V725 Sagittarii is a variable star in the southern constellation of Sagittarius. As recently as a century ago, it was a Population II Cepheid; its transformation was documented by Henrietta Swope beginning in 1937,[7] and is one of the most exciting and instructive events in variable-star astronomy.[8] The star has varied between apparent visual magnitude 12.3 and 14.3.[4]
Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
---|---|
Constellation | Sagittarius |
Right ascension | 18h 11m 59.43904s[2] |
Declination | −36° 06′ 40.3345″[2] |
Characteristics | |
Spectral type | K4III (in 2009)[3] |
Variable type | Unique[4] |
Astrometry | |
Proper motion (μ) | RA: −1.111[2] mas/yr Dec.: −4.396[2] mas/yr |
Parallax (π) | 0.1572 ± 0.0433 mas[2] |
Distance | approx. 21,000 ly (approx. 6,000 pc) |
Absolute magnitude (MV) | −2.4[3] |
Details | |
Mass | 4.25+0.67 −2.40[5] M☉ |
Temperature | 4,413[5] K |
Metallicity [Fe/H] | −0.001[5] dex |
Other designations | |
Database references | |
SIMBAD | data |
Prior to 1926, this star showed the appearance of being an irregular variable. It then became a Population II Cepheid showing a regular light curve with a period of 12 days. Monitoring showed a gradual increase to a 21 day period by 1935,[7] but did not show a corresponding change in brightness. The star was mostly ignored until 1967–68 when it was seen to vary by 0.4 magnitude with a 50 day period.[8] Steady observation thereafter showed that the star had experienced a thermal flash and performed a loop on the H-R diagram. It migrated from the asymptotic giant branch (AGB) to the Cepheid instability strip and then back to the AGB.[3]
In 1973, the spectral class of V725 Sagittarii was estimated to be between F8 and G2 and similar to a type Ib supergiant.[3] In 1994 it was observed to be G8 based on the spectral lines of metals and later than F8 based on the hydrogen lines.[9] In 2006, it was reported that in 2000 V725 Sagittarii was an early M star with emission lines.[10] In 2010, the spectral type was estimated from its colours and other properties to be K4III, although possibly late K.[3]