Canis Major VY

Red supergiant in Canis Major

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VY Canis Majoris (VY CMa) is a Canis Major Extremely oxygen rich type of Red super giant , about 3900 light years (1200 Second gap ）， Apparent magnitude About 6.5~9.6, etc. According to observation, its mass is about 17 times of the sun, and its radius can only be estimated to be 1420-2100 times of the sun due to uncertainty solar system Center, its edge will be located at Jupiter Off track.

Canis Major VY is as bright as 300000 suns, and is classified as Supergiant It is also one of the largest red supergiant stars known. And most of the other stars Supergiant The difference is that it is a single star. Canis Major VY is also a Semiregular variable star 。^[35]

Chinese name: Canis Major VY
Foreign name: VY Canis Majoris
Alias: VY CMa
Classification: Red super giant
Discoverer: Jerome Lalande
Discovery time: March 7, 1801
Average density: 5.33✕10 -3 To 8.38 ✕ 10 -3 g/m³
surface temperature: 3490 K （±90）
Apparent magnitude: 6.5 to 9.6 etc
Right ascension: 07:22:58.33s
Declination: -25°46′03.24″
Distance from ground: 3820 light years （+260 −230）
Spectral type: M3-M4.5（M2.5-M5e Ia）^[1-3]
Apparent magnitude (U): twelve point zero one^[4]
Apparent magnitude (B): ten point one nine^[4]
Apparent magnitude (V): seven point nine five^[4]
Apparent magnitude (J): one point nine eight^[4]

Apparent magnitude (H): zero point four four^[4]
Apparent magnitude (K): eight point one^[5]
U-B color index: +2.32^[6]
B-V color index: +2.057^[7]
V-R color index: +2.20^[6]
Variant type: SRc or Lc^[8-9]
radial velocity: 41 km/s^[10]
Self: Right ascension: 9.84 mas/yr Right latitude: 0.75 mas/yr^[7]
Parallax: 0.83±0.08 mas^[11]
Distance: 1170 (+80 or - 70) pc, 3820 (+260 or - 230) ly^[1]
Absolute thermal magnitude: -9.4
Stellar mass: 17±8 M☉^[1]
Radius: 2,069 R☉^[1]^[12-13]
Thermoluminosity: 270000 ± 400000178000 (+40900 or - 29900) L ⊙^[12-14]
Surface gravity: -0.6±0.4 cgs^[1]
Effective temperature: 3,940±90 K^[1]
Metal abundance [Fe/H]: -0.3 dex^[15]
Autorotation speed: 300 km/s^[11]

catalog

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six evolution

Naming and observation

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Canis Major VY position is not far from the bottom left of NGC 2362

The first observation record of VY Canis Majoris (VY CMa) was made on March 7, 1801 by French astronomer Jerome· Lalande (J é r ô me Lalande) Apparent magnitude About 8 stars are recorded in Catalogue Medium. Later, when studying its luminosity in the 19th and 20th centuries, he thought that its the magnitude It began to decline in 1850. Since 1847, it has been known that Canis Major VY is a Crimson fixed star. Observations in the 19th century found that Canis Major VY has at least 6 bright separation regions, so it is considered that it may be a Aggregate star 。 These bright areas are actually the bright parts of the gas around Canis Major VY. The brightness change of Canis Major VY was first described in 1931, when it was listed as an apparent magnitude range of 9.5 to 11.5 Long-period variable star 。^[16] This variable star was named Canis Major VY in 1939, meaning Canis Major The 43rd variable star of. 1957 Optical observation And the high-resolution observation in 1998 confirmed that Canis Major VY did not companion 。 Canis Major VY is an M-type star with high luminosity. The effective surface temperature is about 3490 degrees Kelvin Herotu In the upper right corner, so it becomes Supergiant The process is considered quite complex. When it was young, it might be an O9 main sequence star with a mass of about 25 ± 10 times Solar mass 。

Peristellar envelope

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The Hubble WFPC2 image shows the asymmetric nebula around VY Canis Major

