Astronomy:VX Sagittarii

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Short description: Star in the constellation Sagittarius
VX Sagittarii
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A visual band light curve for VX Sagittarii, plotted from AAVSO data[1]
Observation data
Equinox J2000.0]] (ICRS)
Constellation Sagittarius
Right ascension  18h 08m 04.04831s[2]
Declination −22° 13′ 26.6327″[2]
Apparent magnitude (V) 6.5 - 14.0[3]
Characteristics
Evolutionary stage Red supergiant[4] or hypergiant[5]
Spectral type M4eIa - M10eIa[6]
Variable type SRc[3]
Astrometry
Radial velocity (Rv)+6.47±3.37[5] km/s
Proper motion (μ) RA: +0.36±0.76[5] mas/yr
Dec.: −2.92±0.78[5] mas/yr
Parallax (π)0.64 ± 0.04[5] mas
Distance5,100 ± 300 ly
(1,560 ± 100 pc)
Details
Mass20 to 40[4] M☉
Radius1,456±108 – 1,556±110[4] R☉
Luminosity195,000±62,000,[5]
275,000+114,000
−52,000[7] L☉
Temperature2,900[8] (near min), 3,200-3,400 (near max)[9], 2,400–3,300[10] K
Other designations
VX Sgr, BD−22°4575, CD−22°12589, HD 165674, HIP 88838, 2MASS J18080404-2213266
Database references
SIMBADdata

VX Sagittarii (abbreviated to VX Sgr) is an extreme red supergiant or hypergiant pulsating variable star with an unusually large magnitude range located in the constellation of Sagittarius and more than one kiloparsec away from the Sun. It is one of the largest stars discovered and also one of the most luminous and massive cool supergiant stars in the Milky Way, with an average radius of 1,450 solar radii (1.01×109 km; 6.7 astronomical unit|au) during its quiescent pulsation phase, and of 1,556 solar radii (1.083×109 km; 7.24 au) during its active pulsation phase.[4] At those radii, an object travelling at the speed of light would take over 6 hours to go around its surface, compared to 14.5 seconds for the Sun.[11] If this star replaced the Sun, its photosphere would extend beyond the orbit of Jupiter and near Saturn.

The overall nature of the star remains uncertain; it has been suggested to be a possible candidate for a Thorne–Żytkow object.[12][13]

Observations

In 1904, it was announced that Henrietta Leavitt had discovered that the star, then known as BD−22°4575, is a variable star.[14] It was given its variable star designation, VX Sagittarii, in 1911.[15] The star is classed as a cool semiregular variable of type SRc with a pulsational period of 732 days. The variations sometimes have an amplitude comparable to a long period variable, at other times they are much smaller. The spectral type varies between M4e around visual maximum and M9.8e at minimum light, and the luminosity class is Ia, indicating a bright supergiant. The spectrum shows emission lines indicating that the star is losing mass through a strong stellar wind.[10]

The annual parallax of VX Sagittarii has been measured as 0.64±0.06 mas, indicating a distance of about 5,100 light years. This is compatible with the distance to Sagittarius OB1, the stellar association that VX Sagittarii is thought to belong to. Its radial velocity and proper motions are also consistent with other members of the association.[5]

Stellar characteristics

The effective temperature of VX Sagittarii is variable from around 2,400 K at visual minimum to around 3,300 K near maximum. Such low temperatures are comparable to the very coolest AGB stars and unprecedented for a massive supergiant.[9][10] Its atmosphere is extended, irregular, and variable during the pulsations of the star, but the bolometric luminosity varies less than the visual brightness and is calculated to be about 195,000±62,000 L☉. At an effective temperature of 3,300 K, the radius is expected to be somewhere between 1,120 R☉ and 1,550 R☉.[5] Older studies frequently calculated higher luminosities.[16][17]

The atmosphere of VX Sgr shows molecular water layers and SiO masers in the atmosphere, typical of an OH/IR star.[18] The SiO masers have been used to derive a distance of 1.57±0.27 kiloparsecs.[19] The spectrum also indicates strong VO and CN. In many respects, the atmosphere is similar to low mass AGB stars such as Mira variables, but with a supergiant's luminosity and size.[9]

