the largest in size include the supergiants VV Cephei A and Mu Cephei (Herschel’s Garnet Star) in the constellation Cepheus, the red supergiant Betelgeuse in the constellation Orion, the red hypergiant VY Canis Majoris in Canis Major, and the yellow hypergiant V766 Centauri A (HR 5171 A) in Centaurus.

Another factor that makes determining stellar sizes difficult is that supergiant stars often have extended atmospheres or are enveloped in thick dust shells, which makes it hard to estimate their effective temperatures.

The red hypergiant VY Canis Majoris is enveloped in a dense reflection nebula formed by material ejected from the star. The nebula’s outer shell is 12,000 astronomical units (0.19 light-years) across.

A comparison showing the red hypergiants Stephenson 2-18 and VY Canis Majoris. Image credit: Wikimedia Commons/ Faren29 (CC BY-SA 4.0)

Supergiants are unstable stars. Many of them pulsate and change in size with the pulsations. Many of the largest stars known show a variation in brightness and are classified as semiregular variables, slow irregular variables, or long-period variables.

UY Scuti is classified as a semiregular variable star with a pulsation period of 740 days. WOH G64 is a long-period variable or carbon-rich Mira, and VY Canis Majoris has been classified as a semiregular variable or slow irregular variable star. AH Scorpii in the constellation Scorpius, NML Cygni in Cygnus, and Mu Cephei, MY Cephei, and RW Cephei in Cepheus are semiregular variables.

Many of the largest stars known reside in the constellations Dorado, Scutum, and Ara. Dorado hosts the Large Magellanic Cloud, the home of WOH G64, WOH S170, HV 888, and many other stars with radii over a thousand times that of the Sun. Scutum hosts the massive open clusters RSGC1 (Red Supergiant Cluster 1), Stephenson 2 (RSGC2), and RSGC3, which are home to the supergiants RSGC1-F01, RSGC1-F06, Stephenson 2-18, Stephenson 2-11, RSGC3-S3 and RSGC3-S15. Ara is home to the compact star cluster Westerlund 1, which contains the red supergiants Westerlund 1-26 (Westerlund 1 W26), Westerlund 1-20, Westerlund 1-237.

While red supergiants are generally the largest stars known, other star types can sometimes briefly increase in size. These include luminous blue variables (LBVs) and luminous red novae. In the 19th century, the famous luminous blue variable Eta Carinae experienced an eruption during which it increased to a size of around 1,400 solar radii.

Luminous red novae, caused by the merging of two stars, expand very rapidly and can reach thousands of solar radii within months. For instance, V838 Monocerotis experienced an event in 2002 during which it expanded to over 1,500 solar radii and engulfed its binary companion. Since the event, the star’s radius has shrunk to around 464 solar radii.

The largest stars in the known universe are not also the most massive stars. For example, the red hypergiant VY Canis Majoris, a previous record holder for the largest known star, has a radius of about 1,420 solar radii and a mass of 17 solar masses. While it is massive enough to be a supernova candidate, it does not come close to the supermassive stars like R136a1 and BAT99-98. These are Wolf-Rayet stars located in the Large Magellanic Cloud. R136a1 has a mass of around 196 solar masses and a radius of only 42.7 solar radii. BAT99-98 has 226 times the Sun’s mass and a radius of 37.5 solar radii. Both are among the most massive stars known.

Most of the largest stars known have a mass over 10 times that of the Sun and will go out as supernovae when they reach the end of their life cycle. Depending on a star’s initial mass, the supernova event will leave behind either a neutron star or a black hole.

None of the brightest stars in the sky are among the largest stars known. The largest first-magnitude stars, the red supergiants Betelgeuse in Orion and Antares in Scorpius, have radii of 640 – 1,021 and 680 solar radii. Betelgeuse was once the largest star known, but recent estimates give it a radius of 640 solar radii, less than half of those of the largest stars discovered to date.

Below is the list of the largest stars currently known, ordered by estimated radius. Stellar size is measured in solar radii. The radius of the Sun is around 695,700 kilometres or 432,300 miles.

LARGEST STARS