Canis Major VY is widely and densely asymmetrical red Reflection nebula Surrounded by stars ejecting into the nebula Total mass It is 0.2~0.4 times the mass of the sun, and the temperature of the gas (formed by the material discharged from the central star) based on the DUSTY model is 800 Kelvin. The diameter of the nebula's inner shell is 0.12 angular seconds, which corresponds to 140 out of 1200 seconds Astronomical unit (0.0022 light year). The diameter of the nebula shell is 10 angular seconds, corresponding to 12000 astronomical units (0.19 light years). This nebula is so bright that it can be found with an 18 cm telescope in 1917. It contains companion Dense area. The Hubble Space Telescope has conducted extensive research on it, and the results show that star clouds have complex structures, including fibrous and arc structures caused by past stellar eruptions. This structure is similar to Post red super giant (Post RSG) or Yellow hypergiant （Yellow hypergiant，YHG） IRC+10420 The surrounding nebula. such Similarity It prompted astronomers to propose that VY Canis Major will Herotu (Hertzsprung Russell diagram) evolved upwards to become a super giant in Huangte, and then became High luminosity blue variable star (Luminous blue variable, LBV), and finally Wolf Rayet star (WR).^[17]

By comparing data from the Hubble Space Telescope with data from the Hawaii Of Keck telescope The data obtained can be combined with the materials around Canis Major VY three-dimensional reconstruction 。 This reconstruction shows that the mass loss of Canis Major VY is greater than that of any Red supergiant or Red super giant Expectations are much more complex. It is obvious that bow shaped and nodular structures appear at different times, which indicates that the jet orientation is random and originates from the explosion in the active region of the stellar photosphere. Spectral data show that the jets leave the stars at different speeds, which indicates that they occurred at different times and originated in different regions of the surface of Canis Major VY.^[18] It is speculated that many asymmetric mass loss events and the ejection of the outermost material have occurred in the past 500~1000 years, while a nodular structure near the star was generated less than 100 years ago. Magnetic field related in the fragile outer layer of stars Strong convection Quality loss is caused.^[18] This is similar to Sunspot and Coronal mass ejection 。

distance

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Star distance can be measured by The earth revolves around the sun Generated parallax To calculate. However, Canis Major VY has smaller parallax due to its long distance, while the standard visual observation results Error range It is too large to use for super large stars with too large expansion and extension of the circumferential envelope (CSE). For example, according to 1997 Hipparcos catalogue The parallax data of Canis Major in Hipparcos Catalog is 1.78 ± 3.54 milliseconds (mas), and the calculated distance is 561.8 seconds (1832.34 light years).^[19]

Map of Giant Molecular Cloud Sharpless 310 and Its Surrounding Environment

In 1976, Charles J. Lada and Mark J. Reid published their research on Molecular cloud The observation result of Sh2-310 is located at 15 points east of Canis Major VY. There is a bright ring at the edge of the molecular cloud, and the CO emission spectral intensity at the edge suddenly drops sharply, and it is observed that twelve CO emission Spectral intensity The rising condition, these two conditions may respectively represent the destruction of the molecular material and the heating between the ring and the molecular cloud. Lada and Reid hypothesized that the distance between the molecular cloud and the earth is Open cluster NGC 2362 The star in is the same distance from the earth, and it is this star cluster that will Molecular cloud The molecules in the edge ring of Sh2-310 are ionized. NGC 2362 is at a distance of Color magnitude diagram （ Herotu ）It is estimated to be about 4890 ± 1630 light years. Canis Major VY is located at one corner of the ring, so it may be related to molecular cloud Sh2-310. In addition, the speed of molecular cloud is very close to that of Canis Major radial velocity 。 This further shows that VY Canis Major is related to the molecular cloud and star cluster NGC 2362, which means that VY Canis Major is also about 4890 light-years away.

