U Sagittarii

Not to be confused with υ (upsilon) Sagittarii.

U Sagittarii is a variable star in the southern constellation of Sagittarius, abbreviated U Sgr. It is a classical Cepheid variable that ranges in brightness from an apparent visual magnitude of 6.28 down to 7.15, with a pulsation period of 6.745226 days.^[3] At its brightest, this star is dimly visible to the naked eye. The distance to this star is approximately 2,080 light years based on parallax measurements,^[2] and it is drifting further away with a radial velocity of 2 km/s.^[6]

A light curve for U Sagittarii, plotted from ASAS data^[12]

U Sagittarii

U Sagittarii (center) is the brightest star in the open cluster Messier 25.[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	18h 31m 53.332s[2]
Declination	−19° 07′ 30.26″[2]
Apparent magnitude (V)	6.28 to 7.15[3]
Characteristics
Evolutionary stage	Supergiant
Spectral type	G1Ib[4]
U−B color index	+0.85[5]
B−V color index	+1.06[5]
Variable type	δ Cep[3]
Astrometry
Radial velocity (Rv)	2.2±0.3[6] km/s
Proper motion (μ)	RA: −1.795 mas/yr[2] Dec.: −6.127 mas/yr[2]
Parallax (π)	1.5693 ± 0.0224 mas[2]
Distance	2,080 ± 30 ly (637 ± 9 pc)
Absolute magnitude (MV)	−3.86 to −3.10[5]
Details
Mass	6.54 to 6.64[7] M☉
Radius	55.5 to 56.3[7] R☉
Luminosity	4,370 to 4,650[7] L☉
Surface gravity (log g)	0.99[8] cgs
Temperature	5,802[8] K
Metallicity [Fe/H]	0.17[8] dex
Rotational velocity (v sin i)	15.4[9] km/s
Age	91[10] Myr
Other designations
U Sgr, BD−19°5047, GC 25287, HD 170764, HIP 90836, HR 6947, SAO 161571, WDS J18319-1908A[11]
Database references
SIMBAD	data

The variability of this star was announced by J. Schmidt in 1866, who found a preliminary period of 6.74784 days.^[13] It was later determined to be a variable of the Cepheid type.^[14] In 1925, P. Doig assumed that the star is a member of the open cluster Messier 25 (M25), but actual evidence of its membership would not be available until 1932 when P. Hayford made radial velocity measurements of the cluster.^[15] Membership in this cluster is now reasonably established,^[16] and as such this Cepheid serves as one of the anchors for the cosmic distance scale since the distance to the cluster can be determined independently from the star.^[17] Indeed, new research indicates U Sgr's host cluster (M25) may constitute a ternary (triple) star cluster together with NGC 6716 and Collinder 394.^[18]

This is an evolved G-type supergiant star with a typical stellar classification of G1Ib.^[4] It appears to be making its third traversal of the instability strip with its period changing at the rate of +0.073±0.010 s·yr⁻¹. Elemental abundances are similar to those in the Sun.^[17] It has an estimated 6.6 times the mass of the Sun and 56 times the Sun's radius. The star is radiating over 4,000^[7] times the Sun's luminosity from its photosphere at an effective temperature of 5,802 K.^[8]

