Gamma Capricorni
A chemically peculiar variable star in Capricornus.
Gamma Capricorni is a star in the constellation of Capricornus, bearing the proper name Nashira. It is notable as a hot chemically peculiar star with disputed classification between Am and Ap types, and as an α² Canum Venaticorum type variable star.
- Bayer designation
- Gamma Capricorni
- Proper name
- Nashira
- Distance
- approximately 171 ly
- Radial velocity
- −31 km/s
- Mean apparent magnitude
- +3.67
- Mass
- 2.44 times the Sun
- Radius
- 2.35 times the Sun
Lore & Background
Gamma Capricorni, also known as Nashira, derives its traditional name from the Arabic 'sa'd nashirah', meaning 'the lucky one' or 'bearer of good news'. The International Astronomical Union's Working Group on Star Names approved the name Nashira for this star on 21 August 2016. In Chinese astronomy, it is part of an asterism called 'Line of Ramparts', which includes several other stars.
The star is classified as a hot chemically peculiar Am star, but its spectrum shows abundance anomalies of strontium, chromium, and europium. It was first classified as an Am star in 1965 by K. Osawa, but in 1974 C. Bertaud and M. Floquet found it to be an Ap star with a strontium abundance anomaly. In 1998, F. A. Catalano and associates found slight variability in the infrared J band with a period of 2.78 days, suggesting the Ap classification is correct. However, no magnetic field is detected, which points to the Am classification, as does the infrared spectrum.
The star displays large radial velocity variations, which could be explained by star spots and rotation, or it may be a double-lined spectroscopic binary viewed from nearly pole-on. It is classified as an α² Canum Venaticorum type variable star, with brightness varying by 0.03 magnitudes. O. J. Eggen included it as a member of the Hyades Stream.
Reader's Guide
Gamma Capricorni's significance lies in its contested classification between Am and Ap star types, reflecting ongoing challenges in stellar spectroscopy. The detection of slight infrared variability with a 2.78-day period supports the Ap classification, yet the absence of a detected magnetic field and the infrared spectrum favor the Am classification. This ambiguity makes it a valuable object for studying chemically peculiar stars. Its large radial velocity variations, possibly due to star spots, rotation, or binary nature, add further complexity. As an α² Canum Venaticorum variable, its small brightness variations contribute to the understanding of such stars. Its membership in the Hyades Stream, as noted by O. J. Eggen, links it to a moving group of stars. The star's position 2.56 degrees south of the ecliptic allows lunar occultations and rare planetary occultations. The name Nashira was officially recognized by the IAU in 2016, and it also names a United States Navy ship, Nashira (AK-85), though that ship was never commissioned.
Did You Know?
- Gamma Capricorni is 2.56 degrees south of the ecliptic, allowing lunar and rare planetary occultations.
- The star's brightness varies by 0.03 magnitudes as an α² Canum Venaticorum type variable.
- Nashira (AK-85) was a United States Navy ship named after the star, though never commissioned.
Frequently Asked Questions
What is Gamma Capricorni?
It is a hot star in the constellation Capricornus, designated Gamma by Bayer and given the IAU proper name Nashira. It shines at a mean apparent magnitude of +3.67 and sits roughly 171 light-years from Earth.
What type of variable star is Gamma Capricorni?
It is an α² Canum Venaticorum type variable, meaning its brightness changes are driven by magnetic-field effects rather than radial pulsation. Its chemical-peculiarity subtype remains contested, with some catalogues listing it as Am and others as Ap.
How massive and how far away is Gamma Capricorni?
The star has a mass of about 2.44 solar masses, making it a moderately heavy upper-main-sequence object. It lies approximately 171 light-years from the Solar System and shows a radial velocity of −31 km/s.
What does 'chemically peculiar' mean in the context of Gamma Capricorni?
It indicates that the star's atmosphere displays element abundances that deviate significantly from standard stellar-composition models. This anomaly is linked to its magnetic structure and is the very feature that fuels the Am-versus-Ap classification debate.
Where does the proper name Nashira come from?
Nashira is the IAU-recognized proper name assigned to Gamma Capricorni, giving it a distinct identity beyond the Greek-letter Bayer designation. It is used in modern popular-astronomy and educational materials to make the star easier to reference.
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