Que Es Stephenson 2-18

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Nancie Fazzari

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Aug 4, 2024, 5:31:44 PM8/4/24

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Theopen cluster Stephenson 2 was discovered by American astronomer Charles Bruce Stephenson in 1990 in the data obtained by a deep infrared survey.[2][7] The cluster is also known as RSGC2, one of several massive open clusters in Scutum, each containing multiple red supergiants.[8]

In a later study, the same star was given the number 18 and assigned to an outlying group of stars called Stephenson 2 SW, assumed to be at a similar distance to the core cluster.[9] The designation St2-18 (short for Stephenson 2-18) is often used for the star, following the numbering from Deguchi (2010).[10][9] To avoid confusion from using the same number for different stars and different numbers for the same star, designations from Davies (2007) are often given a prefix of DFK or D,[8] for example Stephenson 2 DFK 1 or simply D1 where the context is clear.[5]

In 2012, Stephenson 2 DFK 1, along with 56 other red supergiants, was observed in a study regarding the maser emissions from red supergiants across the galaxy. The study derived the properties of those red supergiants using the Australia Telescope Compact Array (ATCA) and the DUSTY model. Stephenson 2 DFK 1 was among the red supergiants mentioned.[10] That same year, it was observed again for a study regarding the types of masers on red supergiant stars in clusters. [5]During 2013, in a study regarding the red supergiants in Stephenson 2, Stephenson 2 DFK 1 (referred to as D1) was observed.[3]In several later studies, the star was described as being a "very late-type red supergiant."[11][6]

When the cluster was originally discovered in 1990, Stephenson 2, and therefore Stephenson 2 DFK 1, was originally estimated to have a distance of around 30 kiloparsecs (98,000 light-years), much further than the cluster is thought to reside today.[7]

Verheyen et al. (2013) used the average radial velocity of the cluster (+109.3 0.7 kilometers per second) to derive a kinematic distance of roughly six kiloparsecs (20,000 light-years) for the cluster. However, Stephenson 2 DFK 1's radial velocity is calculated to be only 89 kilometers per second and therefore leading to the study's statement that the star is a field red supergiant unassociated with the cluster.[5]

An effective temperature of 3,200 K was calculated in a 2012 study by SED integration using the DUSTY model,[10] which would make it much cooler than the coolest red supergiants predicted by stellar evolutionary theory (typically around 3,500 K).[13] However, this effective temperature is unlikely since one beyond the Hayashi line could indicate that it is not in hydrostatic equilibrium.[14]

In 2007, Davies et al. estimated Stephenson 2 DFK 1's spectral type at M5 or M6, unusual and very late for even a red supergiant star, based on its CO-bandhead absorption.[2]Negueruela et al. (2013) identified Stephenson 2 DFK 1's spectral type to be around M6, similar to the spectral type approximated by Davies et al. 2007, based on its spectrum and the characteristics of certain spectral features, such as titanium oxide (TiO) spectral lines.[3]

It has been debated for a while if this star is actually part of its supposed cluster. Due to its radial velocity being below the other cluster stars but with some signs of membership, some sources state that the star is unlikely to be a foreground giant;[2][3] however, more recent papers considered the star an unlikely member due to its extreme and inconsistent properties.[6]

Using radial velocities determined from silicon oxide (SiO) maser emission and IR CO absorption, a study of red supergiant masers in massive clusters considered Stephenson 2 DFK 1 as a field red supergiant, unrelated to Stephenson 2. This is due to its lower radial velocity that is significantly different compared to other stars from Stephenson 2.[5] Despite this, Stephenson 2 DFK 1's membership cannot be ruled out yet.[6]

Stephenson 2-18 (St2-18) is a red supergiant or red hypergiant star located in the constellation Scutum. Also catalogued as Stephenson 2-DFK 1 and RSGC2-18, it is the current record holder for the largest star known, with a size 2,150 times that of the Sun. The star is located at a distance of 18,900 light years from Earth. It appears in the same region as the open cluster Stephenson 2.

Stephenson 2-18 is a red supergiant star of the spectral type M6. It is one of the largest stars ever discovered, with a radius of 2,150 solar radii. It is also one of the most luminous stars in the Milky Way and one of the most luminous red supergiants known.

The radius of Stephenson 2-18 is above what current stellar evolution theory predicts. The theoretical limit of star size in the Milky Way is around 1,500 solar radii. It is believed that stars larger than this do not form and would be too unstable.

Stephenson 2-18 took the title of the largest star known from the previous record holders, the red supergiants WOH G64 in the constellation Dorado and UY Scuti in Scutum. WOH G64 has an estimated radius between 1,540 and 1,730 solar radii, which is considerably smaller than St2-18.