References

1. Luginbuhl, Christian B.; Skiff, Brian A. (1998), Observing Handbook and Catalogue of Deep-Sky Objects, Cambridge University Press, p. 213, ISBN 9780521625562.
2. Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
3. Samus, N. N.; et al. (2017), "General Catalogue of Variable Stars", Astronomy Reports, 5.1, 61 (1): 80–88, Bibcode:2017ARep...61...80S, doi:10.1134/S1063772917010085, S2CID 125853869.
4. Houk, N.; Smith-Moore, M. (1988), Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars, vol. 4, Bibcode:1988mcts.book.....H.
5. Sandage, Allan (May 1960), "Cepheids in Galactic Clusters.VI. U SGR in M25", Astrophysical Journal, 131: 610, Bibcode:1960ApJ...131..610S, doi:10.1086/146873.
6. Breger, M. (1967), "Velocity, radius and light variations of the cepheid U Sgr", Monthly Notices of the Royal Astronomical Society, 136: 61, Bibcode:1967MNRAS.136...61B, doi:10.1093/mnras/136.1.61.
7. Fadeyev, Yu. A. (August 2018), Shustov, B. M.; Wiebe, D. S. (eds.), "Secular period change in the cepheid U Sgr and the theory of stellar evolution", Stars and Satellites, Proceedings of the Memorial Conference Devoted to A.G. Masevich 100th Anniversary, held in Moscow, Russia, in Oct 15-16, 2018, INASAN Science Proceedings (in Russian), Moscow: Yanus-K, pp. 152–156, Bibcode:2018sas..conf..152F, doi:10.26087/INASAN.2018.2.2.024.
8. Prugniel, Ph.; et al. (July 2011), "The atmospheric parameters and spectral interpolator for the MILES stars", Astronomy & Astrophysics, 531: A165, arXiv:1104.4952, Bibcode:2011A&A...531A.165P, doi:10.1051/0004-6361/201116769, S2CID 54940439, A165.
9. Jönsson, Henrik; et al. (2020), "APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South", The Astronomical Journal, 160 (3): 120, arXiv:2007.05537, Bibcode:2020AJ....160..120J, doi:10.3847/1538-3881/aba592, S2CID 220496397.
10. Skowron, Dorota M.; et al. (2019), "A three-dimensional map of the Milky Way using classical Cepheid variable stars", Science, 365 (6452): 478–482, arXiv:1806.10653, Bibcode:2019Sci...365..478S, doi:10.1126/science.aau3181, PMID 31371611, S2CID 199064609.
11. "U Sgr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2022-12-03.
12. "ASAS All Star Catalogue". The All Sky Automated Survey. Retrieved 5 December 2022.
13. Schmidt, Johann Friedrich Julius (March 1868), "Über die Lichtphasen von A, γ und U Sagittarii in den Jahren 1866 und 1867", Astronomische Nachrichten (in German), 71: 139, Bibcode:1868AN.....71..139S.
14. Shapley, H. (December 1918), "Studies based on the colors and magnitudes in stellar clusters. VIII. The luminosities and distances of 139 Cepheid variables", Astrophysical Journal, 48: 279–294, Bibcode:1918ApJ....48..279S, doi:10.1086/142435.
15. Wallerstein, George (April 1957), "The Absolute Magnitude of U Sagittarii and Its Membership in M 25", Publications of the Astronomical Society of the Pacific, 69 (407): 172, Bibcode:1957PASP...69..172W, doi:10.1086/127041, S2CID 121527741.
16. Chen, Xiaodian; et al. (January 2015), "A search for open cluster Cepheids in the Galactic plane", Monthly Notices of the Royal Astronomical Society, 446 (2): 1268–1282, arXiv:1410.4489, Bibcode:2015MNRAS.446.1268C, doi:10.1093/mnras/stu2165.
17. Majaess, D.; et al. (December 2013), "Anchors for the cosmic distance scale: the Cepheids U Sagittarii, CF Cassiopeiae, and CEab Cassiopeiae", Astronomy & Astrophysics, 560: A22, arXiv:1311.0865, Bibcode:2013A&A...560A..22M, doi:10.1051/0004-6361/201322670, S2CID 55934597, A22.
18. Majaess, D.; et al. (August 2024), "A Rare Cepheid-hosting Open Cluster Triad in Sagittarius", Research Notes of the AAS, 8 (8): 205, doi:10.3847/2515-5172/ad7139.

Further reading

• Berdnikov, L. N.; Majaess, D. (April 2014), "Search for evolutionary changes in the periods of Cepheids: U Sgr", Astronomy Letters, 40 (4): 193–200, Bibcode:2014AstL...40..193B, doi:10.1134/S1063773714030025, S2CID 255203281.
• Simon, Norman R. (March 1989), "Application of the Baade-Wesselink inversion technique to SW Dra, X Ari and U Sgr", Monthly Notices of the Royal Astronomical Society, 237: 163–185, Bibcode:1989MNRAS.237..163S, doi:10.1093/mnras/237.1.163.
• Burnham, Robert (1978), Burnham's Celestial Handbook, An Observer's Guide to the Universe Beyond the Solar System, vol. 3, Dover Publications, ISBN 9780486236735.
• Jacobsen, T. S. (February 1970), "A High-Dispersion Velocity Curve of U Sagittarii", Astrophysical Journal, 159: 569, Bibcode:1970ApJ...159..569J, doi:10.1086/150331.
• Wampler, J.; et al. (May 1961), "Cepheids in Galactic Clusters. VIII. a Reinvestigation of U SGR in M25 (= IC 4725).", Astrophysical Journal, 133: 895, Bibcode:1961ApJ...133..895W, doi:10.1086/147092.
• Mitchell, Richard I.; et al. (May 1961), "U, B, V, Observations of U SGR", Astrophysical Journal, 133: 1083, Bibcode:1961ApJ...133.1083M, doi:10.1086/147114.
• Voûte, J.; ten Bruggencate, P. (1927), "U Sagittarii", Annals of the Bosscha Observatory Lembang (Java) Indonesia, 2: B28–B33, Bibcode:1927AnBos...2B..28V.
• Yendell, Paul S. (1895), "On variable stars of short period. IV--The Sagittarius Group", Popular Astronomy, 2: 362–366, Bibcode:1895PA......2..362Y.