VX Sagittarii's pulsations alternate between a longer, active phase and a shorter, quiescent phase. Interferometric observations from 2018 to 2025, carried with the GRAVITY instrument aboard the Very Large Telescope, showed that during a quiescent phase, the star was pulsating in the fundamental mode with a mean radius of 1,556±110 R☉ and an amplitude of about 197 R☉, while in the quiescent phase, it was pulsating in the first overtone with a mean radius of 1,456±108 R☉ and an amplitude of only 60 R☉. In September 2019, the star increased to its maxium radius, at 1,798±127 R☉.[4]

During 2020 and 2021, the star experienced a great dimming event, at the end of the active pulsation phase. This event happened due to extreme mass loss from the star and was characterized by the expansion of carbon monoxide and water atmospheric layers up to 3,200 R☉ in August 2021, appearance of Balmer emission in the spectrum from mid-2020 to mid-2021, and Brγ hydrogen lines in the spectrum in March 2020. This is also the first detection of Brγ lines in a single-system red supergiant.[4]

One paper from May 2018 suggests that VX Sagitarii may be a hypergiant.[5] This would make it one of the very rare red hypergiant stars. A 2021 paper, however, concluded that VX Sagittarii is an extreme AGB star, rather than a red supergiant or hypergiant, which would make it the most luminous known of its kind, exceeding the theoretical limit for the bolometric magnitude at −8.0.[12] Because it displays rubidium in its spectrum and has a high mass loss and luminosity, it may be a type of AGB star known as a super-AGB star, a type of star with masses in between low-mass stars and high-mass stars.[12] A 2026 publication found that it is very unlikely that VX Sagittarii is a super-AGB star, but instead a supergiant, as deduced from its high luminosity, large radius, and pulsational properties, which are only consistent with those of red supergiants.[4]

See also

Other late-type red supergiants

Other Thorne–Żytkow object candidates

  • HV 2112 – A former supergiant TŻO candidate
  • HV 11417 – Another possible supergiant TŻO candidate

