Zeta Puppis - Naos, Suhail Hadar

Zeta Puppis was originally the ζ star of Argo Navis, the ship of Jason's Argonauts. After this huge constellation was broken up in the 19th century, the star became the brightest in Puppis, the Stern, at magnitude 2.25. Zeta Puppis, whose Greek name Naos means "ship", is also known as Suhail Hadar (the "bright star of the ground") in Arabic, since the star appears nearly on the horizon as seen from the latitude of the Arabian peninsula.

Properties and Evolution

Naos is an blue supergiant of spectral class O5 Ia - the rarest and hottest of all normally classified stars. In fact, Naos is the second brightest class O star in the sky. At a distance of 970 light years, it is visually 8,000 times more luminous than the Sun. Its high temperature of 42,000 K, however, causes most of its radiation to be emitted in the invisible ultraviolet, and when that is included, its total luminosity climbs to 360,000 suns.

Zeta Puppis's high luminosity and temperature indicate a radius 11 times solar, and a mass of 40 suns - a third of the maximum allowed for stars. Such stars do not live long, and Naos is an evolved supergiant, fusing helium into carbon and oxygen in its core. Naos will almost certainly explode as a supernova sometime within the astronomically near future.

Typical of its breed, ζ Pup produces a fierce stellar wind that blows erratically at a 2300 km/sec, losing mass at 10 million times the rate in the Sun's solar wind. The star's wind has altered the chemical nature of its surface; it has twice as much helium as normal, and is enriched in nitrogen also.

Runaway Motion

Naos is also a "runaway" star moving away at high speed (nearly 100 km/sec) from its place of origin. Analysis of its motion shows it to have been ejected 2.5 million years ago from the open cluster Trumpler 10 in the neighboring constellation Vela. It is now 400 light years - some 8.5° in the sky - away from the cluster.

Such runaway stars are invariably single and spin much faster than normal O stars; Naos rotates at least 220 km/sec at its equator. No one knows exactly what causes such massive runaway stars. One theory suggests that they form when a companion explodes, another that they are ejected by gravitational interactions within their birth cluster.

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