Alpha Cancri
Alpha Cancri is a white A-type star with a faint companion.
Alpha Cancri, also designated α Cancri and commonly called Acubens, is a star system located in the northern constellation Cancer. With an apparent magnitude of 4.20, it qualifies as a fourth-magnitude star and can be seen without a telescope under clear skies. Parallax measurements place it roughly 178 light-years (55 parsecs) from the Sun, and it is moving closer at a radial velocity of about −12 km/s. Its position near the ecliptic means the Moon can occasionally pass in front of it, an event known as an occultation.
The system’s primary star, Alpha Cancri A, is classified as kA7VmF0/2III/IVSr—a type of Am star where the calcium K-lines resemble an A7 main-sequence star, while the hydrogen lines are more like those of an F0 giant or subgiant. It is a white A-type main-sequence star with an apparent magnitude of +4.26. This star has 2.10 times the Sun’s mass and 3.7 times its radius, and it spins rapidly, with a projected rotational velocity of 75 km/s. Its photosphere radiates 49 times the Sun’s luminosity at an effective temperature of 7,943 K.
The companion, Alpha Cancri B, is an eleventh-magnitude star. In 1836, it was observed at a position angle of 325 degrees and a separation of 11.3 arcseconds from the primary. Alpha Cancri A itself may be a close binary consisting of two stars of similar brightness separated by 0.1 arcsecond, though this is uncertain. A light curve from a 2014 lunar occultation did not confirm the presence of such a close companion.
The Bayer designation Alpha Cancri comes from the Latinized form α Cancri. Johann Bode once labeled this star α² Cancri, assigning α¹ to 60 Cancri, but his superscripts are rarely used today. The traditional name Acubens (also spelled Açubens) derives from the Arabic *al zubanāh*, meaning “the claws.” Another name, Sertan, comes from *al-saraṭān*, “the crab.” Both names were historically shared with ι Cancri, which marks the crab’s other claw. The International Astronomical Union’s Working Group on Star Names selected Acubens as the proper name for the primary component, Alpha Cancri A.
In modern culture, the USS Acubens (AKS-5) was a United States Navy ship.
- Apparent magnitude
- 4.20
- Distance
- 178 light-years (55 pc)
- Radial velocity
- −12 km/s
- Spectral type (primary)
- kA7VmF0/2III/IVSr
- Mass (primary)
- 2.10 solar masses
- Radius (primary)
- 3.7 solar radii
- Luminosity (primary)
- 49 solar luminosities
- Effective temperature (primary)
- 7,943 K
Lore & Background
The primary component, α Cancri A, has a stellar classification of kA7VmF0/2III/IVSr, indicating an Am star with calcium K-lines similar to an A7 main sequence star and hydrogen lines more like an F0 giant or subgiant star. It is a white A-type main-sequence star with an apparent magnitude of +4.26. The star has 2.10 times the mass of the Sun and 3.7 times the Sun's radius. It has a high rate of spin, showing a projected rotational velocity of 75 km/s, and radiates 49 times the luminosity of the Sun from its photosphere at an effective temperature of 7,943 K.
Its companion, α Cancri B, is an eleventh-magnitude star. In the year 1836, its position angle was observed at 325 degrees with a separation from the main star of 11.3 arcseconds. α Cancri A may itself be a close binary, consisting of two stars with similar brightness and a separation of 0.1 arcsecond, though this is questioned. Indeed, a light curve generated during a 2014 lunar occultation failed to demonstrate a close companion.
The traditional name Acubens (Açubens) is derived from the Arabic الزبانى al zubanāh, 'the claws'. A second name, Sertan, derives from the Arabic al-saraṭān, 'the crab'. These names were traditionally shared with ι Cancri, which marks the crab's other claw. The International Astronomical Union's Working Group on Star Names (WGSN) chose Acubens as the proper name for the primary component α Cancri A.
Reader's Guide
Alpha Cancri is notable as a multiple-star system in Cancer, carrying the traditional name Acubens, which refers to the crab's claws. Its primary component is a chemically peculiar Am star with a complex spectral classification, indicating unusual abundances of elements. The system's companion, a faint eleventh-magnitude star, has been observed since at least 1836, with a recorded position angle and separation. A possible close binary within the primary itself remains unconfirmed, as a 2014 lunar occultation light curve failed to show evidence of a close companion. The star's proximity to the ecliptic allows lunar occultations, which have been used to study its properties. Historically, Johann Bode designated this star as α2 Cancri, with α1 assigned to 60 Cancri, but Bode's superscripts are rarely used, and the designations have reverted. The star also bears the alternative name Sertan, shared with ι Cancri. In modern culture, the name Acubens was given to a United States Navy ship, USS Acubens (AKS-5).
Did You Know?
- Alpha Cancri is located 178 light-years from the Sun and is drifting closer at a radial velocity of −12 km/s.
- The primary component has a projected rotational velocity of 75 km/s.
- Its companion, α Cancri B, was observed in 1836 at a position angle of 325 degrees and separation of 11.3 arcseconds.
- A 2014 lunar occultation light curve failed to demonstrate a close companion to α Cancri A.
More in Binary and Multiple Stars, Part 2 1-24
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