References

  1. ↑ "Download Data". AAVSO. https://www.aavso.org/data-download. 
  2. ↑ 2.0 2.1 Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. doi:10.1051/0004-6361:20078357. Bibcode: 2007A&A...474..653V. 
  3. ↑ 3.0 3.1 Samus, N. N. et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S 1. Bibcode: 2009yCat....102025S. 
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Jadlovský, D.; Wittkowski, M.; Chiavassa, A.; Kravchenko, K.; Freytag, B.; Höfner, S.; Krtička, J.; Paladini, C. et al. (2026-04-23). "VLTI-GRAVITY measurements of cool evolved stars: II. Pulsation properties and mass-loss process of the Mira star R Car and the red supergiant VX Sgr". Astronomy and Astrophysics. 
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Xu, Shuangjing; Zhang, Bo; Reid, Mark J; Menten, Karl M; Zheng, Xingwu; Wang, Guangli (2018). "The Parallax of the Red Hypergiant VX Sgr with Accurate Tropospheric Delay Calibration". The Astrophysical Journal 859 (1): 14. doi:10.3847/1538-4357/aabba6. Bibcode: 2018ApJ...859...14X. 
  6. ↑ Kiss, L. L.; Szabó, G. M.; Bedding, T. R. (2006). "Variability in red supergiant stars: Pulsations, long secondary periods and convection noise". Monthly Notices of the Royal Astronomical Society 372 (4): 1721–1734. doi:10.1111/j.1365-2966.2006.10973.x. Bibcode: 2006MNRAS.372.1721K. 
  7. ↑ Davies, Ben; Beasor, Emma R. (March 2020). "The 'red supergiant problem': the upper luminosity boundary of Type II supernova progenitors" (in en). MNRAS 493 (1): 468–476. doi:10.1093/mnras/staa174. Bibcode: 2020MNRAS.493..468D. 
  8. ↑ García-Hernández, D. A; García-Lario, P; Plez, B; Manchado, A; d'Antona, F; Lub, J; Habing, H (2007). "Lithium and zirconium abundances in massive Galactic O-rich AGB stars". Astronomy and Astrophysics 462 (2): 711. doi:10.1051/0004-6361:20065785. Bibcode: 2007A&A...462..711G. 
  9. ↑ 9.0 9.1 9.2 Chiavassa, A.; Lacour, S.; Millour, F.; Driebe, T.; Wittkowski, M.; Plez, B.; Thiébaut, E.; Josselin, E. et al. (2009). "VLTI/AMBER spectro-interferometric imaging of VX Sagittarii's inhomogenous outer atmosphere". Astronomy and Astrophysics 511: A51. doi:10.1051/0004-6361/200913288. Bibcode: 2010A&A...511A..51C. 
  10. ↑ 10.0 10.1 10.2 Lockwood, G. W.; Wing, R. F. (1982). "The light and spectrum variations of VX Sagittarii, an extremely cool supergiant" (in en). Monthly Notices of the Royal Astronomical Society 198 (2): 385–404. doi:10.1093/mnras/198.2.385. ISSN 0035-8711. Bibcode: 1982MNRAS.198..385L. 
  11. ↑ "Solar System Exploration: Planets: Sun: Facts & Figures". NASA. http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric. 
  12. ↑ 12.0 12.1 12.2 Tabernero, H. M.; Dorda, R.; Negueruela, I.; Marfil, E. (2021). "The nature of VX Sagitarii: Is it a TŻO, a RSG, or a high-mass AGB star??". Astronomy & Astrophysics 646: A98. doi:10.1051/0004-6361/202039236. 
  13. ↑ Martínez-Miravé, Pablo; Tamborra, Irene; Vigna-Gómez, Alejandro (2025). "Identifying Thorne-Żytkow Objects through Neutrinos". The Astrophysical Journal 984 (1): L2. doi:10.3847/2041-8213/adc8ab. Bibcode: 2025ApJ...984L...2M. 
  14. ↑ Leavitt, Henrietta; Pickering, Edward C (August 1904). "Sixteen New Variable Stars in Sagittarius". Harvard College Observatory Circular 91: 1. Bibcode: 1904HarCi..91....1L. https://articles.adsabs.harvard.edu/pdf/1904HarCi..91....1L. Retrieved 22 December 2024. 
  15. ↑ Dunér; Hartwig; Müller (December 1911). "Benennung von neu entdeckten veränderlichen Sternen". Astronomische Nachrichten 190 (4): 57–72. doi:10.1002/asna.19111900402. Bibcode: 1911AN....190...57.. https://articles.adsabs.harvard.edu/pdf/1912AN....190...57.. Retrieved 22 December 2024. 
  16. ↑ De Jager, C.; Nieuwenhuijzen, H.; Van Der Hucht, K. A. (1988). "Mass loss rates in the Hertzsprung-Russell diagram". Astronomy and Astrophysics Supplement Series 72: 259. Bibcode: 1988A&AS...72..259D. 
  17. ↑ Mauron, N.; Josselin, E. (2010). "The mass-loss rates of red supergiants and the de Jager prescription". Astronomy & Astrophysics 526: A156. doi:10.1051/0004-6361/201013993. ISSN 0004-6361. Bibcode: 2011A&A...526A.156M. 
  18. ↑ Greenhill, L. J. et al. (1995). "The sio masers and dust shell of VX sgr" (in en). Astrophysics and Space Science 224 (1–2): 469–470. doi:10.1007/BF00667909. ISSN 0004-640X. Bibcode: 1995Ap&SS.224..469G. 
  19. ↑ Chen, X.; Shen, Z. Q.; Xu, Y. (2007). "Measuring the Distance of VX Sagittarii with SiO Maser Proper Motions". Chinese Journal of Astronomy and Astrophysics 7 (4): 531. doi:10.1088/1009-9271/7/4/09. Bibcode: 2007ChJAA...7..531C. 

