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IY Comae Berenices

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IY Comae Berenices

Hello, everyone. 

Some time ago I observed IY Comae for the very first time with my William Optics 158 mm apochromatic refractor. 

Simbad initially accepts that this star is a carbon star and gives it a spectral class of C. But then modifies that to K9. 

However my Guide 9.1 DVD gives a spectral class of M2 - which indicates it is a red star. 

Having observed the star twice I would say it is definitely a carbon star because of its strong and quite intense orange hue. 

Here on AAVSO in the VSX section we have an M sp.class also. 

So what I would like to know is: what is the true spectral class of IY Com?

I would be delighted if some of you on AAVSO would discuss this difficulty. Thank you. 

By the way, IY Comae is very near the famous binary 24 Comae. 

Kind regards, 


Sebastian Otero
Sebastian Otero's picture
M-type star

Hello Aubrey,
Judging from the colours and most of the literature classifications (spanning from K9 to M5) it looks like an M-type giant indeed. Published B-V values range between 1.52 and 1.60 so pretty normal for an early M-type giant. It doesn't show a large amplitude (0.4 mag. at most, usually only 0.2 or so) which makes it a difficult target for visual observations.


Hello, Sebastian. 

Hello, Sebastian. 

Thank you for your response. I do realise B-V values are important. My Guide 9.1 DVD gives a B-V value of IY Com as 1.469 +/--0.019. The website called Universe Guide gives 1.51. I would be pleased to know more about the star's true spectral class. The Simbad website doesn't appear to offer such a value. Is it possible that Simbad is incorrect in suggesting that IY Comae is a carbon star?

Can a spectroscopist help us out here?

Please come back to me, Sebastian.

Thank you from Aubrey. 



Sebastian Otero
Sebastian Otero's picture
Data sources

Hi Aubrey,
it is important to keep in mind what a data source is.
Commercial software or even one-stop databases like SIMBAD are actually not data sources. They take the data from publications or catalogues and those should be the ones we call "source".
SIMBAD compiles data from several sources (I officially wrote the word "sources" too many times already... :) ).
The star is included as a carbon star in (1997A&AS..122..489G) and the SIMBAD object type may come from there. But published spectral types come from other sources so this ends up looking like an inconsistency.
But SIMBAD is not correct or incorrect since it is just providing information from the literature.

Regarding the B-V values you quote, I bet they come from Tycho and Tycho-2. You should look that up in the software documentation or in the headers. This should be mentioned. Remember that quoting a colour as coming from X software is not telling us anything because the planetarium program is using data from different databases.
You can use VizieR to see the data for your star in differente catalogues.
Tycho gives B-V= 1.484 and Hipparcos gives 1.515 (determined from Tycho too) while a Tycho-2 transformed value is 1.47.
So in one way or another, these three values comes from Tycho observations (and different reductions and tranformations) and I am sure this is the source used for the values you quote.
However, Tycho values are not the best for red stars (they are usually too blue). For bright objects you will have (most of the times) GCPD data available. You can enter the object's HD number and get UBV data there.
We have: V= 7.45; B-V= 1.58 and V= 7.42; B-V= 1.60.
So a mean B-V= 1.59.
It is not that the star varies from B-V 1.47 to 1.60, the Tycho values can be disregarded.

So there is a source indicating "Variable star with evolved C-rich dust shell" and several published spectral types poitning to an M-type giant. I can't confirm or deny that the object is a carbon star but for now we will keep it as M2 in VSX due to the large number of sources indicating the M-type nature.

CTX's picture
IY Com Sequence

I just created a small 4 comp sequence for IY Com.

Suggest a 90 arcmin chart (1.5 degree)

Ad Astra,

Tim Crawford, Sequence Team


Hello again, Sebastian. You

Hello again, Sebastian. You certainly are a well informed astronomer with regards this subject matter.

I am greatly surprised that you are encouraging me to disregard Simbad data. I have been thinking all along that Simbad is seriously reliable.

Does it not receive spectral classes from the Gaia spacecraft? Perhaps I have been receiving unreliable information. 

