  | |
| Observation data EpochJ2000.0 EquinoxJ2000.0 | |
|---|---|
| Constellation | Cancer |
| Right ascension | 08h 44m 41.099s[1] |
| Declination | +18° 09′ 15.51″[1] |
| Apparent magnitude (V) | +3.94[2] |
| Characteristics | |
| Evolutionary stage | red clump[3] |
| Spectral type | K0 III[4] |
| U−Bcolor index | +0.99[2] |
| B−Vcolor index | +1.08[2] |
| Astrometry | |
| Radial velocity (Rv) | 16.39±0.25[5] km/s |
| Proper motion (μ) | RA: −18.435mas/yr[1] Dec.: −227.813mas/yr[1] |
| Parallax (π) | 23.8271±0.1853 mas[1] |
| Distance | 137 ± 1 ly (42.0 ± 0.3 pc)[1] |
| Absolute magnitude (MV) | +0.843[6] |
| Details | |
| Mass | 1.71[4] M☉ |
| Radius | 11.7±0.13[7] R☉ |
| Luminosity | 59.5±1.4[7] L☉ |
| Surface gravity (log g) | 2.59[7] cgs |
| Temperature | 4,684±27[7] K |
| Metallicity[Fe/H] | −0.03[7] dex |
| Rotational velocity (v sin i) | 2.8[5] km/s |
| Age | 2.45[4] Gyr |
| Other designations | |
| Asellus Australis,δ Cnc,47 Cnc,BD+18°2027,FK5 326,GC 12022,HD 74442,HIP 42911,HR 3461,SAO 98087,ADS 6967,CCDM 08447+1809 | |
| Database references | |
| SIMBAD | data |
Delta Cancri is a star in theconstellation ofCancer. It has the proper nameAsellus Australis,[8] Delta Cancri is itsBayer designation. This star is visible to thenaked eye with anapparent visual magnitude of +3.94.[2] Based onparallax measurements, it is located at a distance of 137light-years (42 pc) from Earth. It is drifting further away with a line of sight velocity of 16 km/s.[5]
The star is 0.08 degree north of theecliptic, so it can beocculted by theMoon and more rarely byplanets; it is occulted (eclipsed) by the sun from about 31 July to 2 August.[9] Thus the star can be viewed the whole night, crossing the sky at the start of February.
Thespectrum of this star matches aspectral class of K0 III,[4] with theluminosity class III indicating that it is agiant star that has exhausted thehydrogen at itscore. With an estimated age of 2.45 billion years and 1.71 times themass of the Sun,[4] this star has expanded to 11.7 times theSun's radius. It is radiating 60 times theluminosity of the Sun from its enlargedphotosphere at aneffective temperature of 4,684 K.[7] The temperature gives it the orange hue typical ofK-type stars.[10]
This star has an optical companion, named Delta Cancri B. This companion appears close to Delta Cancri A along the line of sight,[11] in reality, it is much farther away than Delta Cancri A and has a differentproper motion.[12]
Component A was believed to be itself abinary star system whose components are Delta Cancri Aa and Ab. This companion would be separated by roughly 0.1" and be 0.96 magnitudes fainter.[11] However, subsequent observations and modern studies suggest the companion does not exist.[7][13][14]
Delta Cancri is the star'sBayer designation. This designation isLatinized fromδ Cancri, and abbreviatedDelta Cnc orδ Cnc.
It bore the traditional nameAsellus Australis which isLatin for "southerndonkey colt".[15] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[16] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entiremultiple systems.[17] It approved the nameAsellus Australis for the component Delta Cancri Aa on 6 November 2016 and it is now so included in the List of IAU-approved Star Names.[8] Together withGamma Cancri, it formed theAselli, flankingPraesepe.[15]
AsArkū-sha-nangaru-sha-shūtu, which means "the southeast star in the Crab", it marked the 13th ecliptic station of the ancientBabylonians.[15]
InChinese astronomy,Ghost (Chinese:鬼宿;pinyin:Guǐ Xiù) refers to anasterism consisting ofTheta Cancri,Eta Cancri,Gamma Cancri and Delta Cancri.[18] Delta Cancri itself is known as the fourth star of Ghost (Chinese:鬼宿四;pinyin:Guǐ Xiù sì).[19]
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 fromHind's The Solar System).
Delta Cancri also marks the famous open star cluster Praesepe (or theBeehive 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 stars
Shining 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 Cancri[13] is hidden by clouds, the wind will be from the south and that situation will be reversed if Asellus Australis 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 guidebookStars 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 theDelta Cancrids meteor shower.
In 1876, the possibility of Delta Cancri having acompanion star was proposed.[20]
Asellus Australis