The parallax of Canis Major VY can be interfered by using a long baseline Measuring instrument Observed Astrophysical shooting conduct High precision measurement 。 In 2008, for H two Observational use of O National Astronomical Observatory of Japan VERA of Interferometer Measured Astrophysical shooting Parallax is 0.88 ± 0.08 millisecond (mas), and 1140 is calculated accordingly +110 −90 Second gap (approximately 3720 +360 -300 Light years). In 2012, use Ultra long baseline array (VLBA) Very long baseline interferometry （ VLBI ）Independent observation of SiO Astrophysical shooting The resulting parallax is 0.83 ± 0.08 millisecond (mas), corresponding to 1200 +130 −100 The distance of the second difference (about 3910 +423 −326 Light years). These new distances Estimated value It means that the distance of Sh2-310 is not as far as commonly estimated, or Canis Major VY is the foreground object of Sh2-310. Gaia The detector mission should have provided sufficiently accurate parallax to determine the distance from Canis Major VY, but the parallax value of − 5.92 ± 0.83 milliangular seconds (mas) in Gaia data release 2 is meaningless.^[20]

Spectral classification

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Canis Major VY is a Variable star , which Apparent magnitude The minimum brightness is about 9.6, the maximum brightness is about 6.5, and the pulsation period is estimated to be about 956 days.^[8-9] stay Variable star list (GCVS), which is classified as SRc subtype Semiregular variable star , which indicates that it is a Cold supergiant^[8] , but in American Association of Variable Star Observers (AAVSO) is classified as LC type slow irregular variable star^[9] 。 In addition, other change cycles are 1600^[21] And 2200^[22] Days.

Canis Major VY is a High brightness M stars. However, its hydrogen line has High luminosity blue variable star Cygnus P The spectrum is dominated by TiO band, and its intensity is classified as M5. The H-alpha (H α) line is not visible yet, and there are also abnormal emission lines of neutral elements such as sodium and calcium. According to different Spectral characteristics The photometric level determined is from II（ Bright superstar ）To Ia (supergiant), the compromise result is M5eIbp. Early attempts at classification were also hampered by nebulous gas, which was regarded as a companion star.^[23]

The spectral grade obtained varies according to the characteristics studied, and the spectral characteristics also change significantly over time. It is considered colder than M2 and therefore redder, and is usually classified as M3 Between and M5^[2] 。 Some people give extreme classifications such as M2.5 and M5. The luminosity level is also very confused, usually only I is given, partly because the luminosity level is in the red and infrared of the spectrum Partial definitions It's very unclear. However, one study gave a luminosity level of Ia+, which means that Canis Major VY is a Supergiant Or extremely bright Red supergiant 。^[24]

Canis Major VY also found Astrophysical shooting At OH (1612 MHz ），H two The transmission line generated at O (22235.08 MHz) and SiO (43122 MHz) is typical OH/IR star features. Other examples HCN ，NaCl，PN，CH， CO ，CH three OH, TiO and TiO two Many molecules have also been detected.^[25] Canis Major VY is sometimes considered as the prototype of a class of OH/IR supergiant with serious mass loss, and the more common asymptotic giant branch OH/IR star Different.

physical characteristics

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Canis Major VY is a very large and very bright star. It is the most extreme star in the Milky Way Effective temperature Below 4000 ON (3730 centigrade ）。 Although its exact brightness and temperature are still uncertain, it does lie in the Herotu The determination of most properties of a star depends directly on its distance.

luminosity

In 2006, Roberta Humphrey calculated the luminosity of Canis Major VY from its spectral energy distribution distance. Because most of the radiation is blocked by the dust in the perigalactic envelope, Han Furui's integral calculation takes the Total radiation The result is that its luminosity is about 560000 times that of the sun.

In addition to volume, the luminosity of Canis Major VY is also a controversial issue. Han Furui believes that visible light Light measurement is not enough, because the dust around the star will emit light to the star Reprocessing , make Visible light wavelength Grow into infrared 。

The thermoluminosity of Canis Major VY can be calculated by spectral energy distribution or thermoluminosity flux, which can be determined by photometry in several visible and infrared wavebands. Earlier based on 1500 Second gap The luminosity calculated from the assumed distance is 200000~560000 times that of the sun^[26-28] , which is quite close to or beyond the empirical Eddington limit 。 In 2006, University of Minnesota Professor Roberta M. Humphreys integrated stellar gas Cladding The total flux of is 430000 times that of the sun through integral calculation, because most of the radiation from stars is reprocessed by the dust in the surrounding clouds, making the wavelength of visible light longer and become infrared.^[29] The latest estimate of luminosity is based on the distance below 1200 seconds, which is 350000 times lower than the sun.