Stephenson discovered the cluster during a space-deep infrared objective-prism survey of the northern Milky Way with the Burrell Schmidt Telescope of the Warner and Swasey Observatory. He estimated a distance of 30 kiloparsecs (98,000 light-years) for the cluster based on the assumption that the stars were all red supergiants. Stephenson 2-18 is now believed to lie approximately 18,900 light-years away, at a similar distance as the cluster, but there are still uncertainties regarding its membership.

The estimated age of the cluster is 17 3 million years. In the study, the brightest star in the K-band that lay in the same line of sight as the cluster (St2-18) was given the identifier 1. The study concluded that the star likely had considerable infrared excess and may be a red hypergiant like VY Canis Majoris.

A 2012 study reported about 80 red supergiants appearing in the same line of sight as Stephenson 2 (RSGC2), 40 of which had radial velocities indicating that they were members of the cluster. Since these supergiants are spread across a wider area of the sky, they likely form an extended stellar association. The cluster has an estimated mass between 30,000 and 50,000 solar masses and a physical radius of about 4 parsecs (13 light-years).

A 2013 study of the class M supergiants in the cluster Stephenson 2 detected maser emission from the two brightest members of the cluster, indicating that the stars have the highest mass loss rates of all the members.

RSGC2 stands for Red Supergiant Cluster 2. DFK 1 comes from the initials of Ben Davies, Don F. Figer, and Rolf-Peter Kudritzki, who identified a core group of 26 physically associated red supergiants in Stephenson 2 in 2007. St2-18 was given the identifier 1 as the brightest star appearing in the region. However, it was believed to be an unrelated red supergiant.

Scutum is one of the smallest and faintest constellations in the sky. It is the 84th constellation in size, stretching across only 109 square degrees of the southern sky. It does not contain any stars brighter than magnitude 3.00. None of its stars make the list of the 300 brightest stars in the sky.

Scutum is best known for being home to the variable white giant star Delta Scuti, which serves as a prototype for its own class of variables, and to the pulsating variable red supergiant UY Scuti, one of the largest known stars. Notable deep sky objects in Scutum include the open clusters Messier 11, better known as the Wild Duck Cluster, and Messier 26, the massive young clusters RSGC1, RSGC2 (Stephenson 2), RSGC3 and RSGC4 (Alicante 8), the globular cluster NGC 6712, and the planetary nebula IC 1295.

The best time of the year to observe the stars and deep sky objects in Scutum is during the month of August, when the constellation rises high above the horizon in the evening. The entire constellation is visible from locations between the latitudes 80 N and -90 S, i.e. from all inhabited places on Earth.

The open cluster Stephenson 2 was discovered by American astronomer Charles Bruce Stephenson in 1990 in the data obtained by a deep infrared survey. The cluster is also known as RSGC2, one of several massive open clusters in Scutum, each containing multiple red supergiants.

The brightest star in the region of the cluster was given the identifier 1 in the first analysis of cluster member properties. However, it was not considered to be a member of Stephenson 2 due to its outlying position, abnormally high brightness, and slightly atypical proper motion. In a later study, the same star was given the number 18 and assigned to an outlying group of stars called Stephenson 2 SW, assumed to be at a similar distance to the core cluster. The designation St2-18 (short for Stephenson 2-18) is often used for the star, following the numbering from Deguchi (2010). To reduce confusion from using the same number for different stars and different numbers for the same star, designations from Davis (2007) are often given a prefix of DFK or D, for example Stephenson 2 DFK 1.

Stephenson 2-18Image of a star cluster, with Stephenson 2-18 in center of the pictureDesignationsSemi-major axisVery close to Sagittarius AOrbital periodA few million years around Sgr APhysical characteristicsMassUnknownRotation periodPossibly some yearsCompositionHydrogen, helium, lithium, carbon, nitrogen, oxygen, neon, silicon and ironSurface temperature2,926 CPositionSuperior systemScutumEquivalent systemRed Supergiant StarInferior systemNone(?)

Stephenson 2-18, also known as RSGC2-18 and Stephenson 2 DFK 1, is a red supergiant star in the Stephenson 2 supercluster, in the Scutum constellation, about 20,000 light years or 6,000 parsecs away and it is located near the Supermassive black hole, Sagittarius A. The star is extremely large, having a gigantic diameter of nearly 2150 times the diameter of the Sun, and it is probably approaching the potential largest size of Red Supergiant stars and probably is one of the largest stars in the Universe.