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Determination of stellar temp and spectral class from (B-V) photometry

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Determination of stellar temp and spectral class from (B-V) photometry

Dear AAVSO colleagues;

I recently performed CCD photometry on KP 301510 with B and V filters.  Reviewing the stars VSX information, KP 301510 is a DSCT (Delta Scuti) type star with stellar spectral class A0-F5 III-V.  Employing the transformed B and V photometry data and data from VStar, (B-V) results in 0.67.  Converting the B-V (0.67) to temp using the graph on page 1076 provided by Sekiguchi and Fukugita: A study of the B-V Color Temp Relation, AJ, 120: 1072-1084, 2000, the resulting temp is 5700 K, a G type star.  Also, referring to Johnson and Morgan’s Astrophysical Journal, 117(3), 313-352, 1953, page 346 shows that a B-V of 0.67 is in the G star range.

Is there an offset I also need to calculate to obtain the correct temp and stellar class?

I would greatly appreciate assistance if you can provide publications and or textbooks on the method to convert B-V to temp and spectral class.

Thank you;


Joe Kalen

Color excess

Hi Joe,

One of the shortcomings of inferring temperature and spectral class from the B-V index is that you need a good estimate of the color excess -- denoted as E(B-V). Interstellar extinction can have a huge effect on the observed B-V, causing a star to appear redder and cooler than it actually is. The color excess is the difference between the observed B-V and the B-V in the absence of interstellar reddening. For example, if the star's intrinsic B-V is 0.2 and you measure B-V = 0.5, then E(B-V) = 0.3. So the unexpectedly red color of KP 301510 might be caused (at least in part) by interstellar reddening.


PVEA's picture
Stellar temp and spectral class from (B-V)

According Schlafly & Finkbeiner 2011 (ApJ 737, 103) the mean value of redeening for KP 301510 is:
E(B-V)S and F =  0.0322 +/-  0.0007 (mag)

According Schlegel et al. 1998 (ApJ 500, 525) the mean value of redeening for KP 301510 is:
E(B-V)SFD =  0.0374 +/-  0.0009 (mag)

So after appliing dereddening for your estimated  B-V=0.67 the collor should be convert to:

(B-V)S and F = 0.67 - 0.0322 = 0.6378

(B-V)SFD = 0.67 - 0.0374 = 0.6326

so the temperature of that star should be:

Teff S and F = 5 730 K

Teff SFD =  5 740 K

or spectral type: G4

Not a big difference with your calculations.



P.S. by the way in VSX database I could not find a spectral type of that star

PVEA's picture
2MASS (J-K) color

P.S. Acc. 2MASS (J-K) color = 0.385 so acc. Tokunaga A.T.: 2000, Allen's astrophysical quantities, ed. by Arthur N. Cox, p. 143, Springer, New York, the temperature of KP 301510 is: clas is

T = 5 760 K

or spectral class is the same as in previous post: G4



From APASS color index B-V = 0.601 I would estimate Teff app 5 910 K (without correcting for metallicity; M.Saxner, G.Hammarbäck, Astron. Astrophys. 151, 372-390, 1985) and from 2MASS J-H color index 0,311 I wolud estimate 5 830 K (A. Collier Cameron et al., Mon. Not. R. Astron. Soc. 380, 1230-1244, 2007; see Appendix B). This puts KP 301510 in a temperature regime recently drawn attention to by Qian S.-B . et. al. Note figure 3 and figure 4 in for further details.


Not a DSCT

The conclusion to draw from all this is that KP 301510 is likely not a DSCT, but probably a W UMa type binary (with twice the period given in VSX).  A detailed light curve (in B and V) will be able to prove this: the phase plot might show different minima and/or maxima and a similar amplitude in B and V (a DSCT would show a larger amplitude in B than in V).


Thank you

Thanks to everyone that replied.  It was all very helpful.


Sebastian Otero
Sebastian Otero's picture
Interesting project

I was the one who revised this star some time ago. I must have missed the color facts... But I see I had some doubts because I added an uncertainty mark. In fact the amplitude as a little large for a normal DSCT.
Even when the period would be okay for an EW, the light curve looks weird, too shallow minima.
Multicolor data are welcomed!


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