No evidence has been found that it is part of a multiple star system. Its great infrared (IR) excess makes it one of the brightest objects in the local part of the galaxy at wavelengths of 5 to 20 microns (µm) and indicates a dust shell or heated disk.[14][15] It is about 17±8 times the mass of the Sun (M☉). It is surrounded by a complex asymmetric circumstellar envelope (CSE) caused by its mass loss. It produces strong molecular maser emission and was one of the first radio masers discovered. VY CMa is embedded in the large molecular cloudSh2-310, a large, quite local star-formingH II region—its diameter: 480 arcminutes (′) or 681ly (209pc).[16][17]
The radius of VY CMa is very roughly 1,420 times that of the Sun (R☉), which is close to the modelled maximum, the Hayashi limit, a volume nearly 3 billion times that of the Sun.[3] Taking this mid-point estimate as correct, an object travelling at the speed of light would take 6 hours to go around its surface, compared to 14.5 seconds for the Sun.[18] If this star replaced the Sun its surface would, per this approximation, be beyond the orbit of Jupiter.[3]
The first known-recorded observation of VY Canis Majoris is in the star catalogue of the French astronomer Jérôme Lalande in 1801,[lower-alpha 2] which lists it as a 7th order of magnitude star. Further quite frequent studies of its apparent magnitude imply the light of the star as viewed from Earth has faded since 1850, which could be due to emission changes or a denser part of its surrounds becoming interposed (extinction).[19] Since 1847, VY Canis Majoris has been described as a crimson star.[19] During the 19th century, observers measured at least six discrete components, suggesting that it might be a multiple star. These are now known to be bright zones in the host nebula. Observations in 1957 and high-resolution imaging in 1998 all but rule out any companion stars.[19][20]
The variation in the star's brightness was first described in 1931, when it was listed (in German) as a long-period variable with a photographic magnitude range of 9.5 to 11.5.[28] It was given the variable star designationVY Canis Majoris in 1939, the 43rd variable star of the constellation Canis Major.[29]
Map of the giant molecular cloud Sharpless 310 and its surroundings
Combining data from the mentioned telescope with others from the Keck in Hawaii it was possible to make a three-dimensional reconstruction of the envelope of the star. This reconstruction showed that the star's mass loss is much more complex than expected for any red supergiant or hypergiant. It became clear that the bows and nodules appeared at different times; the jets are randomly oriented, which prompts suspicion they derive from explosions of active parts of the photosphere. The spectroscopy proves the jets move away from the star at different speeds, confirming multiple events and directions as with coronal mass ejections.[30] Multiple asymmetric mass loss events and the ejection of the outermost material are deduced to have occurred within the last 500 to 1,000 years, while that of a knot near the star would be less than 100 years. The mass loss is due to strong convection in the tenuous outer layers of the star, associated with magnetic fields. Ejections are analogous to—but much larger than—coronal ejections of the Sun.[9][30][31]
In 1976, Lada and Reid[lower-alpha 3] published observations of the bright-rimmed molecular cloud Sh2-310, which is 15″ east of the star. At its edge bordered by the bright rim, an abrupt decrease in the CO emission and an increase in brightness of the 12 CO emission were observed, indicating possible destruction of molecular material and enhanced heating at the cloud-rim interface, respectively. They assumed the distance of the cloud is approximately equal to that of the stars, which are members of the open clusterNGC 2362, that ionize the rim. NGC 2362 could be anywhere in the ranges of 1.5±0.5kiloparsecs (kpc) or 4,890±1,630light-years (ly) away as determined from its color-magnitude diagram.[32] This star is projected onto the tip of the cloud rim, strongly suggesting its association. Furthermore, all the vectors of velocity of Sh2-310 are very close to those of the star. There is thus a near-certain physical association of the star with Sh2-310 and with NGC 2362 in all standard models.[33]
Melnik and others later prefer a range centred on 1.2 kiloparsecs (about 3,900 light-years).[34]
