NAOS (Zeta Puppis). All stars amaze, from the brightest to dimmest, all remarkable concentrations of matter that run -- or have run -- on some form of nuclear fusion that converts matter to energy. But some amaze more than others. Below Sirius, in the great craft of the Argonauts, Argo, lies Naos, the Greek name meaning "the ship." Argo is so large that in the nineteenth century, astronomers took to breaking it into parts, Carina, the Keel (which contains Canopus), Vela, the Sails, and Puppis, the Stern. Naos, the Zeta star of Argo, fell into Puppis, and is now known also as Zeta Puppis. At the dim end of second magnitude (2.25), Naos seems to be just one more star scattered below the Larger Dog. In fact it is one of the brightest O stars (O5) in the sky, class O being the rarest and hottest of all normally classified stars. Its large distance of 1400 light years shows it to be visually 22,000 times more luminous than the Sun. Its high temperature of 42,000 Kelvin, however, causes most of the star's radiation to be emitted in the invisible ultraviolet, and when that is taken into account, the total luminosity comes in at three-quarters of a million times solar. For us to be comfortable with such a star, our Earth would have to orbit at a distance 20 times that of Pluto from the Sun. Its high luminosity and temperature show it to have a mass as much as 60 times solar, almost half the maximum allowed for stars, and to be an evolved supergiant. With a radius 17 times solar, the star is dying and is most likely now fusing helium into carbon and oxygen in its core. Typical of its breed, the enormous brightness helps produce a wind that blows erratically at a fierce 2300 kilometers per second, the star losing over a millionth of a solar mass a year. While that may sound small, it is 10 million times the rate lost in the solar wind, which is powerful enough to produce aurorae and comet tails. The star's evolutionary state and winds have altered the chemical nature of the surface, Naos having twice as much helium as normal and probably enriched in nitrogen too. Naos is a fine example of a class O "runaway" star, one that has been ejected at high speed (nearly 100 km/s) from its place of origin, Naos's in the neighboring constellation Vela. Over the past one to two million years, much of its short life, the star has moved some 500 light years. Such stars are invariably single and spin much faster than normal O stars, Naos at least 220 kilometers per second at its equator, 100 times the Sun's rotation speed. No one knows exactly how massive stars can get such a high kick. One theory suggests that runaways are sent flying when a companion explodes, another that these stars are ejected by gravitational interactions within a birth cluster. Whatever the case, Naos itself will almost certainly explode as a supernova sometime within the astronomically near future.

Update 2008: Analysis of Naos's motion shows it to have been ejected two and a half million years ago from a cluster called Trumpler 10, and that its distance is 970 light years rather than 1400. The star is now some 8.5 degrees, roughly 400 light years, away from the cluster. The new distance lowers the star's luminosity to 360,000 Suns, the radius to 11 times solar, and the mass to 40 Suns (though others derive 22.5 solar masses).
Zeta Pup The triangle that makes the lower part of Canis Major is at upper left. Naos, Zeta Puppis, seems to have been ejected 2.5 million years ago from the cluster Trumpler 10, which now lies 8.5 degrees, or some 400 light years, away.
Written by Jim Kaler 3/26/99; last updated 7/25/08. Return to STARS.