Alpha Trianguli Australis - Atria

At magnitude 1.92, Alpha Trianguli Australis is the brightest star in Triangulum Australe, and is also the 41st brightest in the sky. It has the traditional name Atria, a contraction of its Bayer designation. Atria is one of the closest bright stars to the south celestial pole, offset from it by just 21°.

Properties

Atria is an orange K2 II bright giant, fusing helium into carbon and oxygen in its deep core. Atria's surface temperature is not well known, estimates spanning 3970 to 4400 K. From the average of 4200 K, and the star's distance of 415 light years, we derive a substantial luminosity of 4900 suns, and a radius 130 times solar. Luminosity, temperature, and stellar evolutionary theory yield a mass of 7 suns, and an age of 45 million years. Its metallicity is most likely close to solar.

Anomalies

Atria has been classed as barium star. Such stars have been contaminated by heavy elements from a more massive companion that evolved to a red giant, then transferred matter enriched by nuclear reactions to the other star, and finally became a white dwarf. Atria's youth implies that the hypothesized white dwarf should be hot enough to see in ultraviolet light, but none has been detected.

More significantly, Atria is a "hybrid star," a giant that generates a cool stellar wind from its surface, yet also has a hot magnetic corona. The star is a notable source of X-rays, which are best explained as coming from a young, sunlike, class G companion.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]