Distances can be calculated by measuring the change in position against very distant background objects as the telescope orbits the Sun. However, this star has a small parallax due to its distance, and standard visual observations have a margin of error too large for a hypergiant star with an extended CSE to be useful, for example, the Hipparcos Catalogue of 1997 gives a purely notional parallax of 1.78±3.54milliarcseconds (mas), in which the "central" figure equates to 562pc (1,832ly).[35] Parallax can be measured to high accuracy from the observation of masers using a long baseline interferometry. In 2008, such observations of H 2O masers using VERA interferometry from the National Astronomical Observatory of Japan gave a parallax of 0.88±0.08mas, corresponding to a distance of 1.14+0.11 −0.09kpc (about 3,720+360 −300ly).[36] In 2012, observations of SiO masers using very-long-baseline interferometry (VLBI) from Very Long Baseline Array (VLBA) independently derived a parallax of 0.83±0.08mas, corresponding to a distance of 1.20+0.13 −0.10kpc (about 3,910+423 −326ly).[10] These imply the cloud (Sh2-310) is less remote than thought or that the star is a foreground object.[16]
The Gaia mission provides highly constrained parallaxes to some objects, but the data release 2 value of −5.92±0.83mas for this star is not meaningful.[37]
The spectrum of VY Canis Majoris is that of a high-luminosity M-class star. The hydrogen lines, however, have P Cygni profiles fit for luminous blue variables. The spectrum is dominated by TiO bands whose strengths suggest a classification of M5. The H-alpha (Hα) line is not visible yet and there are unusual emission lines of neutral elements such as sodium and calcium. The luminosity class as determined from different spectral features varies from bright giant (II) to bright supergiant (Ia), with a compromise being given: as M5eIbp. Old classifications were confused by the interpretation of surrounding nebulosity as companion stars.[41]
The present spectral classification system is inadequate to this star's complexities. The class depends on which of its complex spectral features are stressed. Further, key facets vary over time as to this star. It is cooler and thus redder than M2, and is usually classified between M3 and M5. A class as extreme as M2.5 appeared in a study of 2006.[4] The luminosity class is likewise confused and often given only as I, partly because luminosity classes are poorly defined in the red and infrared portions of the spectrum. One study though, gives a luminosity class of Ia+ which means a hypergiant or extremely luminous supergiant.[42]
A very large and luminous star, VY CMa is among the most extreme stars in the Milky Way and has an effective temperature below 4,000K (3,730°C; 6,740°F). It occupies the upper-right hand corner of the HR diagram although its exact luminosity and temperature are uncertain. Most of the properties of the star depend directly on its distance.
Luminosity
The bolometric luminosity (Lbol) of VY CMa can be calculated from spectral energy distribution or bolometric flux, which can be determined from photometry in several visible and infraredbands. Earlier calculations of the luminosity based on an assumed distance of 1.5kpc (4,900ly) gave luminosities between 200,000 and 560,000 times the Sun's luminosity (L☉).[14][32][43] This is considerably very close or beyond the empirical Humphreys–Davidson limit. One study gave nearly one millionL☉ at a distance of 2.1kpc (6,800ly).[44] In 2006 a luminosity of 430,000L☉ was calculated by integrating the total fluxes over the entire nebula, since most of the radiation coming from the star is reprocessed by the dust in the surrounding cloud.[31] More recent estimates of the luminosity extrapolate values below 350,000L☉ based on distances below 1.2 kpc.[3][36][45]
Most of the output of VY CMa is emitted as infrared radiation, with a maximum emission at 5–10μm, which is in part caused by reprocessing of the radiation by the circumstellar nebula.[9][31] Many older luminosity estimates are consistent with current ones if they are re-scaled to the distance of 1.2 kpc.[36] Despite being one of the most luminous stars in the Milky Way, much of the visible light of VY CMa is absorbed by the circumstellar envelope, so the star needs a telescope to be observed. Removing its envelope, the star would be one for the naked eye.[24]
Mass
Since this star has no companion star, its mass cannot be measured directly through gravitational interactions. Comparison of the effective temperature and bolometric luminosity compared to evolutionary tracks for massive stars suggest its initial mass was 25±10M☉ for a rotating star but current mass 15M☉—or 32M☉ at first if non-rotating falling to present-day 19M☉,[3] and an age of 8.2 million years (Myr).[10] Older studies have found much higher initial masses (thus also higher current masses) or a progenitor mass of 40–60M☉ based on old luminosity estimates.[14][46]
Mass loss
Image taken by the ESO's Very Large Telescope showing the asymmetric nebula around VY CMa using SPHERE instrument. The star itself is hidden behind a dark disk. Crosses are artifacts (lens effects) due to the characteristics of the instrument.
