Delta Cancri
Delta Cancri is a star in the constellation of Cancer. It has the proper name Asellus Australis, Delta Cancri is its Bayer designation. This star is visible to the naked eye with an apparent visual magnitude of +3.94. Based on parallax measurements, it is located at a distance of from Earth. It is drifting further away with a line of sight velocity of 16 km/s.
The star is 0.08 degree north of the ecliptic, so it can be occulted by the Moon and more rarely by planets; it is occulted by the sun from about 31 July to 2 August. Thus the star can be viewed the whole night, crossing the sky at the start of February.
Properties
The spectrum of this star matches a spectral class of K0 III, with the luminosity class III indicating that it is a giant star that has exhausted the hydrogen at its core. With an estimated age of 2.45 billion years and 1.71 times the mass of the Sun, this star has expanded to 11.7 times the Sun's radius. It is radiating 60 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,684 K. The temperature gives it the orange hue typical of K-type stars.This star has an optical companion, named Delta Cancri B. This companion appears close to Delta Cancri A along the line of sight, in reality, it is much farther away than Delta Cancri A and has a different proper motion.
Component A was believed to be itself a binary 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. However, subsequent observations and modern studies suggest the companion does not exist.
Nomenclature
Delta Cancri is the star's Bayer designation. This designation is Latinized from δ Cancri, and abbreviated Delta Cnc or δ Cnc.It bore the traditional name Asellus Australis which is Latin for "southern donkey colt". In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. 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. Together with Gamma Cancri, it formed the Aselli, flanking Praesepe.
As Arkū-sha-nangaru-sha-shūtu, which means "the southeast star in the Crab", it marked the 13th ecliptic station of the ancient Babylonians.
In Chinese astronomy, Ghost refers to an asterism consisting of Theta Cancri, Eta Cancri, Gamma Cancri and Delta Cancri. Delta Cancri itself is known as the fourth star of Ghost.
Observations
Delta Cancri was involved in the first recorded occultation by Jupiter:Delta Cancri also marks the famous open star cluster Praesepe. In ancient times M44 was used as a weather gauge as the following Greek rhyme from Aratos' Prognostica reveals:
The meaning of this verse is that if Asellus Borealis or Gamma Cancri 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."
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:
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.
Books
Category:Cancer
Cancri, Delta
3461
Category:Durchmusterung objects
Cancri, 47
074442
042911
Asellus Australis