A sketch of the four components of Gamma Velorum that can be seen through a small telescope. Image and copyright Harry Roberts ©, all rights reserved
We often take common objects for granted – despite some being truly unique. Sometimes it helps to remember their exotic nature -such a star is Gamma Velorum.
Sited at 47º south declination – it’s in our night sky most of the year, and highest in summer. Only when searching the naked eye stars for O types did I find that it is the brightest O type AND (by far) the brightest Wolf-Rayet type in the whole sky. As such it is of interest to northern viewers whose brightest O and WR types are very much fainter.
At magnitude 1.7 Gamma Vel. is a bright star, and in almost any amateur ‘scope it is a multiple – four components are seen in a small refractor at 60X. All are part of the same system – not line-of-sight members. They comprise the primary A, aka Gamma 2 Velorum, with the others B, C and D. The sketch (Fig1) shows the view through a small scope – and there are some colour contrasts worth noting.
The colour of primary A is hard to explain – I’ve always seen it as violet maybe, but often with a green tint. When David Malin published “Colours of the Stars” he noted there were no green stars. And due to the physics of black bodies – I’m sure he’s right. Except to say that this particular star emits so much yellow and blue light in its carbon emission bands, as well as strong green and violet continua, that we have, with Gamma2 perhaps, an exception to the “no-green” rule.
Star B by contrast is a normal B type, of 4.2 magnitude – and looks blue in a range of scopes. Stars C and D are white, as they are type A stars, of 8 and 9 magnitude.
Historic accounts of colour in doubles were made with older refractors that often had strong false-colour – modern reflectors or “cats” show more accurate colours. What colour do you see?
Most southern viewers know the star group dubbed the “False Cross”, like a lop-sided version of the Southern Cross, This handy grouping blends stars from constellations Carina (“the keel”) and Vela (“the sails”) of ancient Argo Navis. A projection of the short arm of the false cross westward leads to Gamma Velorum. It is worth a look with almost any scope.
It is true that the southern sky lacks the common names of the north – and there have been attempts to remedy this. Gamma Velorum is quite a mouthful – and the ancient Arabic name “Suhail” has not caught-on, and is used for other stars in the region. Recently it seems, the name “Regor”, “roger” spelt backwards (!) has been applied to Gamma Vel. I can’t see this working either, despite the suggestion that it honours one of the Apollo astronauts lost in the capsule fire. Let’s persist with Gamma Velorum!
The chief claim to fame of this star is its extraordinary spectrum – which I’ve described elsewhere. Both star types O and WR are rare, the latter in particular. Yet here we have a naked eye example. WR stars have such strong stellar winds that they have blown away much of their mass in the short time (astronomically) of a million years or less – and have lost most of their hydrogen fuel. As a result they now burn helium, at ~70,000K surface temperature, creating carbon in the fusion process – the cause of Gamma2’s bright carbon bands. Perhaps 800 Ly away, the WR star is said to be 100,000 times brighter than our Sun – and, apparently, a supernova candidate.
All up, this southern star can claim to be strangest of the naked-eye stars in the whole night sky. Take a look – almost any scope will do.
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers