Harry has another look at the extraordinary southern multiple star, Gamma Velorum

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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

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8 responses to “Harry has another look at the extraordinary southern multiple star, Gamma Velorum

  • Hi there, just became aware of your blog through Google, and
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  • A. James.
    First of all sorry to my english, i know that it´s terrible.
    You said that Gamma 2 Velorum is about 157 parsec of distance, i use to know that this star was about at least 250 parsec of distance. My question is: how Gamma 2 Velorum will produce a type Ic supernova we will be safe at this distance. Thanks to atention.
    Rafael Londero

  • Dr.Lomb and Harry,

    Just a couple of additional historical and general points here;

    – The system was discovered by J. Dunlop in 1826 from his home at Paramatta, with Δ64 [DUN 64] being the AB stars and Δ65 [DUN 65] for AC stars. The Washington Double Star Catalogued (WDS) listed all these stars as Δ65, which according to Dr. Mason of the US Naval Observatory was decided as Δ65 for some unknown reason.

    – The Paramatta Star Catalogue lists A as PSC 1916 @ 4.9 magnitude, but calls it Gamma Argus  

    – Respighi was the first to find the hydrogen spectrum and some variability in star A in 1885 (I think of the top of my head.)

    – John Tebbutt made measures of these stars in 1881 and published it in the Royal Astronomical Society Journal “Observatory , vol. 4, pg.211.

    – Gamma (1 or A) Velorum is also slightly variable in brightness and is highly periodic. (called V* gam01 Vel) It is listed as He3-127 for its prominence Hα (Appreciated here for Harry’s solar work!!) and is astrong X-ray emitter. Some postulate that if this star goes supernova, it will brighten to -9 to -10 magnitude, shining as a point source almost as bright as the moon! It will be dangerous to look in a telescope as the light source will damage the eyes like a laser beam! Astronomers in the past have been concerned about even measuring such brightness in true magnitudes (telescopes are designed to measure faint stars not blindingly bright ones!There is quite huge problems in distance., the most recent being 157 parsec from the CaII H & K lines in 2009. 

    – Gamma (2 or B) Velorum (2) or HR3206 is still designated the variable NSV3930 or as V* gam02 Vel. It has the spectral type of B1IV (sub-giant), whose distance is still not well known. It has been estimated to be 3-4 million years old

    – According to Abt et al., AJ, 81, 541 (1976), the more distant D and E components, lying 63 and 93 arcsec away, respectively from C star are probably associated.

    – The whole system is mostly considered as a multiple star with six or seven components, and is now sometimes called the Gam Vel Subcluster, being part of the gravitationally weaker star Association known as Vela OB2 (2009) [Really advanced readers might like to read the highly technical  paper “The Stellar Association around Gamma Velorum and its Relationship with Vela OB2” by R.D. Jeffries  (and others.) It is really the best source for current observational parameters and what we presently know on the system. /Download as 7.8 Mb pdf/]

    – Innes in 1899 says it is “one of the most brilliant fields in the Heavens“, but in 1911 says; “[Gamma] Velor(um)… has a bright companion at 42″ and two other stars near it, making it one of the most beautiful objects in the heavens. Spect.(rum) of bright lines.

    – According to the WDS (Washington Double Star Catalogue) “Gamma (1) Vel. Both components are spectroscopic binaries, and B is variable. A is probably the nearest Wolf-Rayet system. Spectrum: WC8+O7.5e. Spectrum of B B1IV.

    – The Innes pair is I 1175 DE or sometimes RST 4878. It lies PA 144 degrees (Angle from north through east.) and separated by 1.5 arcsec. Magnitude is 12.8, and is just visible in 25cm under good conditions.

    I have seen the four components at Sydney Observatory through the main 29cm. refractor under good observatory conditions. (The colours I saw was mostly blue for all stars except “A” which was a slightly deeper blue. The issue with the refractor is the colours are not quite right, which probably has something to do with the colour of the glass of the main achromatic lens and/or the original eyepieces. [likely with absorption features – probably testable with a spectrophotometer – and being a great project for some new up or budding astronomer!]) H.C. Russell and Lawrence Hargrave saw several stars as green.

    As for violet stars is is likely impossible in stars. (I have a discussion of these colour issues at d.) Dubious or Interpretative Colour Descriptors : Purple and “Indigo” Stars (violet stars are considered as purple or indigo stars in some sources.) My general reasoning is;

    The possible existence of purple coloured stars is something quite perplexing. Purple is interpreted as the combination of red and blue light — colours lying on the exact opposite ends of the visible spectrum. Stars producing such colours are simply likely due to contrasting effects.What I believe is some observers actually mean either blue or deep blue and that the red tint is simply an aberration of the used descriptors. However, another very distinct possibility might be with visual purple or its other formal name rhodopsin, being the chemical responsible for our vision via the photoreceptor cells. Here this light sensitive compound is actually related to the monochromatic sensitive rods and not the colour cones.

    Studies find that [the chemical] rhodopsin itself will absorb strongly the wavelengths of green-blue light thus appearing more reddish-purple – the origin of the perceived colour of the visual purple.

    (Apologies for the length of the reply!!)

    Cheers, Andrew James

    • Oh! I should have said…. Thanks for this interesting article. The stars look very much the same as I remember in the same kind of aperture 20cm telescope (brand and type.)

      • Thank you for that Andrew. You are a fountain of information! All interesting and useful to have available and published. Nick

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