Delta Scuti variable in the context of Rapidly oscillating Ap star

Sigma Octantis is a solitary star in the Octans constellation that forms the pole star of the Southern Hemisphere. Its name is also written as σ Octantis, abbreviated as Sigma Oct or σ Oct, and it is officially named Polaris Australis (/poʊˈlɛərɪs ɔːˈstreɪlɪs/). The star is positioned one degree away from the southern celestial pole of the Southern Hemisphere, lying in a nearly opposite direction to the North Star on the celestial sphere.

Located approximately 294 light-years from Earth, it is classified as a subgiant with a spectral type of F0 IV. Sigma Octantis has an apparent magnitude of 5.5, but is slightly variable and is classified as a Delta Scuti variable.

The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars: Delta Scuti variables, SX Phoenicis variables, and rapidly oscillating Ap stars (roAps) near the main sequence; RR Lyrae variables where it intersects the horizontal branch; and the Cepheid variables where it crosses the supergiants.

RV Tauri variables are also often considered to lie on the instability strip, occupying the area to the right of the brighter Cepheids (at lower temperatures), since their stellar pulsations are attributed to the same mechanism.

Gamma Doradus variables are variable stars which display variations in luminosity due to non-radial pulsations of their surface. The stars are typically young, early F or late A type main sequence stars, and typical brightness fluctuations are 0.1 magnitudes with periods on the order of one day. This class of variable stars is relatively new, having been first characterized in the second half of the 1990s, and details on the underlying physical cause of the variations remains under investigation.

The star 9 Aurigae was first noticed to be variable in 1990. However, none of the currently-accepted explanations were adequate: it pulsated too slowly and was outside of the Delta Scuti instability strip, and there was no evidence for any eclipsing material, although Gamma Doradus and HD 96008 were noted to be similar. These three stars, as well as HD 224638, were soon hypothesized to belong to a new class of variable stars in which variability was produced by g-mode pulsations rather than the p-mode pulsations of Delta Scuti variables. HD 224945 and HD 164615 were noticed to be similar as well, while HD 96008 was ruled out on the basis of its more regular period. Eclipses and starspots were soon ruled out as the cause of the Gamma Doradus' variability, and the variability of 9 Aurigae was confirmed to be caused by g-mode pulsations a year later, thus confirming the stars as the prototypes of a new class of variable stars. Over ten more candidates were quickly found, and the discoverers dubbed the group the Gamma Doradus stars, after the brightest member and the first member found to be variable.

Rho Virginis (ρ Vir, ρ Virginis) is the Bayer designation for a star in the constellation Virgo. It has an apparent visual magnitude of +4.9, making it a challenge to view with the naked eye from an urban area (according to the Bortle Dark-Sky Scale). The distance to this star has been measured directly using the parallax method, which places it 118.3 light-years (36.3 parsecs) away with a margin of error of about a light year.

Rho Virginis is an A-type main sequence star with a stellar classification of A0 V. It is larger than the Sun with a radius 60% larger and about twice the mass. As such it is generating energy at a higher rate than the Sun, with a luminosity 14 times greater. The outer atmosphere has an effective temperature of 8,930 K, which is what gives it the white-hued glow of an A-type star. It is classified as a Delta Scuti type variable star and its brightness varies by 0.02 magnitudes over periods of 0.5 to 2.4 hours.