Most of the radiation of Canis Major VY is infrared light The radiation peak is 5~10 μ m, which is caused by the reprocessing of radiation by the surrounding nebula^[10] If the distance is considered to be 1200 seconds, many earlier photometric estimates are consistent with the current photometric estimates. Although VY Canis Major is one of the brightest stars in the Milky Way, most of its visible light is Gas absorption Therefore, it cannot be seen by the naked eye, and can only be observed with a telescope. If no light is absorbed, it will be a star visible to the naked eye.

quality

Because Canis Major VY does not companion , its quality cannot pass Gravitational interaction direct measurement 。 The effective temperature and thermal luminosity of Canis Major VY High quality The comparison of the evolution trajectories of stars shows that if it was originally a fast rotating star, its initial mass may be sunlight 25 (± 10) times, the current mass is about 15 times the solar mass. If we assume that it is a slowly rotating star, then according to the calculation, its initial mass may be about 32 times that of the sun, while the current mass is about 19^[1] Times the mass of the sun, about 8.2 million years old^[11] Years. Earlier studies have given a higher initial mass, or according to old photometric estimates, the initial mass of the parent nebula of VY Canis Major is 40 to 60 times that of the sun.^[11]

Under the newly measured radius, some people believe that such a large star is caused by an overweight with a mass of 150 times more than the mass of the sun Stellar evolution However, in fact, a star whose mass is more than 60 times that of the sun cannot experience Red supergiant Phase. A star whose mass is 150 times that of the sun has an effective temperature of more than 30000 Kelvin when it is born Main sequence star It will also expand to a certain extent and become a super giant of Lante. But even if it expands to the maximum extent, its radius is only about 400 times that of the sun, surface temperature It is still above 10000. Such a star will not go through the stage of red supergiant. Its short life is not enough to support its expansion of its shell to more than 2000 times the diameter of the sun, and it is too violent nuclear fusion The reaction will also cause its outer layer to be violently ejected into space, instead of steadily expanding to thousands of times the diameter of the sun like a star with about 20 times the mass of the sun.

Pseudo color photos of large amount of gas emitted by Canis Major VY

Canis Major VY has powerful constancy Stellar wind Because of its extremely high luminosity and relatively low surface gravity, it is losing a lot of material. Its average Mass loss rate The solar mass is about 0.0006 per year, which is one of the stars with the highest known mass loss rate. Even for a red supergiant, it is extremely huge, which can be proved by its wide peristellar envelope^[21]^[30] 。 Therefore, it is to understand the great quality Stellar evolution The final mass loss event is one of the most important stars.^[21] In the most violent mass loss In the event, its mass loss rate may exceed 0.001 solar mass per year.^[30] The updated observation results suggest that the mass of Canis Major VY is about 17 times that of the sun, passing through the material Loss rate The initial mass of Canis Major VY can be inferred. This red super giant may have lost more than 1/3 of the mass of the main sequence star stage, so Canis Major VY may be an O9 star with a mass of about 25 times the mass of the sun in the main sequence star stage.

temperature

The effective temperature of Canis Major VY is uncertain. Because its spectral grade is changing, its temperature may also change. Early estimates of its temperature were based on M5 Assuming its temperature is below 3000 Kelvin.^[29]^[31] In 2006, its temperature was calculated as high as 3650 ± 25 degrees Kelvin, and the corresponding spectral grade was M2.5^[2] , although Canis Major VY is generally considered as a M4 -M5 stars. Using the spectral grade and temperature scale of M4-M5 proposed by Emily Levesque, the temperature range obtained is 3450 to 3535 Kelvin.

