Sigma Octantis
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 Octans and around, with Sigma Octantis in the middle marked in cyan. (Click on the image to see it full size.) |
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| Observation data Epoch J2000 |
|
|---|---|
| Constellation (pronunciation) |
Octans |
| Right ascension | 21h 08m 46.8s |
| Declination | -88° 57′ 23″ |
| Apparent magnitude (V) | 5.42 |
| Characteristics | |
| Spectral type | F0 III |
| U-B color index | 0.13 |
| B-V color index | 0.27 |
| Variable type | Delta Scuti |
| Astrometry | |
| Radial velocity (Rv) | +11.9 km/s |
| Proper motion (μ) | RA: 25.96 mas/yr Dec.: 5.02 mas/yr |
| Parallax (π) | 12.07 ± 0.48 mas |
| Distance | 270 ly (83 pc) |
| Absolute magnitude (MV) | 0.83 |
| Other designations | |
| Database references | |
| SIMBAD | data |
Sigma Octantis (σ Oct / σ Octantis) is a magnitude 5.6 star in the constellation Octans most notable for being the current South Star. Sigma Octantis is approximately 270 light years from Earth, and is classified as a Giant, with a spectral type of F0 III. It is a Delta Scuti variable, with magnitude varying by about 0.03 magnitudes over 2.3 hours.
Sigma Octantis is the dimmest star to be represented on a national flag. It appears on the flag of Brazil, symbolising the capital city Brasilia.[1]
[edit] Southern Pole Star
Its position near the southern celestial pole makes it the southern hemisphere's current pole star and counterpart to Polaris, the current North Star. To an observer in the southern hemisphere, Sigma Octantis appears almost motionless and all the other stars in the Southern sky appear to rotate around it. It is part of a small "half hexagon" shape. It is over a degree away from the true south pole, and the south celestial pole is moving away from it due to precession of the equinoxes.
Sigma Octantis is sometimes called Polaris Australis, or South Star, though it has never been officially recognised as such and this name is certainly a modern invention.
[edit] Locating Sigma Octantis
The star itself, it is a faint star—at magnitude 5.5 it is barely visible to the naked eye—which makes it a rather poor pole star, especially by comparison with the much brighter and easily visible Polaris. This is why the Crux constellation is often preferred for the role of determining the position of the South Celestial Pole.
Once Sigma Octantis' approximate position has been determined, either by the major stars in Octans or using the Southern Cross method, it can be positively verified using an asterism. Sigma Octantis, Kappa Octantis, Tau Octantis and Upsilon Octantis are all stars of around magnitude 5.5, and form the distinctive shape of a trapezoid.
[edit] References
- ^ Astronomy of the Brazilian Flag. FOTW Flags Of The World website.
- HD 177482 -- Variable Star. SIMBAD Astronomical Database. Retrieved on 2005-11-05.
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