I would like to be able to show some (telescopic) coloured stars at an upcoming public event.
On the red end of the spectrum, the carbon star EsB 365 near beta crux is by far the reddest in my experience, but since it is not visible in the mid evening at the moment, I thought I would go for r leporis.
On the blue end, I have struggled to find a list of 'bluest stars', or a list of stars with the most negative BV indices to check out. zeta puppis at BV -0.27 is as low as I have found, but it needs a lot of imagination to truly call it blue.
Maybe a bright blue nebula (which one?) would be a better way to go.
Hi Andrew.
I was shown the double star h3945 in canis major at a star party. It consists of a red/orange star and a blue star. The contrast was amazing.
Would be a good one to show.
Many of the bright stars in Orion are hot O-class blue giants e.g. Mintaka, Alnilam, Alnitak and Meissa. Rigel itself is a blue supergiant, spectral type B8. But you are quite right, it is hard to distinguish the blue from a white A-class star like Sirius or Fomalhaut. Acamar is a nice white pair.
You might get some contrast difference between the components of gamma Velorum (Regor). The primary is a [massive blue Wolf-Rayet and O-class blue supergiant] spectroscopic binary, while the B component is blue-white with the C and D components whitish. The four visual components form a Y shape.
Betelgeuse is a red supergiant but appears orange-reddish. You could try W Orionis for a red carbon star. Aldebaran in Taurus is orange in colour.
The alpha Centauri AB binary is a nice yellow pair.
The Saturn Nebula (NGC 7009) in Aquarius is a blue-green planetary nebula.
Other than that you could go for the planet Uranus near Jupiter which is blue-green.
Hi Andrew.
I was shown the double star h3945 in canis major at a star party. It consists of a red/orange star and a blue star. The contrast was amazing.
Would be a good one to show.
Also goes by the tag 145 CMa. Lovely contrast. Often the contrast makes the orange star more orange and in this case, the white star appear bluish. Spectral types K4/A5.
x Velorum is a nice yellow and blue pair. Spectral types G2.5/B8.
I agree with Robh, Gamma Velorum is a good challenge. We have to remember that this is also the brightest Wolf-Rayet star in the sky, its class is "W", that means just before O class (ultra-violet radiation is very intense).
Otherwise, I suggest some O or W stars belonging to OB associations:
*Orion (explecially Alnitak and Sigma Orionis);
*the Plaskett stars in Monoceros (two O-type stars in a system);
*S Monocerotis (O7, belonging to Mon OB1);
*29 Canis Majoris (O7, close to NGC 2362);
*Many stars of the Tr 14 cluster, inside to the Carina Nebula; HD 93129 (6th magn.) is O2 class.
*HD 92740, magn. 6.3, Wolf-Rayet star near Tr 14.
Here is an observing list of Colourful Double Stars that demonstrates the observation planning capability of SkyTools.
For anyone requiring an observation planning application, you should definitely take a good look at SkyTools which was designed from the ground up to be an observation planning, observing, charting and logging tool.
The attached screen shot is full of useful information and the application is highly customisable. It would take way too much time to list and describe all the capabilities and I’d probably do SkyTools an injustice, so it might be worth your while in dropping by their website to have a good look at how it works and assess its suitability for your needs.
Cheers
Dennis
PS – this is the usual disclaimer that I have no association with SkyTools other than being a very satisfied end user!
Andrew, what a fabulous question. I've been curious about the same question as you. Thank you for your starting up this thread and I will be watching it closely and printing it out.
I had a look at R leporis, W ori and the winter albireo (h3945) last night.
For red stars, R leporis definitely had the best colour, being richer than W ori, but the latter is nevertheless also a good subject.
The winter albireo (we should really call it the summer albireo) is delightful as a contrasting double - putting the pair slightly out of focus so that the discs of light are muted in brightness and overlap really shows up the colour contrast. Thanks for the tip on this one.
Don't forget to make a note to look at EsB365 near beta crux though when the opportunity arises, for those of you who haven't seen it - it is without doubt the reddest in the sky (b-v 5.8) - not just orange red but ruby red. The reason it doesn't get much publicity is that it is not visible from the northern hemisphere.
Don't forget to make a note to look at EsB365 near beta crux though when the opportunity arises, for those of you who haven't seen it - it is without doubt the reddest in the sky (b-v 5.8) - not just orange red but ruby red. The reason it doesn't get much publicity is that it is not visible from the northern hemisphere.
Cheers,
Andrew
Yep Andrew, Ruby Crucis (DY Crucis, EsB365) is a ripper and has been given heaps of publicity in here
but for Blue stars, I reckon Achernar ( Alpha Eridani) looks fairly Blue through a scope.
I also notice that blue stars tend to actually look bluer at very high magnification.
Try it. Look at some proposed blue targets then up the magnification.
Probably because the light is being spread out more thus diffusing the brightness.
