Delta Cancri

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δ Cancri
Cancer constellation map.svg
Red circle.svg
Location of δ Cancri (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cancer
Right ascension  08h 44m 41.09921s[1]
Declination +18° 09′ 15.5034″[1]
Apparent magnitude (V) +3.94[2]
Spectral type K0 III[3]
U−B color index +0.99[2]
B−V color index +1.08[2]
Radial velocity (Rv)16.39±0.25[4] km/s
Proper motion (μ) RA: −17.67[1] mas/yr
Dec.: −229.26[1] mas/yr
Parallax (π)24.98 ± 0.24[1] mas
Distance131 ± 1 ly
(40.0 ± 0.4 pc)
Absolute magnitude (MV)+0.843[5]
Mass1.71[3] M
Radius11[4] R
Luminosity52[3] L
Surface gravity (log g)2.7[4] cgs
Temperature4,637±27[3] K
Metallicity [Fe/H]−0.13[4] dex
Rotational velocity (v sin i)2.8[4] km/s
Age2.45[3] 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

Delta Cancri (δ Cancri, abbreviated Delta Cnc, δ Cnc) is a double star about 180 light-years from the Sun in the constellation of Cancer.

Its two main constituents are designated Delta Cancri A and B. A is itself a binary star whose components are Delta Cancri Aa (formally named Asellus Australis /əˈsɛləs ɔːsˈtrlɪs/, the traditional name of the entire system)[6] and Ab.

The star system is 0.08 degree north of the ecliptic, so it can be occulted by the Moon and more rarely by planets; it is occulted (eclipsed) by the sun from about 31 July to 2 August.[7] Thus the star can be viewed the whole night, crossing the sky at the start of February.


δ Cancri (Latinised to Delta Cancri) is the system's Bayer designation. The designations of the two constituents Delta Cancri A and B, and those of A's components - Delta Cancri Aa and Ab - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[8]

It bore the traditional name Asellus Australis which is Latin for "southern donkey colt".[9] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[10] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[11] It approved the name Asellus 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.[6] Together with Gamma Cancri, it formed the Aselli, flanking Praesepe.[9]

As Arkū-sha-nangaru-sha-shūtu, which means "the southeast star in the Crab", it marked the 13th ecliptic station of the ancient Babylonians.[9]

In Chinese astronomy, Ghost (Chinese: 鬼宿; pinyin: Guǐ Xiù) refers to an asterism consisting of Theta Cancri, Eta Cancri, Gamma Cancri and Delta Cancri.[12] Delta Cancri itself is known as the fourth star of Ghost (Chinese: 鬼宿四; pinyin: Guǐ Xiù sì).[13]


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 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 Cancris[14] 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 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.[15]


  1. ^ a b c d e van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357.
  2. ^ a b c Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99), Bibcode:1966CoLPL...4...99J.
  3. ^ a b c d e Luck, R. Earle (September 2015), "Abundances in the Local Region. I. G and K Giants", The Astronomical Journal, 150 (3): 23, arXiv:1507.01466, Bibcode:2015AJ....150...88L, doi:10.1088/0004-6256/150/3/88, 88.
  4. ^ a b c d e Massarotti, Alessandro; et al. (January 2008), "Rotational and radial velocities for a sample of 761 HIPPARCOS giants and the role of binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209
  5. ^ Soubiran, C.; et al. (March 2008), "Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants", Astronomy and Astrophysics, 480 (1): 91–101, arXiv:0712.1370, Bibcode:2008A&A...480...91S, doi:10.1051/0004-6361:20078788.
  6. ^ a b "Naming Stars". Retrieved 16 December 2017.
  7. ^ In the Sky Earth astronomy reference utility showing the ecliptic and relevant date as at J2000 - present
  8. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  9. ^ a b c Allen, Richard Hinckley, Star Names: Their Lore and Meaning, 1898.
  10. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  11. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  12. ^ 陳久金 (2005). Zhōngguó Xīngzuò Shénhuà 中國星座神話 [Chinese Constellation Mythology]. 台灣古籍出版有限公司. p. 394. ISBN 978-986-7332-25-7.
  13. ^ "香港太空館 - 研究資源 - 亮星中英對照表" [Hong Kong Space Museum - English-Chinese Glossary of Bright Stars] (in Chinese). Retrieved February 3, 2018. Asellus Australis
  14. ^ Kaler, 2009:"ASELLUS BOREALIS". Archived from the original on 2011-07-16.
  15. ^ Burnham, S. W. (1878). "The companion to delta Cancri". The Observatory. 2. Bibcode:1878Obs.....2...60B.