U Sagittarii

Not to be confused with υ (upsilon) Sagittarii.
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]
Distance2,080 ± 30 ly
(637 ± 9 pc)
Absolute magnitude (MV)−3.86 to −3.10[5]
Details
Mass6.54 to 6.64[7] M☉
Radius55.5 to 56.3[7] R☉
Luminosity4,370 to 4,650[7] L☉
Surface gravity (log g)0.99[8] cgs
Temperature5,802[8] K
Metallicity [Fe/H]0.17[8] dex
Rotational velocity (v sin i)15.4[9] km/s
Age91[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
SIMBADdata

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]

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]

  1. ^ Cite error: The named reference Luginbuhl_Skiff_1998 was invoked but never defined (see the help page).
  2. ^ a b c d e Cite error: The named reference GaiaEDR3 was invoked but never defined (see the help page).
  3. ^ a b c Cite error: The named reference Samus_et_al_2017 was invoked but never defined (see the help page).
  4. ^ a b Cite error: The named reference Houk_Smith-Moore_1988 was invoked but never defined (see the help page).
  5. ^ a b c Cite error: The named reference Sandage_1960 was invoked but never defined (see the help page).
  6. ^ a b Cite error: The named reference Breger_1967 was invoked but never defined (see the help page).
  7. ^ a b c d Cite error: The named reference Fadeyev_2018 was invoked but never defined (see the help page).
  8. ^ a b c d Cite error: The named reference Prugniel_et_al_2011 was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference johnsson2020 was invoked but never defined (see the help page).
  10. ^ Cite error: The named reference skowron2019 was invoked but never defined (see the help page).
  11. ^ Cite error: The named reference SIMBAD was invoked but never defined (see the help page).
  12. ^ Cite error: The named reference ASASServer was invoked but never defined (see the help page).
  13. ^ Cite error: The named reference Schmidt_1868 was invoked but never defined (see the help page).
  14. ^ Cite error: The named reference Shapley_1918 was invoked but never defined (see the help page).
  15. ^ Cite error: The named reference Wallerstein_1957 was invoked but never defined (see the help page).
  16. ^ Cite error: The named reference Chen_2015 was invoked but never defined (see the help page).
  17. ^ a b Cite error: The named reference Majaess_et_al_2013 was invoked but never defined (see the help page).
  18. ^ Cite error: The named reference Majaess2024 was invoked but never defined (see the help page).