volume

Comparison chart of Canis Major VY and Sun

The calculation of the radius of Canis Major VY is very complicated due to the gas cloud in the surrounding envelope of the star. At the same time, Canis Major VY is also a Pulsating variable star , so its size will change over time. Early in infrared（ K-band , the wavelength is 2.2 μ m), the direct measurement of its radius gives 18.7 ± 0.5 millisecond (mas) Angular diameter If the distance is 1500 seconds, its radius is 3000 times larger than that of the sun, greatly exceeding that of conventional red supergiant stars^[31] 。 However, this is probably larger than the actual size of the star（ Stellar photosphere Radius), and the estimated angular diameter seems to be too large due to the interference of the peri satellite envelope.^[1]^[10]^[32] In 2006-2007, a study based on an estimated 430000 times Solar luminosity And the effective temperature of 3200-3535 degrees Kelvin, the radius of which is 1800~2100 times of the sun.

VY image of Canis Major taken by SPHERE instrument of Very Large Telescope

March 6 and 7, 2011, using Very large telescope Of Interferometry VY Canis Major was observed in the near infrared band, and the size of the star was calculated using the Rotherland radius. Of this band Optical depth 2/3, and according to the re measurement of the distance, measurement result 1140 (+110 or - 90) and 1200 (+130 or - 100) seconds respectively^[11] The angular diameter measured directly is 11.3 ± 0.3 mm Angular second 。 The height of these observations spectral resolution So that the impact of the pollution of the ring envelope is minimized. Then, according to the radius, its effective temperature is 3490 ± 90 ° C, the corresponding spectral grade is M4, and the luminosity is 27 ± 40000 times the solar luminosity, which is based on the distance and the measured flux (6.3 ± 0.3) × 10 -13 W/c㎡^[1] Calculated. At the end of 2013, Songpu et al competitiveness The final result is about 2069 times Solar radius^[13] , based on Cold end The estimated temperature is 2800 Kelvin, and the luminosity is 237000 times the value of the sun.^[34]

Compared with other well-known stars, VY Canis Major is often the largest one

Most radius estimates for Canis Major VY are considered optical photosphere The size of the, and the Stellar radio The size of the photosphere is estimated to be twice the size of the optical photosphere.^[3] Although Canis Major VY has a very large volume, its average density is only 5.33~8.38 mg/m3, less than the Earth sea level Of Atmospheric density (1.2 kg/m3).

Since the middle of the 20th century, Canis Major VY has been considered as Extreme celestial body , although its true nature is still uncertain. The size of Canis Major VY has been calculated more accurately, while some other galactic and extragalactic red supergiant stars (and Supergiant ）, such as Stevenson 2-18 （Stephenson 2-18）、 Westelu 1-26 （ Westerlund 1 -26) and Large Magellanic Galaxy Of WOH G64 , there is a larger volume. However, VY Canis Major is still described as the largest radius among stars with obvious characteristics. Another other estimate is based on the Wittkowski radius and Monnier radius, and the average size of Canis Major VY is 2000 times the solar radius.^[25]

evolution

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Scientists found that Canis Major VY is bloated Red super giant , less than 10 million years old, but Red super giant Your life is short. It has lost a lot of quality, and the quality lost every year is equivalent to 0.0006 Solar mass , and its mass is about 17 times that of the sun. The giant in the universe has entered the end of its life, approaching the final stage of its star life, and it is likely to occur once in 100000 years Polar supernova The explosion ended. In this process China Council Rapid outward expansion leads to the eruption of a large amount of material into space, forming Planetary nebula 。

Astronomers use Hubble Space telescope 、 European Southern Observatory Very large telescope After tracking and observing Canis Major VY, it is found that its surroundings are full of lost mass, dust clouds and Gas cloud Discrete distribution Around. According to the findings observed data With theoretical deduction, the study of Canis Major VY is helpful to understand such super massive stars (such as Pismis 24-1 ）Of Death process 。 The mass of Canis Major VY may be about 17 times the mass of the sun, which will theoretically produce Polar supernova Burst to form a black hole or neutron star , will release at the same time Gamma ray bursts , make Galaxy Some areas were destroyed, destroying lives in thousands of light years nearby. Because of its long distance, the earth will not be disturbed and life will not be disturbed. Because of its amazing energy release Galaxy Will see it explode.^[33]

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