Pyxis: The Mariner's Compass

Pyxis sits right next to the broken-up pieces of an ancient ship and looks, at a glance, like it belongs to the same story. It doesn't. It's a much younger addition that just happens to share a neighborhood.

A compass with no connection to the ship beside it

Nicolas-Louis de Lacaille introduced Pyxis in the 1750s to represent the mariner's compass, and he placed it near Argo Navis — the old constellation later split into Carina, Puppis, and Vela — but Pyxis was never actually part of that ship. It's a separate, modern invention that landed in the same stretch of sky by coincidence of geography rather than mythology. There's no classical myth behind it, the way there is none behind most of Lacaille's instrument constellations. It's simply a compass, named for a compass, sitting beside a ship it has no story in common with.

Its lone named star is Alpha Pyxidis, a blue-white giant at magnitude 3.68 — faint, but the clear standout in an otherwise sparse patch of sky. There's no runner-up to compare it against, no Beta or Gamma sitting nearby to argue over which one is actually the constellation's centerpiece. Alpha Pyxidis is Pyxis, more or less, at least as far as naked-eye stars go.

A nova that keeps coming back

The real reason to know Pyxis by name is T Pyxidis, a recurrent nova that flares roughly every few decades. A regular nova is a one-time event as far as casual observation goes. A recurrent nova is different: the same star brightens dramatically, fades back down, and then does it again, on a rough cycle, because the physical process driving the outburst resets rather than exhausting itself in one go. T Pyxidis is one of the better-known examples of this, a star that has earned its place in the astronomical record by repeating a dramatic event instead of only doing it once.

Pyxis also holds NGC 2818, a planetary nebula that happens to sit in the same line of sight as an open cluster — two unrelated objects, at different distances, that from Earth appear to overlap in the same small patch of sky. It's a reminder that "next to each other" on a star chart doesn't always mean "actually near each other" in three dimensions. A planetary nebula, despite the name, has nothing to do with planets — it's the puffed-out outer layers of a dying star, expanding outward and glowing under whatever light the star's exposed core still puts out. The open cluster sharing its line of sight is a separate, unrelated clump of young stars that just happens to sit on the same visual line from where we're standing.

Sitting beside a ship it never sailed on

Because Pyxis sits right next to Puppis, it's easy to assume the compass was part of the original Argo Navis before the ship got broken apart. It wasn't. Puppis, Carina, and Vela all share genuine history as pieces of one older constellation; Pyxis is a later, separate addition that Lacaille tucked in beside them.

What this means for you tonight

Pyxis is best placed in March and, like most of Lacaille's southern instruments, it's a target for the southern hemisphere rather than the north. Start with Alpha Pyxidis at magnitude 3.68 — it's dim enough that you'll want a reasonably dark sky, and it's the only named star in the constellation, so once you find it you've found the anchor for the whole shape. T Pyxidis won't be visible to you most nights; it only becomes a naked-eye or binocular target during one of its periodic outbursts, so check current reports before you go looking for it specifically. For the background on why a magnitude 3.68 star needs a darker sky than a magnitude 1 star to spot at all, how star brightness works covers the scale, and what is a constellation explains why a small, recent invention like Pyxis counts as a full constellation right alongside something ancient like the ship next door.