Seeing Red - A Selection Of Lesser-Known Carbon Stars

[cloudsweeper]

In Main Sequence stars, as the Hydrogen fusion changes from core to shell, they cool and expand to Red Giants/Supergiants.  The Hydrogen eventually runs out, and the Giants get compressed and heat up, allowing Helium to fuse to Carbon and Oxygen.  In intermediate mass stars, there is more Carbon than Oxygen in their atmospheres, and Carbon molecules form, scattering blue light from inside, while allowing red light to pass through, which is why red is enhanced.  (It's like the red sky at night effect.)

These Carbon stars are relatively uncommon since the process is short-lived.  Their appearance depends on magnification/contrast, transparency, aperture, and perception. 

I have just viewed a selection of them, as follows.  (Not many doubles and variables are programmed into the Celestron handset, so I used their SAO numbers and GoTo, or "hopping" aided by Stellarium, and using nearby stars whose SAO numbers are  in the handset.)

8SE from the shed – well cooled.  5.35pm – Capella appeared, followed very soon after by Betelgeuse.  Aligned on them.

RT Ori – noticeably orange.

BL Ori – distinctly orange x56, deeper colour x113.  (Greater contrast.)

W Ori – distinctly orange.

TT Tau – orange, not as clear, magnification helped.

TU Tau – as above.

Y Tau – another that stood out in a field of silvery stars.

TU Gem  – orange.

VW Gem – noticeably orange.

BM Gem – just noticeably orange, aided with magnification.  Transparency poor however.

FU Aur (next to the double O Sigma 117) – slightly orange.  The double immediately below was mismatched and tight.  Its primary is said to be orange, but this was not detected.

V Aur – the only “failure” of the carbons – no colour detected, even with more mag.

UU Aur – clearly orange – it is also a tight double, but not split.  (Quite a lot of cloud, poor seeing also.)

So – two and a half hours of picking out these colourful lovelies, most of which stood out quite clearly.  There’s a lot of enjoyment to be had even when there are lots of clouds about!

Doug.

[cotterless45]

Brilliant , there's nothing like the cheery warm site of red and orange stars and the science behind them. My favourites are the "Garnet star", "Hind's Crimson" and "La Superba",

Nick.

[MarsG76]

The red stars do jump out at you in the eyepiece... especially when within a cluster of white and blue stars..... good to see a fellow 8SE user getting results.

 

[cloudsweeper]

9 hours ago, cotterless45 said:

Brilliant , there's nothing like the cheery warm site of red and orange stars and the science behind them. My favourites are the "Garnet star", "Hind's Crimson" and "La Superba",

Nick.

Thanks, Nick.  Yes, carbons and red giants/supergiants can be delightful.  Hind's Crimson Star/Vampire Star is one that really stands out for me.

Another great red s/g is Ahad's Ruby Star (119 Tau), named by Abdul Ahad just 14 years ago.  Colour more intense than Betelgeuse and Antares.

Doug.

[25585]

Pocket Sky Atlas includes about 150. Interstellarium has them, but does not identify as R,N,S,C.

[cloudsweeper]

54 minutes ago, 25585 said:

Pocket Sky Atlas includes about 150.

Yes, and likewise the Cambridge Double Star Atlas.  A more detailed source (presumably with many less prominent carbons) is The Astronomical League Carbon Star Observing Program.  (Click on the link to the lists, at the bottom of that page.)

Most of the stars listed are in Stellarium, but rarely by name/code, and not always by SAO number.  The hunt is part of the fun!

Doug.

[Stu]

Great report Doug, I've been trying a few of these lately, it's good fun.

Hind's seems to be the most extreme in terms of red, the rest are orangey to a greater or lesser degree as you say.

[David Levi]

Like the other observers contributing to this thread I have really been enjoying carbon star observation. I've been using Sky & Telescope's Pocket Sky Atlas as my source of targets. The carbon stars are marked as '(c)'.

I have a question which I hope contributes to the discussion on this thread, rather than starting a new topic. Last night I observed several red and carbon stars but none of them appear as red as Hind's Crimson Star (R Lep) - at least to my eye. However, according to Stellarium the B-V colour index of Hind's Crimson Star is 2.71. I've observed several stars with B-V colour indices greater than this, for example W Ori (3.34), Y Tau (3.09), X Cnc (2.96) and there's an unnamed star in Gemini (3.34 - this star is just north of M35 and east of 139 Gem). Why is this? 

