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Asellus Australis is the central star in the "crab" |
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| Observation data Epoch J2000.0 Equinox J2000.0 |
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|---|---|
| Constellation | Cancer |
| Right ascension | 08h 44m 41.1s |
| Declination | +18° 09′ 15″ |
| Apparent magnitude (V) | +3.94 |
| Characteristics | |
| Spectral type | K0III |
| U−B color index | 0,99 |
| B−V color index | 1,08 |
| Variable type | unknown |
| Astrometry | |
| Radial velocity (Rv) | +17.1 km/s |
| Proper motion (μ) | RA: -17.10 mas/yr Dec.: -228.46 mas/yr |
| Parallax (π) | 23.97 ± 0.97 0.64 95 mas |
| Distance | 136 ± 5 ly (42 ± 1 pc) |
| Absolute magnitude (MV) | 0.824 |
| Other designations | |
| Database references | |
| SIMBAD | data |
Delta Cancri (δ Cnc, δ Cancri) is an orange giant star approximately 180 light-years away in the constellation Cancer. It has the traditional name Asellus Australis which in Latin means "southern donkey colt". It also has the longest of all known star names, Arkushanangarushashutu, derived from ancient Babylonian which means "the southeast star in the Crab".[1]
In Chinese, 鬼宿 (Guǐ Su), meaning Ghost (asterism), refers to an asterism consisting of δ Cancri, θ Cancri, η Cancri and γ Cancri.[2] Consequently, δ Cancri itself is known as 鬼宿四 (Guǐ Su sì, English: the Fourth Star of Ghost.)[3]
Since it is near the ecliptic, it can be occulted by the Moon and very rarely by planets.
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Delta Cancri was involved in the first recorded occultation by Jupiter:
"The most ancient observation of Jupiter which we are acquainted with is that reported by Ptolemy in book X, chap. iii (sic), of the Almagest, ...when the planet eclipsed the star known as (Delta) Cancri. This observation was made on September 3, B.C. 240, about 18h on the meridian of Alexandria."—Allen, 1898, quoting from Hind's The Solar System).
Delta Cancri also marks the famous open star cluster Praesepe (or the Beehive Cluster, also known as Messier 44). In ancient times M44 was used as a weather gauge as the following Greek rhyme from Aratos' Prognostica reveals:
A murky manger with both starsShining unaltered is a sign of rain.
While if the northern Ass is dimmed
By vaporous shroud, he of the south gleam radiant,
Expect a south wind: the vaporous shroud and radiance
Exchanging stars harbinger Boreas.—Allen, 1898
The meaning of this verse is that if Asellus Borealis or Gamma Cancris[4] is hidden by clouds, the wind will be from the south and that situation will be reversed if Arkushanangarushashutu is obscured. There is some doubt however as to the accuracy of this as Allen notes: "Our modern Weather Bureau would probably tell us that if one of these stars were thus concealed, the other also would be." (Allen, 1898)
But Delta Cancri also acts as more than just a dubious weather guide: it is a reliable signpost for finding the vividly red star X Cancri as Patrick Moore notes in his guidebook Stars of the Southern Skies:
“In the same binocular field with Delta [Cancri] you will find one of the reddest stars in the sky: X Cancri. It is a semi-regular variable; at maximum it rises to magnitude 5 and it never falls below 7.3 so that it can always be seen with binoculars. It looks rather like a tiny glowing coal.”—Page 146, Moore, 1994.
Delta Cancri also marks the radiant of the Delta Cancrids meteor shower.
In 1876, the possibility of Delta Cancri having a companion star was proposed.[5]
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