VY CMa has a strong stellar wind and is losing much material due to its high luminosity and quite low surface gravity. It has an average mass loss rate of 6×10−4M☉ per year, among the highest known and unusually high even for a red supergiant, as evidenced by its extensive envelope.[47][39] It is thus an exponent for the understanding of high-mass loss episodes near the end of massive star evolution.[48] The mass loss rate probably exceeded 10−3M☉/yr during the greatest mass loss events.[47]
The star has produced large, probably convection-driven, mass-loss events 70, 120, 200, and 250 years ago. The clump shed by the star between 1985 and 1995 is the source of its hydroxyl maser emission.[49]
Temperature
The effective temperature of this star is uncertain. Some signature changes in its spectrum correspond to temperature variations. Early estimates of the mean temperature assumed values below 3,000 K based on a spectral class of M5.[43][44] In 2006, its temperature was calculated to be as high as 3,650±25K, corresponding to a spectral class of M2.5,[4] yet this star is usually considered as an M4 to M5 star. Adopting the latter classes with the temperature scale proposed by Emily Levesque gives a range of between 3,450 and 3,535 K.[50]
Size
Right to left: VY Canis Majoris compared to Betelgeuse, Rho Cassiopeiae, the Pistol Star, and the Sun (too small to be visible in this thumbnail). The orbits of Jupiter and Neptune are also shown.
The calculation of the radius of VY CMa is complicated by the extensive circumstellar envelope of the star. VY CMa is also a pulsating star, so its size changes with time. Earlier direct measurements of the radius at infrared (K-band = 2.2µm) wavelength gave an angular diameter of 18.7±0.5mas, corresponding to radii above 3,000R☉ (2.1×109km; 14au; 1.3×109mi) at an assumed distance of 1.5 kpc, considerably larger than expected for any red supergiant or red hypergiant.[43] However, this is probably larger than the actual size of the underlying star and the angular diameter estimate appears exceedingly large due to interference by the circumstellar envelope.[31][9][3] In 2006–2007 radii of 1,800–2,100R☉ have been derived from the estimated luminosity of 430,000L☉ and temperatures of 3,450–3,535 K.[31][9]
On 6 and 7 March 2011, VY CMa was observed at near-infrared wavelengths using interferometry at the Very Large Telescope. The size of the star was calculated using the Rosseland Radius, the location at which the optical depth is 2⁄3,[51] with two modern distances of 1.14+0.11 −0.09 and 1.20+0.13 −0.10kpc.[36][10] Its angular diameter was directly measured at 11.3±0.3mas, which corresponds to a radius of 1,420±120R☉ at a distance of 1.17+0.08 −0.07kpc. The high spectral resolution of these observations allowed the effects of contamination by circumstellar layers to be minimised. An effective temperature of 3,490±90K, corresponding to a spectral class of M4, was then derived from the radius and a luminosity of 270,000±40,000L☉ which is based on the distance and a measured flux of (6.3±0.3)×10−13W/cm2.[3] In late 2013, a radius of 2,069R☉ was determined, based on a rather cool adopted temperature of 2,800 K and a luminosity of 237,000L☉.[52]
Most radius estimates of the VY CMa are considered as the size for the optical photosphere while the size of the star for the radio photosphere is calculated to be twice that of the size of the star for the optical photosphere.[5] Despite the mass and very large size (though some estimates give smaller sizes), VY CMa has an average density of 5.33 to 8.38mg/m3 (0.00000533 to 0.00000838kg/m3), it is over 100,000 times less dense than Earth's atmosphere at sea level (1.2kg/m3).
