World's Biggest Star

Updated: Oct 19

The Stephenson 2-18 (abbreviated to St2-18), also known as the Stephenson 2 DFK 1 or the RSGC-2-18 in the constellation of Scutum, is a Red Supergiant star (RSG). It lies next to the open cluster Stephenson at a distance of 5.8 kiloparsecs (19,000 light years) from the earth.


It is among the most prominent known stars, one of the most luminous red supergiants, and one of the brightest stars in the Milky Way.


Image credit- https://en.wikipedia.org


This star is estimated to have a radius of about 2,150 solar radii, or about 10 billion times the solar system's volume. If this estimate is correct, traveling around the star at light speed would take almost 9 hours, compared to 14.5 seconds around the Sun. Its photosphere would engulf Saturn's orbit if placed at the Solar System's center.

Stephenson 2 is one of the several massive open clusters in Scutum, each containing multiple red supergiants, discovered by American astronomer Charles Bruce Stephenson in 1990.


When the first analysis of cluster member properties was conducted, the brightest star in the region was given the identifier 1. However, due to its location, abnormally high brightness, and slightly atypical proper motion, it was categorized as a red supergiant unrelated to Stephenson 2.


Comparison of the sizes of selected different stars.

Image Credit- https://en.wikipedia.org


Later, the same star was given the number 18 and assigned to an outlying group called Stephenson 2 SW. This group is believed to be at a similar distance from the core cluster. Stephenson 2-18 (St2-18) is often referred to by the designation St2-18, following the numbering from Deguchi (2010). Designations from Davies (2007) are often preceded by DFK so that there is no confusion caused by using the same number for different stars and different numbers for the same star.


A study concerning the maser emissions from red supergiants across the galaxy observed Stephenson 2-18 along with 56 other red supergiants in 2012. The study derived the properties of those red supergiants using the Australia Telescope Compact Array (ATCA) and the DUSTY model. The red supergiant Stephenson 2-18 was also mentioned.


That same year, it was used for a study regarding the types of masers on red supergiant stars in clusters. The study identified Stephenson 2-18 as an unrelated star to Stephenson 2, based on its differing radial velocity. In 2013, in a study regarding the red supergiants in Stephenson 2, Stephenson 2-18 (referred to as D1) was observed and measured, with its spectral type identified.[3] In several later studies, the star was described as being a "very late-type red supergiant".

Humphreys et al. (2020) also referred to it as RSGC1-01, another large and luminous red supergiant in the constellation of Scutum. In addition to noting the unique spectral energy distribution, doubtful membership, and extreme properties of Stephenson 2-18, the study calculated a more accurate luminosity estimate for Stephenson 2-18.

Image credit- https://en.wikipedia.org


Stephenson 2, and therefore Stephenson 2-18, were originally estimated to have a distance of approximately 30 kiloparsecs (98,000 light years), much farther than they are thought to reside today. This distance was calculated by assuming the cluster stars were all M-type supergiants and then calculating the distance modulus using their typical absolute magnitudes.


The distance between the stars in the cluster was estimated by Nakaya et al. in 2001 at 1.5 kiloparsecs (4,900 light-years), significantly closer than any previous distance estimate for the star and the cluster. Alternatively, a study around a similar timeframe gave a distance of about 5.9 kiloparsecs (19,000 light-years).


According to a 2007 study, the kinematic distance is 5.83 + 1.91.

19000+6200 kiloparsecs = 0.78 kiloparsecs


The cluster's radial velocity is considerably closer to the original distance of 30 kiloparsecs (98,000 light years) given by Stephenson (1990). Due to Stephenson 2-18's doubtful membership, its distance could not be directly estimated. This distance was later adopted in a recent study of the cluster.


A similar kinematic distance of 5.5 kiloparsecs (18,000 light-years) was reported in a 2010 study, derived from the average radial velocity of four of the cluster's members (96kilometress per second) and an association with a clump of stars near Stephenson 2, Stephenson 2 SW, locating it near the Scutum–Centaurus Arm of the Milky Way This value was later adopted in a 2012 study, which used the aforementioned distance to calculate the star's luminance but noted that the distance was uncertain by more than 50 percent. Despite this, it is also stated that distances to massive star clusters will be improved in the future.

The average radial velocity of the cluster(+109.3 kilometers per second), Verheyen et al. (2013) calculated the cluster's kinematic distance as 6 kiloparsecs (20,000 light-years). Stephenson 2-18, however, has a radial velocity of only 89 kilometers per second, indicating that it is not a member of the cluster, but a field red supergiant.