Further reading

  • Chiavassa, A. et al. (February 2022). "The extended atmosphere and circumstellar environment of the cool evolved star VX Sagittarii as seen by MATISSE". Astronomy & Astrophysics 658: A185. doi:10.1051/0004-6361/202142514. A185. Bibcode: 2022A&A...658A.185C. 
  • Su, Jiang-Bo et al. (December 2020). "Observational characteristics of SiO masers around VX Sgr". Research in Astronomy and Astrophysics 20 (12): 192. doi:10.1088/1674-4527/20/12/192. 192. Bibcode: 2020RAA....20..192S. 
  • Su, J. B. et al. (January 2018). "Dynamics of SiO Masers around VX Sgr". The Astrophysical Journal 853 (1): 42. doi:10.3847/1538-4357/aaa12b. 42. Bibcode: 2018ApJ...853...42S. 
  • Su, J. B. et al. (September 2014). "Outward Motions of SiO Masers around VX Sgr". Journal of Astrophysics and Astronomy 35 (3): 539–540. doi:10.1007/s12036-014-9280-2. Bibcode: 2014JApA...35..539S. 
  • Su, J. B. et al. (July 2012). "High-resolution VLBA Observations of Three 7 mm SiO Masers toward VX Sgr at Five Epochs". The Astrophysical Journal 754 (1): 47. doi:10.1088/0004-637X/754/1/47. 47. Bibcode: 2012ApJ...754...47S. 
  • Su, J. B. et al. (June 2011). "Pumping Mechanisms for SiO Masers around VX Sgr". Journal of Astrophysics and Astronomy 32 (1–2): 261–264. doi:10.1007/s12036-011-9002-y. Bibcode: 2011JApA...32..261S. 
  • Lu, R. -S. et al. (2010). "High-frequency VLBI Imaging of Sgr A* and VX Sgr". Proceedings of 10th European VLBI Network Symposium and EVN Users Meeting: VLBI and the new generation of radio arrays — PoS(10th EVN Symposium). 10. p. 087. doi:10.22323/1.125.0087. Bibcode: 2010evn..confE..87L. 
  • Chen, Xi et al. (April 2006). "Inward Motions of the Compact SiO Masers around VX Sagittarii". The Astrophysical Journal 640 (2): 982–994. doi:10.1086/500168. Bibcode: 2006ApJ...640..982C. 
  • Kamohara, Ryuichi et al. (April 2005). "Time Variation of SiO Masers in VX Sagittarii over an Optically Quiescent Phase". Publications of the Astronomical Society of Japan 57 (2): 341–345. doi:10.1093/pasj/57.2.341. Bibcode: 2005PASJ...57..341K. 
  • Murakawa, K. et al. (September 2003). "The radially expanding molecular outflow of VX Sagittarii". Monthly Notices of the Royal Astronomical Society 344 (1): 112. doi:10.1046/j.1365-8711.2003.06626.x. Bibcode: 2003MNRAS.344....1M. 
  • Yates, J. A.; Richards, A. M. S.; Gray, M. D. (2002). "The H2O maser proper motions of RT Vir and VX Sgr". in Mineese, Victor; Reid, Mark. Cosmic Masers: From Proto-Stars to Black Holes. 206. San Francisco: Astronomical Society of the Pacific. p. 298. ISBN 1-58381-112-5. Bibcode: 2002IAUS..206..298Y. 
  • Szymczak, M. et al. (June 2001). "Magnetic field structure in the outer OH maser envelope of VX Sagittarii". Astronomy and Astrophysics 371 (3): 1012–1017. doi:10.1051/0004-6361:20010451. Bibcode: 2001A&A...371.1012S. 
  • Pashchenko, M. I.; Rudnitskii, G. M. (May 1999). "Observations of late-type variable stars in the water-vapor radio line. The supergiant VX Sagittarii". Astronomy Reports 43 (5): 311–324. Bibcode: 1999ARep...43..311P. 
  • Berulis, I. I. et al. (1999). "H2O maser emission of the M-type supergiant VX Sgr". Astronomical and Astrophysical Transactions 18 (1): 77–82. doi:10.1080/10556799908203038. Bibcode: 1999A&AT...18...77B. 