I can hazard a guess that as the R filter magnitude is not included in the index then this affects the result? It would be good to know if someone knows the answer for sure.

[Martin Meredith]

Very interesting account, thanks.

For later in the year  V Corona Borealis with a B-V index of +4.4 is worth a look.

David, I don't know for sure but I'd agree with your guess. What matters is the entire spectrum (or at least the part that overlaps with that of our visual system for eyepiece observing) and the B-V index is just a crude one-dimensional approach to classify colour, and other colour indices can be used. It would be interesting to dig out spectra for R Lep, W Ori etc and check this out.

Also, I note that Wikipedia has the B-V for R Lep at +5.74 but presumably this is at or near minimum. The value in Stellarium I guess is some kind of average?

Martin

[25585]

1 hour ago, David Levi said:

Like the other observers contributing to this thread I have really been enjoying carbon star observation. I've been using Sky & Telescope's Pocket Sky Atlas as my source of targets. The carbon stars are marked as '(c)'.

I have a question which I hope contributes to the discussion on this thread, rather than starting a new topic. Last night I observed several red and carbon stars but none of them appear as red as Hind's Crimson Star (R Lep) - at least to my eye. However, according to Stellarium the B-V colour index of Hind's Crimson Star is 2.71. I've observed several stars with B-V colour indices greater than this, for example W Ori (3.34), Y Tau (3.09), X Cnc (2.96) and there's an unnamed star in Gemini (3.34 - this star is just north of M35 and east of 139 Gem). Why is this? 

I can hazard a guess that as the R filter magnitude is not included in the index then this affects the result? It would be good to know if someone knows the answer for sure.

According to Wiki Hind's has a B-V of +5.74 and R-I of +1.47. Being a Mira variable it's apparent magnitude goes down to 11.7, so you were lucky to see it. Perhaps getting fainter made it appear redder. 

 

[DHEB]

Sorry for being picky. The PSA states that there are 55 carbon stars marked with "(c)". Not 150. See introduction, page viii.

Nice thread by the way

[David Levi]

Yes, @25585 as @Martin Meredith pointed out there seems to be some discrepancy in the B-V colour index value between Wikipedia and Stellarium. The R-I value sounds interesting. I'll have to look up these values for the carbon stars that I observe as well. That is, if they are easily available. A lot of the carbon stars are, as this thread suggests, lesser known.

I can see down to magnitude 12 stars from my back garden with direct vision. I've been viewing Hind's Crimson Star for a few weeks now and without being an expert in the matter I would say that it's a lot brighter than it's minimum at the moment.

[cloudsweeper]

35 minutes ago, DHEB said:

The PSA states that there are 55 carbon stars marked with "(c)". Not 150. See introduction, page viii.

Nice thread by the way

Thanks for that!  Further to what we said before, I now notice the CDSA has similar variables to the PSA, but does not indicate carbon stars as such.  Useful to know!

Doug.

[David Levi]

Here's an interesting article from Sky & Telescope that also gives the B-V colour index of Hind's Crimson Star as +5.7. There's a short list of carbon stars at the bottom of the article that gives the B-V for T Cancri as +5.3. That will be on the list for my next observing session.

http://www.skyandtelescope.com/observing/carbon-stars-will-make-see-red1203201401/

 

[Mr Spock]

A nice list of targets. Shows there's something interesting to look at if you make the effort.

[25585]

2 hours ago, David Levi said:

Yes, @25585 as @Martin Meredith pointed out there seems to be some discrepancy in the B-V colour index value between Wikipedia and Stellarium. The R-I value sounds interesting. I'll have to look up these values for the carbon stars that I observe as well. That is, if they are easily available. A lot of the carbon stars are, as this thread suggests, lesser known.

I can see down to magnitude 12 stars from my back garden with direct vision. I've been viewing Hind's Crimson Star for a few weeks now and without being an expert in the matter I would say that it's a lot brighter than it's minimum at the moment.

 

Starry Night Enthusiast states Hind's Crimson Star is B-V 2.70 Spectral Class 1647 Kelvins M4 IA Mag 8.06.

Prev SGL thread

also info

http://stars.astro.illinois.edu/sow/hinds.html