Now regarding VisieR: I am having great trouble in discovering how this website can be used to discover the spectral class of IY Comae or HD 108815. I appear to be drawing a blank at the moment. It seems to be a most complex website. VisieR is not as simple as Simbad to get the information I require. So I am willing to wait for full instructions how to use VisieR. 

Moving onto our friend CTX whose name is Tim Crawford. You are very welcome to add any additional contributions. 

If I may ask you one question for now. 

Where can I find your comp. sequence in regards to IY Com?

Thank you very much, Tim.  

Best regards from Aubrey. 

Simbad data

Hello Aubrey,

Simbad is by no means a complete and totally reliable source of stellar information (except coordinates and names and maybe publications, too). Spectral class there is a hint, but it can be sometimes very off, a'la reddened hot objects shown as cool stars etc - encountered that several times during VARMAS mini-survey. Probably things are moving in a better direction (regarding spectral data!) when Gaia and other large scale spectroscopic surveys (Weave, 4MOST, DESI ...) provide their data in published tables.

Some objects have the best data one can find but for many others an user has to be careful. Simbad team has created wonderful database, but they can't multicheck every single object...

Best wishes,

CTX's picture
Variable Star Plotter

While possibly a bit dated, the below link will take you to a pdf that should allow you to be able to use the VSP:

If you have problems using the VSP you can email me direct:

Ad Astra,


Sebastian Otero
Sebastian Otero's picture
SIMBAD, VizieR and spectral types

Hello Aubrey,
I haven't encouraged you to disregard SIMBAD data, I just encouraged you to take a critical approach to all the information you may find anywhere, putting special attention to find the original source of that information. So, again, there is no such a thing as "SIMBAD data" because the data come from elsewhere. Spectral types or object types may be wrong because they were wrong in the publications SIMBAD is adopting to display the star parameters.
There are no spectral types published by Gaia so there is no such information to offer.

Regarding VizieR searches, all the catalogue data available are returned, unlike SIMBAD where they take information from different sources to show a single star data page.
In VizieR you need to find the relevant data yourself.
Astrometric, spectroscopic, photometric and any kind of catalogue including the star will be listed here with their information.
You need to be careful, making sure that the different entries correspond to the same star by checking the identifiers and especially the coordinates and the distance from the catalogue object to the requested object's position. Each catalogue entry may show several stars: the deeper the catalogue, the more objects you will find.
E.g. if you search using a 30" radius there will be many faint stars displayed in the Gaia DR2 catalogue entry. The star listed with a r distance close to 0.0" will be the target you are looking for. Objects listed at 5, 10, 15, 20" etc. will be nearby stars.
It takes some getting used but it is extremely useful.
There is no special order to find spectroscopic entries but in general old catalogues are near the top while spectral types from more recent sources are found near the bottom.

I hope you find this information useful.

Sebastian's picture
IY Com Magnitude

I just visually observed IY Com using the comp sequence Tim Crawford created and then uploaded my observation. Assuming the magnitude of 7.29 - 7.70 V is accurate, the star must be at maximum brightness right now because I estimated the star to be around a 7.0 visually comparing it to the 7.4 and 8.6 magnitude comparative stars. It would be interesting to hear a more exact measurement of magnitude from someone with the equipment for it. 

Sebastian Otero
Sebastian Otero's picture
Small amplitude variables and visual observations

Hi Aubrey,
As I mentioned in an earlier comment, these small amplitude variable stars are very difficult to estimate visually. I personally do not encourage observing them.
Also it is important to note that if you want to detect 0.1 mag. differences (this object varies by 0.2 or 0.3 mag. so you need to be very accurate) you can't use two comparison stars with a magnitude difference of 1.2 mag. That magnitude difference will set a limit to the accuracy of your estimate. And there is no comparison star brighter than the variable so you have extrapolated the magnitudes using steps and that adds up even more uncertainty to the result.
I have observed binocular variables for years (I assume you are using binoculars here due to the star's magnitude?) and I have always used 0.1 or 0.2 mag. steps (mag. difference between comparison stars) if I want to get accurate results for small amplitude objects.
If this star doesn't have bright comparison stars available nearby, probably other comparison stars further away will have to be chosen and you will have to move your binoculars somewhat to adjacent fields.
But all these are additional sources of error.
So we have:
1) the star has a very small amplitude, 2) the star has a red color, which makes visual estimates dependent on the red sensitivity of the observer and the observing technique used and 3) the comparison stars available are poor.
All this makes a strong argument to say that youd should better focus your efforts on other objects.
Even when you particularly like this star because of a number of reasons, you have to consider different aspects (like those mentioned above) to decide if a star is worth observing or not.
This one doesn't meet those criteria.

