The Bluest Star

An artist's impression of the outer layers of the star Naos (zeta Puppis). Credit: Wikipedia.

An artist’s impression of the outer layers of the star Naos (zeta Puppis). Credit: Wikipedia.

Scattered in a thick band south of Canis Major lie the stars and star clusters of the constellation Puppis. There are no stars here to visually rival the brilliant stars of the Big Dog or Orion further to the north and west. But visual appearances are deceiving because among the stars of Puppis is one of the most luminous and hottest stars in our part of the galaxy, the star Naos or zeta Puppis.

Naos (the ancient Greek word for “ship”) was once part of the sprawling and ancient constellation Argo Navis, the legendary ship that carried Jason and his Argonauts. In the 18th century, the constellation was parceled into three smaller groups called Carina (the Keel), Vela (the Sail), and Puppis (the Stern). The star Naos, which was designated ζ (zeta) in Argo Navis, ended up as ζ Puppis.

In a pair of binoculars, you can tell at a glance that Naos is hot because of its remarkably blue color. A blue star is a hot star, and the outer atmosphere of Naos measures a blazing 42,000 K, an astonishing temperature. This places it well into the hottest class of stars, the so-called O-type stars, and Naos is formally classified as an O41f supergiant. By comparison, the big blue-white star Rigel at the western foot of Orion is just 11,000K while our Sun is a modest 5,700 K. What’s more, a hot star is a massive star because mass squeezes the core of the star to drive a rapid rate of energy formation through nuclear fusion. More energy in the core means more energy escapes through the star’s outer layers.

The location of the star Naos, or zeta Puppis, in the constellation Puppis.

The location of the star Naos, or zeta Puppis, in the constellation Puppis.

How massive is zeta Puppis? It’s about 23 times as massive and has a radius 14 times greater than our Sun. It’s also more than 20,000 times more luminous than our Sun in visible light, and if you count ultraviolet light, as you should, then the star is more than 500,000 times brighter than our Sun. If our little Earth was a planet around Naos, we’d have to be about twenty times the distance of Pluto to the Sun to get the same amount of light.

Not that a planet like Earth would have time to form around Naos. Big stars like this don’t live long enough, as far as we know, for planets to settle down to harbor life. The star will only live a total of 5 million years or so, and it’s already well on its way to running out of fuel and blowing up as a supernova.

Naos appears modest in our sky because of its great distance of about 1,080 light years. If it was at the same distance as Sirius, about 8.6 light years, it would shine at magnitude -7.8, a full three magnitudes (or 15 times) brighter than Venus.

A nightscape image of the constellation Puppis showing the position of Naos (zeta Puppis) from a latitude of 24.6 N in the Florida Keys.

A nightscape image of the constellation Puppis showing the position of Naos (zeta Puppis) from a latitude of 24.6 N in the Florida Keys.

Such a remarkable star has attracted the attention of astronomers. They have studied the star in detail and found it blasts winds of charged particles into space at 2,300 km/s. It also appears to be a runaway star, one that was ejected at high speed from its birth cluster about 2 million years ago. Naos likely originated in the small open cluster Trumpler 10 in the constellation Vela and has traveled about 500 light years since it was whipsawed out of the cluster by a gravitational interaction with another massive star.

For us backyard stargazers, it’s good fun to compare this star to a red supergiant like Betelgeuse. Even a pair of binoculars will show a stunning difference in color if you move back and forth between the two. Or if you have a telescope, have a look at a carbon star like R Leporis just under the feet of Orion. R Lep looks like a dull red coal from a campfire. Zeta Puppis is a beautiful azure blue. You can see the color of these stars better if you slightly de-focus their image to spread out the light on the back of your retina.

A word of warning– Naos is well south of the celestial equator at a declination of -40°, south and east of the lower regions of Canis Major, so it is a tough (to impossible) sight from northern Europe, the northern U.S., and Canada, but an easy target for southern stargazers.

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