VY Canis Majoris has been known to be an extreme object since the middle of the 20th century, although its true nature was uncertain.[41][53] In the late 20th century, it was accepted that it was a post-main sequence red supergiant. Its angular diameter had been measured and found to be significantly different depending on the observed wavelength. The first meaningful estimates of its properties showed a very large star.[54][55]
Early direct measurements of the radius at infrared (K-band = 2.2µm) wavelength gave an angular diameter of 18.7±0.5mas, corresponding to radii above 3,000R☉ (2.1×109km; 14au; 1.3×109mi) at a still very plausible distance of 1.5 kiloparsecs; a radius dwarfing other known red hypergiants.[43] However, this is probably larger than the actual size of the underlying star—this angular diameter estimate is heightened from interference by the envelope.[3][9][31] In 2006–07, radius between 1,800–2,100R☉ has been derived from the preferring luminosity of 430,000L☉ and the still-preferred temperature range of 3,450–3,535kelvin.[9][31]
In contrast to prevailing opinion, a 2006 study, ignoring the effects of the circumstellar envelope in the observed flux of the star, derived a luminosity of 60,000L☉, suggesting an initial mass of 15M☉ and radius of 600R☉ based on an assumed effective temperature of 3,650 K and distance of 1.5kpc. On this basis they considered VY CMa and another notable extreme cool hypergiant star, NML Cygni, as normal early-type red supergiants.[4][56] They assert that earlier very high luminosities of 500,000L☉ and very large radii of 2,800–3,230R☉[14][57] (or even 4,000R☉[20]) were based on effective temperatures below 3,000 K that were unreasonably low.[4]
Almost immediately another paper published a size estimate of 1,800–2,100R☉ and concluded that VY CMa is a true hypergiant. This uses the later well-reviewed effective temperature 3450–3535kelvin, and a luminosity of 430,000L☉ based on SED integration and a distance of 1.5kpc.[31]
In 2011,[lower-alpha 4] the star was studied at near-infrared wavelengths using interferometry at the Very Large Telescope. The size of the star was published at its Rosseland Radius, outside of which optical depth falls below 2⁄3,[51] given the mean of two most modern, similar but distinct distances.[lower-alpha 5][10][36] Its angular diameter was directly measured at 11.3±0.3mas, thus radius of 1,420±120R☉ given a distance of 1.17+0.08 −0.07kpc. The high spectral resolution of these observations allowed the effects of contamination by circumstellar layers to be minimised. An effective temperature of 3,490±90K, corresponding to a spectral class of M4, was then derived from the radius and a luminosity of 270,000±40,000L☉ which is based on the distance and a measured flux of (6.3±0.3)×10−13W/cm2.[3]
Most such radius estimates are considered as the size for the mean limit of the optical photosphere while the size of the star for the radio photosphere is calculated to be twice that.[5] Despite the mass and very large size (though some estimates give smaller sizes), VY CMa has an average density of 5.33 to 8.38mg/m3 (0.00000533 to 0.00000838kg/m3). It is over 100,000 times less dense than Earth's atmosphere at sea level (1.2kg/m3).
In 2012, the size was calculated more accurately to be somewhat lower, for example 1,420R☉,[3] which leaves larger sizes published and in-date for other galactic and extragalactic red supergiants (and hypergiants) such as Westerlund 1 W26 and WOH G64. Despite this, VY Canis Majoris is still often described as the largest known star, sometimes with caveats to account for the highly uncertain sizes of all these stars.[58][lower-alpha 6] A 2013 estimate based on the Wittkowski radius and the Monnier radius put mean size at 2,000R☉,[27] and later that year, Matsuura and others put forward a competing method of finding radius within the envelope, putting the star at 2,069R☉, based on a cool-end of estimates adopted temperature of 2,800 K and a luminosity of 237,000L☉.[52] However, these values are not consistent with its spectral types, leaving the 2012 values in better match.