  • Trigilio 7, C. et al. (June 1998). "The circumstellar magnetic field of VX Sagittarii". Monthly Notices of the Royal Astronomical Society 297 (2): 497–501. doi:10.1046/j.1365-8711.1998.01538.x. Bibcode: 1998MNRAS.297..497T. 
  • Doeleman, S. S. et al. (1998). "86 GHz VLBI Imaging of the SiO Masers in VX SGR". in Zensus, J. A.; Taylor, G. B.; Wrobel, J. M.. IAU Colloquium 164: Radio Emission from Galactic and Extragalactic Compact Sources. ASP Conference Series. 144. p. 363. ISBN 1-886733-64-3. Bibcode: 1998ASPC..144..363D. 
  • Zell, Philip J.; Fix, John D. (July 1996). "The Spatial Distribution of Circularly Polarized 1612 MHz OH Maser Emission From VX SGR". Astronomical Journal 112: 252. doi:10.1086/118007. Bibcode: 1996AJ....112..252Z. 
  • Trigilio, C. et al. (1996). "Magnetic field measurements in the circumstellar envelope of VX SGR". in Pallavicini, Roberto; Dupree, Andrea K.. Cool Stars, Stellar Systems, and the Sun, 9th Cambridge Workshop. ASP Conference Series. 109. San Francisco: Astronomical Society of the Pacific. p. 549. Bibcode: 1996ASPC..109..549T. 
  • Greenhill, Lincoln J. et al. (June 1994). "Circumstellar envelope of VX SGR resolved with long-baseline interferometry at millimeter and midinfrared wavelengths". in Breckinridge, James B.. Amplitude and Intensity Spatial Interferometry II. 2200. pp. 304–315. doi:10.1117/12.177248. Bibcode: 1994SPIE.2200..304G. 
  • Greenhill, L. J. et al. (May 1993). "The Circumstellar Envelope of VX SGR". Bulletin of the American Astronomical Society 25: 824. Bibcode: 1993AAS...182.2203G. 
  • Yu, Zhiyao (June 1992). "The Circumstellar Envelope of VX-Sgr". Astrophysics and Space Science 192 (1): 53–62. doi:10.1007/BF00653259. Bibcode: 1992Ap&SS.192...53Y. 
  • Pijpers, F. P. (November 1990). "A model for the wind of the M supergiant VX Sagittarii". Astronomy and Astrophysics 238: 256. Bibcode: 1990A&A...238..256P. 
  • Netzer, Nathan (July 1989). "Interaction of Dust and Radiation in Circumstellar Envelopes. I. The Expansion Velocity Gradient in the Envelope of VX Sagittarii". The Astrophysical Journal 342: 1068. doi:10.1086/167663. Bibcode: 1989ApJ...342.1068N. 
  • Chapman, J. M.; Cohen, R. J. (May 1986). "MERLIN observations of the circumstellar envelope of VX Sagittarius". Monthly Notices of the Royal Astronomical Society 220 (3): 513–528. doi:10.1093/mnras/220.3.513. Bibcode: 1986MNRAS.220..513C. 
  • Smith, Horace A. (February 1978). "Beat Phenomena in the Light Curve of VX Sgr". Journal of the American Association of Variable Star Observers 6 (2): 63. Bibcode: 1978JAVSO...6...63S. 
  • Dinerstein, Harriet (1973). "VX Sagittarii: A Variable at Many Wavelengths". Journal of the American Association of Variable Star Observers 2 (2): 52–59. Bibcode: 1973JAVSO...2...52D. 
  • Humphreys, Roberta M.; Lockwood, G. W. (March 1972). "Spectroscopic and Photometric Changes in the Peculiar Infrared Star VX Sagittarius". The Astrophysical Journal 172: L59. doi:10.1086/180891. Bibcode: 1972ApJ...172L..59H. 
  • Robinson, B. J. et al. (March 1971). "Similarity of the OH Emissions from VX Sagittarii and VY Canis Majoris". Astrophysical Letters 8: 171. Bibcode: 1971ApL.....8..171R. 



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