Last edited by ballaratdragons; 25-11-2010 at 02:53 PM.
I think defocussing as per my earlier post achieves the same effect as high mag - the eye is relatively insensitive to colour from point sources, so any way of diffusing the same amount of light over a wider area will help counteract this (see http://outreach.atnf.csiro.au/educat...ry_colour.html)
Wow, this is all very interesting.
I can't wait to try these techniques. In winter, I saw my first "coloured pair" in Albireo, the gorgeous yellow and blue pair. I did see definite colour, but it wasn't as spectacular as I hoped for. Now, to wait another year for it to return ...
Try the 'winter albireo' mentioned below - the pair is almost as good as albireo itself (I'm sure someone will have a view on that) and it is in a good position (not far from Sirius) over the summer.
that concludes that the very centre of the fovea in the (normal) human eye - the point of greatest visual acuity - actually has an area of about 25 arc minutes in diameter that 'lacks functioning blue sensitive cones' !!
This suggests to me that, in addition to the situation that there appear to be no stars with a strong hue in the visible blue part of the spectrum (comparable to those at the red end), we don't have any chance of seeing true any amount of blue colour in a star or any other nearly point source unless the image is distorted in some way (such as by defocussing, poor seeing, diffraction spikes), or by averted vision, so that it shows outside this central zone.
The colour indicies are okay as a guide but can be misleading particularly at the extreme ends as to how a star looks visually. For example, you mention my favourite star of all Zeta Puppis (Naos) with an apparently thumping B-V of -2.7. It really doesn't look that blue at all because 90% of its light is in the ultra-violet part of the spectrum that we can't see. If we could see ultra-violet, Naos would shine at about magnitude -3 even from 1000-odd light-years distance. Of the bright stars in the sky (say 1st & 2nd magnitude), it has the highest spectral classification of all (O5). But sadly it doesn't look real blue. Virtually all the O-stars so far mentioned are the same -- too much ultra-violet to really look blue -- to us anyway.
What you need is a star with a spectrum between about B0 and B4 to look as blue as it can to the naked eye. In the winter sky, Spica is about as blue as you can see.
Of the ten brightest stars in the sky, the bluest is Achernar. Rigel I'd describe as "cold white". Another quite blue one through the 'scope is Theta Carinae -- as a bonus there are about another dozen quite blue ones around it.
One colour-contrasting pair I think no-one has mentioned yet is Dunlop 94 just outside the Eta Carinae complex in the centre of the sparse cluster Vdb-Ha 99. A little wider than both Alberio and h3945, it is equally as colourful but sadly, overshadowed by the nebula complex to the point almost no-one looks at it -- and it is also quite bright. But it is also a bit low in the sky if your event is soon.
I had zeta puppis at B-V -0.27 rather than -2.7. Most of the other very hot stars seem to hover about that same value. I've not seen reference to any stars with a significantly more negative B-V.
On that basis, B-V seems a reasonable guide to the degree of visible blueness.
I had zeta puppis at B-V -0.27 rather than -2.7. Most of the other very hot stars seem to hover about that same value. I've not seen reference to any stars with a significantly more negative B-V.
Oooops ... my typo, -0.27 is correct as to your eariler post. Theta Carinae is -0.197 from memory, Achernar -0.058 and Spica -0.136. But these three are as blue as you will see naked-eye or eyeball to eyepiece. According to Megastar v5 at least, (and without tedious trawling through photometric catalogues) Zeta Puppis is recorded there at -0.203
The only caveat to that is that each person's vision and colour perception is different due to a host of reasons. Colour is a highly subjective experience.
However, for most person's vision and colour, a star around the B0-B4 spectral class is as blue as it gets. Hotter than that, it's just more U.V, not more blue light. It becomes more "technically" blue, but not more "actually" blue -- if you take my meaning.
Mu Columbae (HD 38666), spectral type O9.5, has a B-V index around -0.28.
Also, you can't rely solely on the colour (B-V) index for the "real" colour of the star. A star can have an observed B-V that is actually higher than its intrinsic B-V. It might appear redder from the Earth but its spectra indicate that it is a hot blue star. A blue supergiant in the plane of the galaxy can have its light dimmed by interstellar materials. The light of the star is shifted towards the red end. Eta Carinae looks more orange in a scope even though it has blue components, one a Wolf-Rayet star. This is due to the surrounding gases and materials.
Interesting Rob , I've been wondering why E.Carina shows up yellow where as infact it is a LBV. I'm only just starting my journey on learning spectral lines and all things stars. That Cambridge Encyclopedia of Star is a big book to digest and one that will take me time to absorb and learn. The want and enthusiasm is very much there.
, I've been wondering why E.Carina shows up yellow where as infact it is a LBV. I'm only just starting my journey on learning spectral lines and all things stars. That Cambridge Encyclopedia of Star is a big book to digest and one that will take me time to absorb and learn. The want and enthusiasm is very much there. 
