March 22, 2008


The constellation Pyxis is probably the Southernmost constellation I intend to cover. Constellations further south are at least partially or in some cases totally blocked by the horizon at all times of the year, but Pyxis is just above the Southern horizon in evenings during the Spring, and I want to be thorough. And even minor constellations have interesting objects in them to look at!

So... Pyxis represents the compass of a larger ship that was once the sprawling constellation Argo Navis but was divided into Pyxis (the compass), Carina (the keel), Puppis (the poop deck), and Vela (the sails). The top of the ship can be found by proceeding Southward around the horizon following the shallow arc of stars along the back of Canis Major. From Sirius to Wezen to Aludra, you follow this arc through Puppis about 20 degrees around the horizon until you are facing due South (in the Spring), and you will be looking at the 4th magnitude star Alpha Pyxidis, a hot B star that is dimmed somewhat by all of the gas and dust along our line of sight through the disk of the Milky Way.

Probably the most interesting star in this constellation is the variable star T Pyxidis, which has quite a story to tell. T Pyx is about 6000 light years away and is a cataclysmic variable (CV), a kind of star I actually did some research on and published about in graduate school. CV's are eclipsing binary systems in which a giant star is transferring its matter to a white dwarf companion, leading to a wildly varying light curve (hence the name), much greater in amplitude than a typical eclipsing binary. Every so often, the matter on the surface of the white dwarf will attain a high enough density and temperature that it will ignite (fuse), creating a burst of energy called a nova.

These repeat but not completely predictably since each eruption changes the parameters of the system slightly. T Pyx has gone off in 1890, 1902, 1920, 1944 and 1967, so in some sense we can say it is overdue to erupt by about 20 years. Astronomers expect that because the interval between outbursts is so long, we should be in store for a very bright outburst this time as more fuel has been able to build up on the white dwarf. This star was recently a target of the Hubble Space Telescope as astronomers were trying to understand the behavior of the shells of expelled material surrounding this star and figure out how to predict when the next blast would occur. That was 10 years ago, and still no blast!

Astronomers believe that such a nova eruption may be responsible for the biblical Star of Bethlehem phenomenon, a long story for another time, but T Pyx is likely too faint and in the wrong part of the sky to account for this legend, in my opinion.

There are a couple of deep sky objects here. First, a nice image of NGC 2613, a nearly edge-on spiral galaxy about 60 million light years away with a faint bar feature, very similar in size and type to our own Millky Way. This is found about 10 degrees due North of Alpha Pyx and 2 degrees West. There is also an open galactic cluster here known as NGC 2818, very low on the Southern horizon, about 8 degrees East of Alpha Pyx and 5 degrees South. Within it is a small but striking planetary nebula known as PN 261+8.1.

Posted by Observer at March 22, 2008 08:56 PM

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