Stevenson's star 2-18
It is the largest star discovered so far in terms of size, and is 20 thousand light years away from us
Stephenson 2-18 is a bright supergiant star located in the direction of the gear constellation 20,000 light-years away within the open star cluster Stephenson 2. It is about 2,158 solar diameters and is currently among the largest stars known, if not the largest.
It is a red giant] (RHG) star in the constellation Gear. It is located near the open star cluster Stevenson 2, which lies about 5.8 kiloparsecs (19,000 light-years) away from Earth in the Centaur Arm of the Milky Way, and is presumed to be one of a group of stars at a similar distance, although some sources consider it to be Unattached red giant. He is among the biggest known stars, One of the brightest stars is the red supergiant, and one of the brightest stars in the Milky Way.
The giant star Stevenson 2-18 has a radius of about 2,150 solar radii (1.50 x 109 km; 10.0 AU), which is approximately 10 billion times the size of the Sun. If we take this estimate correctly, it would take approximately 9 hours to travel around its surface at the speed of light, compared to just 14.5 seconds around the Sun. If it were placed at the center of Earth's solar system, its photosphere would engulf Saturn's orbit.
Compare the sizes of different selected stars. From left to right are Cygnus OB2 #12, V382 Base, V915 Scorpii, UY Scotty, and Stephenson 2-18. The orbits of Saturn and Neptune are also shown for comparison.
American astronomer [Charles Bruce Stevenson] discovered the open cluster Stevenson 2 in 1990 in data obtained through a deep infrared survey. Also known as RSGC2, the cluster is one of several massive open clusters in Scutum, each containing several red giant stars.
The brightest star in the cluster region was given the identification "1" in the first analysis of the properties of the stellar cluster members. However, it has not been considered a member of Stevenson Cluster 2 due to its remote location, abnormally intense brightness, and slightly atypical proper motion, such that it is classified as an unrelated red giant.
In a later study, the same star was given the number 18 and assigned to a distant group of stars called Stephenson 2 SW, assumed to be at a similar distance from the main cluster. The designation St2-18 (short for Stevenson 2-18) is often used for the star, following the notation from Deguchi (2010).[3][4] To avoid confusion from using the same number for different stars and different numbers for the same star, the data from Davies (2007) are often given a prefix of DFK or D, for example Stephenson 2 DFK 1.
distance
When the cluster was originally discovered in 1990 at Stevenson 2, the cluster was therefore originally estimated to be about 30 kiloparsecs (98,000 light-years) apart, much further than the cluster is thought to exist today. A supergiant, then calculate the distance coefficient based on the absolute magnitude data.] In 2001, Nakaya et al. The distance of the stars in the cluster has been estimated at 1.5 kiloparsecs (4,900 light-years), which is much closer than any other distance estimate given for the star and the cluster. Instead, a study on a similar time frame gave an additional distance of about 5.9 kiloparsecs (19,000 light-years).
A study in 2007 assigned a motor distance of 5.83+1.91
−0.78 kiloparsecs is equivalent to 19,000 light-years from the comparison with the radial velocity of the star cluster, considerably closer than the original distance of 98,000 light-years reported by Stevenson (1990). However, due to the questionable membership of Stevenson 2-18, the distance has not been directly estimated. This value was later adopted in a recent study of the cluster.
A similar kinetic 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 members (96 kilometers per second) and from an association with a cluster of stars near Stevenson 2 SW, with its location Near the Centaur Arm of the Milky Way. This value was later adopted in a 2012 study, which used the above distance to calculate the star's luminosity. However, the uncertainty in the distance was greater than 50%. However, it is also reported that distances to massive star clusters will be improved in the future
St2-18 exhibits the features and characteristics of an extremely bright red giant, with a spectral type of M6, which is unusual for a giant star. This makes it one of the most extreme stars in the Milky Way. It occupies the upper corner of the Hertzsprung-Russell diagram, and it occupies a region of stars with an exceptionally large temperature and luminosity.
Stevenson 2-18 is usually classified as a red giant with a broad red spectral line. But a significant increase in infrared radiation (which indicates a previous phase of significant mass loss) may
It prompted the astronomer authors Davies (2007) to suggest that the star might be a red giant, like VY Canis Major. It is also reported that Stevenson 2-18 is about to shed its outer layers and evolve into a luminous blue variable or Wolf-Rayet star.
Gloss
Calculating to find the bolometric luminosity by fitting the spectral energy distribution using a dust dust model gives the star a luminosity approximately 440,000 times that of the Sun.
On the other hand, an old calculation from 2010 assumes that Stephenson 2-18 is part of the Stephenson 2 group, that is, at a distance of 5.5 kpc and based on 12 and 25 km flux densities, giving it a relatively lower and modest brightness that its luminosity is more than 90 thousand times that of luminosity. the sun.
A recent calculation based on SED integration (based on published data) and assuming a distance of 5.8 kpc gives it a bolometric luminosity of 630,000 solar luminosities. However, it has been observed that its SED is disproportionate to the standard table. Which makes it brighter. Because of this unusually high brightness, whether or not the star is part of the Stevenson 2 star cluster (i.e., alone) is still up for debate.
As stated in the scientific thesis Negueruela et al. (2012), the star cluster is spread over a large area.
temperature
An effective temperature of 3,200 K was calculated in a 2012 study by SED integration using a destiform model, which made it cooler than red giant stars predicted by stellar evolution (typically around 3,500 K).
Spectral type
Astronomer Neguer et al. (2013) identified a Stevenson spectral type of 2-18, which is unusual even for a red giant star based on its spectrum and some spectroscopic features. They found broad lines in the Stevenson spectrum 2-18 on the spectral lines of titanium oxide (TiO). <reference name="negueruela2013" />
the size
A radius of about 2,150 times the Sun's luminosity has been derived, estimated from the Sun's bolometric luminosity approximately 440,000 times greater than the Sun's luminosity, and an estimated effective temperature of about 3,200 K, much larger than theoretical models of the largest red giants predicted by the theory of stellar evolution ( About 1,500 radii of the Sun).
Assuming this value is correct, this would make it larger than other famous red giant stars, such as Antares, Betelgeuse, VY Canis Major, and UY Scotti.
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