V725 Sagittarii
Star in the constellation Sagittarius
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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 | |
| V725 Sgr[6] | |
| Database references | |
| SIMBAD | data |
In 1937, Henrietta Hill Swope announced her discovery that the star is a variable star, after she had analysed 40 years of Harvard College Observatory photographic plates.[9][10] It was given its variable star designation in 1939.[11] 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.[12] In 2006, it was reported that in 2000 V725 Sagittarii was an early M star with emission lines.[13] In 2010, the spectral type was estimated from its colours and other properties to be K4III, although possibly late K.[3]