Surroundings
WFPC2/HST image showing the asymmetric nebula surrounding VY CMa, which is the central star
VY Canis Majoris is surrounded by an extensive and dense asymmetric red reflection nebula, with a total ejected mass of 0.2-0.4M☉ and a temperature of 800kelvin, based on a DUSTY model atmosphere that has been formed by material expelled from its central star.[14][47] The inner shell figures as 0.12″ across, corresponding to 140AU (0.0022ly) for a star 1,200 parsecs away, whereas that of the outer one is at 10″, corresponding to 12,000AU (0.19ly).[47] This nebula is so bright that it was discovered in a dry night sky in 1917 with an 18cm telescope, and its condensations were once regarded as companion stars.[20] It has been extensively studied with the aid of the Hubble Space Telescope (HST), showing that the nebula has a complex structure that includes filaments and arcs, which were caused by past eruptions; the structure is akin to that around post-red supergiant (Post-RSG) or yellow hypergiant (YHG) IRC +10420. The similarity has led at least two professional articles to propose a model that the star might evolve blueward on the Hertzsprung–Russell diagram (HR diagram) to become a yellow hypergiant, then a luminous blue variable (LBV), and finally a Wolf–Rayet star (WR star).[14][20]
Evolution
VY Canis Majoris is a highly evolved star yet less than 10 million years (Myr) old. Some old writings envisaged the star as a very young protostar or a massive pre-main-sequence star with an age of only 1 Myr and typically a circumstellar disk.[15] It has probably evolved from a hot, dense O9main sequence star of 5–20R☉ (solar radii).[30][32][59] The star has evolved rapidly because of its high mass. The time spent to the red hypergiant phase is estimated to be between 100,000 and 500,000 years, and thus VY CMa most likely left its main sequence phase more than a million years ago.[10][30]
The future evolution of VY CMa is uncertain, but like the most cool supergiants, the star will certainly explode as a supernova. It has begun to fuse helium into carbon en masse.[lower-alpha 7] Like Betelgeuse, it is losing mass and is expected to explode as a supernova within the next 100,000 years — it will probably revert to a higher temperature beforehand.[3][58][60] The star is very unstable, having a prodigious mass loss such as in ejections.
VY Canis Majoris is a candidate for a star in a second red supergiant phase, but this is mostly speculative and unconfirmed.[61]
From this star CO emission is coincident with the bright KI shell in its asymmetric nebula.
The explosion could be associated with gamma-ray bursts (GRB), and it will produce a shock wave of a speed of a few thousand kilometers per second that could hit the surrounding envelope of material, causing strong emission for many years after the explosion. For a star so large, the remnant would be probably a black hole rather than a neutron star.[60]
Notes
↑ The given stellar metallicity is given as the solar metallicity ([Fe/H] = approx. +0.0 dex).
↑ 1.14+0.11 −0.09 and 1.20+0.13 −0.10kpc, see above at Distance
↑ Alcolea et al 2013 refer to VY CMa as having the highest radius "among well-characterised stars in our galaxy", referring to the Wittkowski et al. 2012 value of 1,420R☉ which is based on the distances from Choi et al. 2008 and Zhang et al. 2012 plus an angular diameter. Several red supergiants (or hypergiants) are possibly larger, although they could have less accurate radius estimates.
↑ a main sequence star fuses hydrogen into helium.
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Red supergiants (RSGs) are stars with a supergiant luminosity class and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.
A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They have luminosity class I and spectral class B9 or earlier, although sometimes A-class supergiants are also deemed blue supergiants.
Mu Cephei, also known as Herschel's Garnet Star, Erakis, or HD 206936, is a red supergiant or hypergiant star in the constellation Cepheus. It appears garnet red and is located at the edge of the IC 1396 nebula. Since 1943, the spectrum of this star has served as a spectral standard by which other stars are classified.
Sigma Canis Majoris, also named Unurgunite, is a variable star in the southern constellation of Canis Major. It is approximately 1,120 light-years from the Sun and has an average apparent visual magnitude of +3.41.
VV Cephei, also known as HD 208816, is an eclipsing binary star system located in the constellation Cepheus, approximately 5,000 light years from Earth. It is both a B[e] star and shell star.
A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (MV) around −9, but also one of the rarest, with just 20 known in the Milky Way and six of those in just a single cluster. They are sometimes referred to as cool hypergiants in comparison with O- and B-type stars, and sometimes as warm hypergiants in comparison with red supergiants.
MY Cephei is a red supergiant located in open cluster NGC 7419 in the constellation of Cepheus. It is a semiregular variable star with a maximum brightness of magnitude 14.4 and a minimum of magnitude 15.5.
RW Cephei is a K-type hypergiant and a semirregular variable star in the constellation Cepheus, at the edge of the Sharpless 132 H II region and close to the small open cluster Berkeley 94. It is among the largest stars known with a radius of almost 1,000 times that of the Sun (R☉), nearly as large as the orbit of Jupiter.