Best wishes,

IY Comae Berenices

Let me firstly thank each and everyone of you who has taken the time to discuss IY Comae Berenices. It has been a real honour to discover the true spectral class of this enigmatic star. 

I am now firmly believing that the star is NOT a carbon star. 

I always knew that its magnitude varies very little as the time goes by. 

By the way, I have observed other variable stars in recent years; and I have used repeatedly. 
I love carbon stars; but I also adore double and multiple stars too. 

If you refer back to post #1, I did state I own a William Optics 158 mm F/7 apochromatic refractor which I have had for almost 10 years. So I don't use binoculars at all. My finder scope is a William Optics 70 mm F/6 apochromatic refractor. With a 2" 28 mm eyepiece I get a full 6 degrees true field of view. All apochromatic refractors are known to kill false colours. These 2 telescopes are no exception. I do see red stars alright. But IY Com is not one of them. 

So getting back to IY Comae: I now fully accept that it is an M class star, and that is despite the fact I see no redness in its appearance. My only source of suggesting it is a C class carbon star was taken from Simbad. All other sources say it is an M class star. has a brilliant format where we can list off the entire number of carbon stars in any single constellation. I am now of the persuasion that this way of finding carbon stars is totally reliable. So I will be practising this in the immediate future at all times. 

Oh, I did find IY Com on VisieR last night. The website states that the star has a spectral class of M2. 

So my thanks to Sebastian, Tim, Tonisee and J Tully. I especially thank you, Mr. Tully for taking the time to observe IY Comae Berenices and giving it a current estimation of its magnitude. I can see you enjoyed the experience. It is a delightful star to check out because of its colour.  

I hope I have covered this star adequately, and will be checking out any further comments regarding IY Com. 

Best regards from Aubrey.'s picture
IY Com

I know everyone is already satisfied with this subject, but I took another look. IY Com looked more orangey than the nearby magnitude 7.4 comparison star HD 108468. This would seem to add evidence that IY Com is more likely towards the M5 end than the K9 end of the classifications quoted in the literature as you all discussed. HD 108468 is quoted as being a G8 star and it seems it could be less likely for there to be a noticable visible difference between a G8 and a K9 than say a G8 and an M2.

I use an Orion XT10 Dobsonion reflector telescope. 

pox's picture
Hi, The problem here is that

Hi, The problem here is that not only are colours subjective things visually, but observers' eyes are different too, dependent on several factors such as age, genetics and so on. I would agree with Sebastian that stars like this are simply not suitable for visual observation (and I am 100% a visual observer)'s picture
IY Com color

I would also add that my wife happened to be outside at the time and I asked her what she thought without telling her what I thought and she saw the same thing. This of course does not change that visual observation is not exact. 

IY Comae Berenices Spectral Class

Thank you again to you all who are discussing IY Com - especially J Tully and pox. 

However I am now fully accepting that IY Com is an M class star as I stated above in #12.But I would still like to invite a spectroscopist to give a final judgement. That would clear up the problem altogether. If the experts cannot agree as to what the spectral class of IY Com is, how can anyone of us mere mortals possibly agree with one another. 

There ought to be one truth! 

How can one single star have spectral classes of K, G, M or C? It just doesn't make sense! Which spectral class is correct?

Do spectroscopists exist here on

Perhaps one could step up to the mark soon. Can it be all that complicated?

Best regards from Aubrey. (Gath)

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