Asellus Australis (δ Cancri) | Facts, Distance, Size, Mass & Definition

Asellus Australis, designated as Delta Cancri, is the second-brightest star in the zodiacal constellation of Cancer. Asellus Australis is also a double star.

Key Facts & Summary

Asellus Australis is located at around 131 light-years / 40 parsecs away from the Solar System.
This star system has a combined visual magnitude of +3.94, being the second-brightest object in the zodiacal constellation of Cancer.
The absolute magnitude has been recorded at +0.843.
The stars that make up the Delta Cancri star system are Delta Cancri A, which is also known as Asellus Australis, and Delta Cancri B.
Delta Cancri A, however, is a binary star system composed out of the primary star Delta Cancri Aa, and Delta Cancri Ab.
The fact that Delta Cancri had a companion star was speculated since 1876.
The primary star of this system is of spectral type K0 III. It has around 1.71 solar masses, and 11 solar radii, thus it is more than 20 times bigger than our Sun.
Asellus Australis is however cooler than our Sun, having surface average temperatures of around 4,637 K.
The surface gravity on this star is 2.7 cgs.
Asellus Australis is 52 times brighter than our Sun.
The star system has a radial velocity of 16.39 km / 10.18 mi per second, while Asellus Australis has a rotational velocity of 2.8 km / 1.7 mi per second.
Asellus Australis is around twice as young as our Sun, being 2.45 billion years old.
The best time of the year to observe Asellus Australis, the other stars, and deep-sky objects in the constellation of Cancer, is during the month of March.
The zodiacal constellation of Cancer is the 31^st largest constellation in the sky, out of the 88 modern constellations. Cancer is also the second faintest of the zodiacal constellations.

δ Cancri

The star, Delta Cancri, was named Asellus Australis, which comes from Latin and translates to “southern donkey colt”. This name was approved by the IAU in 2016.

The star was also known as Arku-sha-nangaru-sha-shutu, which means “the southeast star in the Crab”. It marked the 13^th ecliptic station of the ancient Babylonians.

Formation

Asellus Australis / Delta Cancri formed around 2.45 billion years ago from an interstellar medium of dust and gas. Gravity pulled the swirling gas and dust together and resulted in the second-brightest star of the constellation of Cancer, Asellus Australis.

It is unknown if the other stars around Asselus Australis formed from the same interstellar medium at the same time. Asellus Australis is almost twice as young as or Sun.

Distance, Size, and Mass

Asellus Australis is located at around 131 light-years / 40.0 parsecs away from Earth. It is easy to spot with the use of binoculars.

Asellus Australis / Delta Cancri has around 1.71 solar masses, or 171% of our Sun’s mass, and 11 solar radii, or 1,100% of our Sun’s radius. It is thus more than 20 times bigger than our Sun.

Other Characteristics

Asellus Australis is a giant star of spectral type K0 III. It has an apparent magnitude of +3.94, and an absolute magnitude of +0.843. This star is 52 times brighter than our Sun.

Asellus Australis has a surface gravity of 2.7 cgs, and it has a rotational velocity of around 2.8 km / 1.7 mi per second. This star is cooler than our Sun, having surface temperatures of around 4,637 K.

Star System

The Delta Cancri star system consists of two stars, designated as Delta Cancri A – also known as Asselus Australis, and Delta Cancri B.

Delta Cancri A has yet another star orbiting around it, designated as Delta Cancri Ab, while the primary star, Asellus Australis, is designated as Delta Cancri Aa. This star system has a radial velocity of around 16.39 km / 10.18 mi per second.

The fact that Delta Cancri had a companion star was speculated since 1876.

Location

Asellus Australis / Delta Cancri is located in the zodiacal constellation of Cancer, the celestial crab. It is the second-brightest object in the constellation after Beta Cancri / Tarf.

Asellus Australis marks the famous open star cluster Praesepe, also known as the Beehive Cluster, or Messier 44. In antiquity, Messier 44 was used as a weather gauge, if Asellus Australis was hidden by clouds, the wind would come from the south.

Asellus Australis is also used as a guide in finding one of the reddest stars in the sky, X Cancri. The constellation of Cancer is one of the first 48 Greek constellations, listed by Ptolemy in his 2^nd century Almagest.

Cancer is now part of the zodiac family of constellations, being the second faintest in the sky, and it is the 31^st largest constellation in the sky out of the 88 modern constellations.

There are many interesting stars and deep-sky objects in the zodiacal constellation of Cancer, such as Asellus Australis, Tarf, X Cancri, YBP 1194, the Messier 67 open cluster, Messier 44, the QSO J0842+1835 quasar, or the OJ 287 BL Lacertae objects, among many others.

The best time to view Asellus Australis, the other stars and deep-sky objects in the constellation of Cancer, is during the month of March when the constellation is the most prominent in the sky.

The Future

Asellus Australis will continue to exist for many millions of years. It is not massive enough to explode as a supernova, and thus it may end up one day as a white dwarf star.

Did you know?

Asellus Australis was involved in the first recorded occultation by Jupiter around 240 BC. This was observed by the famous Greek astronomer Ptolemy, in Alexandria.
Asellus Australis marks the radiant of the Delta Cancrids meteor showers, which is in turn associated with the zodiacal constellation of Cancer itself.
The Chinese know Asellus Australis as Gui Xiu si – the Fourth Star of Ghost.
In Chinese astronomy, Ghost – Gui Xiu – is an asterism comprised out of Asellus Australis, Theta Cancri, Eta Cancri, and Gamma Cancri.