WOH G64 is an unusual red supergiant (RSG) star in the Large Magellanic Cloud (LMC) satellite galaxy in the southern constellation of Dorado. It is one of the largest known stars, being described as possibly being the largest star known. It is also one of the most luminous and massive red supergiants, with a radius calculated to be around 1,540 times that of the Sun (R☉) and a luminosity around 282,000 times the solar luminosity (L☉).
A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants are classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially star formation, stability, and their expected demise as supernovae.
VX Sagittarii is an asymptotic giant branch star located more than 1.5 kiloparsec away from the Sun in the constellation of Sagittarius. It is a pulsating variable star with an unusually large magnitude range. It is one of the largest stars discovered, with a radius varying between 1,350 and 1,940 solar radii (940,000,000 and 1.35×109 km; 6.3 and 9.0 au). It is the most luminous known AGB star, at bolometric magnitude –8.6, which is brighter than the theoretical limit at –8.0.
RSGC1 is a young massive open cluster in the Milky Way galaxy. It was discovered in 2006 in the data generated by several infrared surveys, named for the unprecedented number of red supergiant members. The cluster is located in the constellation Scutum at the distance of about 6.6 kpc from the Sun. It is likely situated at the intersection of the northern end of the Long Bar of the Milky Way and the inner portion of the Scutum–Centaurus Arm—one of its two major spiral arms.
NML Cygni or V1489 Cygni is a red hypergiant or red supergiant (RSG) in the constellation Cygnus. It is possibly one of the largest known stars currently known by radius, and is also possibly one of the most luminous and massive cool hypergiants, as well as one of the most luminous stars in the Milky Way.
PZ Cassiopeiae is a red supergiant star located in the constellation of Cassiopeia, and a semi-regular variable star.
Westerlund 1 W26 or Westerlund 1 BKS AS is a red supergiant located at the outskirts of the Westerlund 1 super star cluster. It is one of the largest known stars and the most luminous supergiant stars discovered so far with radius calculated to be in excess of a thousand times the solar radius, and a luminosity of over 200,000 times the solar luminosity. If placed at the center of the Solar System, its photosphere would engulf the orbit of Jupiter.
AH Scorpii is a red supergiant variable star located in the constellation Scorpius. It is one of the largest stars known by radius and is also one of the most luminous red supergiant stars in the Milky Way.
HR 5171, also known as V766 Centauri, is a yellow hypergiant in the constellation Centaurus. It is said to be either an extreme red supergiant (RSG) or recent post-red supergiant (Post-RSG) yellow hypergiant (YHG), both of which suggest it is one of the largest known stars. The star's diameter is uncertain but likely to be between 1,100 and 1,600 times that of the Sun, while its distance is 3.6 kpc from Earth. According to a 2014 publication, the star is a contact binary, sharing a common envelope of material with a smaller yellow supergiant and secondary star, the two orbiting each other every 1,304 ± 6 days.
Roberta M. Humphreys is an American observational stellar astrophysicist. She is Professor Emerita at the University of Minnesota. Her work has included Galactic structure, observational stellar evolution, stellar populations, and large databases. She is best known for her research on massive stars in the Milky Way and in nearby resolved galaxies.
Stephenson 2 DFK 1, also known as RSGC2-01 or St2-18, is a red supergiant (RSG) or possible extreme red hypergiant (RHG) star in the constellation of Scutum. It lies near the open cluster Stephenson 2, which is located about 5.8 kiloparsecs away from Earth in the Scutum–Centaurus Arm of the Milky Way galaxy, and is assumed to be one of a group of stars at a similar distance, although some studies consider it to be an unrelated or foreground red supergiant. It is potentially among the largest known stars, one of the most luminous red supergiants, and one of the most luminous stars in the Milky Way.
Stephenson 2 DFK 49 or St2-11 is a putative post red supergiant star in the constellation Scutum. It is located in the massive open cluster Stephenson 2. It is possibly one of the largest known stars with radius estimates ranging from 884 solar radii to 1,300 solar radii. If the upper estimate is correct, then Stephenson 2 DFK 49 has a volume 2.2 billion times that of the Sun. If it was placed at the center of the Solar System, its photosphere will potentially approach or engulf Jupiter's orbit. It loses mass at a very high rate, resulting in large amounts of Infrared excess.
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