Get to know the stars of Carina — and learn how to spot the False Cross instead of being fooled by it.

Southern Hemisphere sky view
The constellation Carina rides high in March skies. The isolated pink nebulosity just to the left of centre is the Carina Nebula; a little to its left is the Southern Cross.
ESO/P. Horálek

Continuing our tour of southern sky, we pick up where we ended last month in Carina and spend a bit more time there. We’ve already had a look at some of its best nebulae and clusters, but now let’s get to know some of the constellation’s stars.

Carina has three of the 50 brightest stars in the sky — Canopus (α Car), Miaplacidus (β Car) and Avior (ε Car). The first two we covered last month, but what about the third?

Avior is a binary system with a combined apparent magnitude of 1.86, making it the 39th brightest in the whole sky. Located roughly 600 light-years from Earth, the pair comprises a magnitude 2.2 K-type evolved giant and a hot B-type of magnitude 4.1 (which might itself be a binary). The origin of its name is interesting — unlike many other star names, Avior is a fairly recent moniker, dating to the 1930s, when a proper name was given to the star when it was put on a list used by the Royal Air Force for aerial navigation.

The crosses of the southern sky
The Southern Cross has two nearby imposters — the Diamond Cross and False Cross — which are a trap for casual stargazers.
Image courtesy Akira Fujii, with modifications from the author

Avior is also of note because it is one of the four stars that make up the “False Cross” asterism, a larger imitation of the nearby (and smaller) Southern Cross. The stars of the False Cross — Avior, Aspidiske (ι Car), Alsephina (δ Velorum) and Markeb (κ Vel) — trace out a large crucifix shape, which is why beginner stargazers and casual observers often mistake it for the true Crux constellation.

Our next stop is Aspidiske, which ranks at number 64 on the list of brightest stars, shining at magnitude 2.2. It’s a bit further away than Avior, around 690 light-years, and is classed as an A9 lb-type star that is heading towards the red supergiant phase. Yet it’s already big enough — more than 40 times as wide as the Sun and intrinsically almost 5,000 times brighter.

Another fascinating star is Theta (θ) Carinae, a magnitude-2.75 spectroscopic binary located about 460 light-years from Earth. Its component stars are thought to be very different to each other. The primary is around 15 times the mass of the Sun and has a surface temperature of more than 30,000 kelvin (compared to 5770K for the Sun). It’s a blue straggler — that is, brighter and bluer than expected — most likely because it has pulled gaseous material from its companion star. That companion is suspected to be a nondescript main-sequence star with less than a hundredth of its partner’s luminosity.

The Theta system is also the brightest member of open cluster IC 2602, a brilliant group of about 6 dozen stars sometimes called the Southern Pleiades because of their resemblance to the namesake in Taurus.

And, not to be outdone by Avior, Theta, too, is part of an asterism that can masquerade as the Southern Cross. This one is the Diamond Cross, and it’s comprised of four Carina stars — Theta, Beta, Upsilon (υ) and Omega (ω). Although it’s about the same size as the False Cross, the Diamond’s short axis is more centred along the long axis, giving it less of a resemblance to a crucifix. Joining up the stars via the ‘outside route’ rather than along the axes, reveals the diamond shape.

On the topic of Upsilon, about 8,100 years from now both it and its sibling Iota will be able to stake claims for being the new southern ‘pole star’. This is because of the long, slow precession of the Earth’s axis of rotation, which makes the northern and southern celestial poles trace out circles on the celestial sphere over a period of about 26,000 years. At the moment the southern title is held by Sigma Octantis, a dim star (magnitude 5.5) that is nowhere near as handy for finding the pole as bright Polaris is for the northern sky.

Carina constellation
Canopus is the brightest star in the southern constellation Carina, the Keel.
IAU / Sky & Telescope (Roger Sinnott & Rick Fienberg)

Variable Stars in Carina

Carina is awash with variable stars of many types, led, of course, by the mighty Eta Carinae itself (see last month’s column). Here are a few more of them:

AG Carinae is a rare luminous blue variable some 20,000 light-years from Earth, which varies between magnitudes 5.7 and 9 in just over a year (371 days). Its mass is thought to be more than 50 solar masses, and it likely has a luminosity one million times greater than that of the Sun.

An easy-to-observe Cepheid variable, U Carinae goes from magnitude 5.7 to 6.9 and back with a period of around 39 days. This is one of the longest-period Cepheids known, and a member of the Diamond Cross.

Another Carina Cepheid is I Carinae, located around 1,600 light-years from Earth. This one varies from magnitude 3.4 to 4.1 over a period of just under 36 days, so it’s an easy one to follow.

Another variable that’s about 1,600 light-years from us is the red giant S Carinae, a Mira-type variable star which fluctuates between magnitude 4.5 and 10 over 150 days.

Finally, there’s QZ Carinae. This is a multiple system comprised of at least nine — yes, nine! — stars, including an eclipsing binary pair that gives QZ its observable period of six days ranging from magnitude 6.2 to 6.5.

This is still just the tip of the iceberg. Carina is loaded with objects to observe, and one can spend an entire evening exploring the area without seeing all it has to offer.