Sail into the Southern Skies with Vela

There once was a mighty constellation deep in the south called Argo Navis, named after Jason’s ship in Greek mythology. In the mid-18th century it was split into several smaller constellations: Vela (the Sails), Puppis (the Stern) and Carina (the Keel). All three ride high on April evenings, but in this column we’ll focus on Vela.

The splitting of Argo Navis resulted in a few oddities. For one, Vela does not have Alpha or Beta stars — those two brightest of Argo Navis’ stars ended up in Carina instead, so Vela’s brightest is Gamma (γ) Velorum. Seemingly a single star to the naked eye, it is in fact a quadruple-star system made up of two pairs with a combined apparent magnitude of 1.72. Gamma’s first pair, γ¹ Vel, comprises a blue giant and another, undetermined star, while the second pair, γ² Vel, contains a Wolf-Rayet star (the brightest and one of the closest examples of its kind) and a blue supergiant.

Gamma has long been known by the Arabic Suhail al Muhlif (the ‘oath taker’), but during the space race of the 1960s it became unofficially known as “Regor,” in honor of astronaut, Roger Chaffee, who tragically perished in the Apollo 1 fire. Regor is Roger spelled backwards.

The most prominent of Vela’s other stars have their own charm. Delta (δ), Vela’s second brightest (mag. 1.96) and one of the member stars of the False Cross, is a triple-star system containing an magnitude-2.48 eclipsing binary, known as Kappa (κ). Their companion, Lambda (λ), is a solitary supergiant with a lovely orange colour; it’s very slowly variable between magnitudes 2.14 to 2.30.

With the majority of Vela having the Milky Way’s diffuse disk of stars as its background, it’s no surprise that this constellation contains its fair share of open star clusters. First up is the brightest: IC 2391. This cluster contains around 30 stars with a combined magnitude of 2.4 and is visible to the unaided eye as a misty patch. Expect to identify around six to eight scattered stars through 10x50 binoculars, while a dozen or more should be seen through a small telescope.

Next is IC 2395, a group of about four dozen stars some 2,300 light-years away with a combined apparent magnitude of 4.6; a small telescope will show it well. Another magnitude-4.6 cluster is Trumpler 10, which contains around 30 stars. It’s much closer at 1,100 light-years, and a good target for binoculars.

Roughly the same distance away as Trumpler 10 is NGC 2547, a fine open cluster about 17 arcminutes across with a combined magnitude of 4.7. Astronomers think it’s only about 30 million years old, which puts it at about the same age as Trumpler 10 and some other clusters in Puppis and Canis Major, so it’s possible they all had a common origin.

At a combined magnitude of 6.0, open cluster NGC 3228 is visible with binoculars, although bear in mind that its brightest star is magnitude 7.9. It contains around 50 stars about 1,800 light-years away and spans an area of sky about 20 arcminutes across. A small telescope is best for this one.

Our last object for this month is NGC 3201, a globular star cluster of magnitude 8.2 that has some intriguing characteristics. For a start, it’s unusual among globulars in being not very dense, i.e. having a fairly low concentration of stars in its center. It also has a retrograde orbit, meaning it moves along its orbit in the Milky Way in the direction opposite to most other objects in our galaxy. Its retrograde motion has led to suspicions that it might have been an extragalactic interloper gravitationally stripped from another galaxy at some point in the distant past; oddly enough, however, its chemical make-up is similar to that of other galactic globular clusters with normal prograde orbits.

There are other interesting sights to be seen within Vela, but we’ll leave those for a future column. For now, take a stroll outside, look up with binoculars, a small scope, or just your own eyes, and let yourself sail away across this splendid expanse of the southern sky.