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Analysis and Interpretation of Astronomical Spectra 81

18 The Measurement of the Stellar Photosphere Temperature

18.1 Introduction

Depending on the spectral class, stellar spectra and their measurable features reflect to a
different extent also the physical state of the photosphere. For the spectroscopic determi-
nation of the effective temperature (sect. 3.2), numerous methods exist with different

degrees of accuracy but also corresponding complexity.

18.2 Temperature Estimation of the Spectral Class

The spectral class reflects directly the sequence of the corresponding photospheric tem-
peratures (sect. 14.2). It is therefore the most direct, simple, but relatively imprecise way to
estimate the effective temperature . For main sequence stars of intermediate and late
spectral classes, the accuracy, realistically achievable by amateurs, should lie within some
100 K. In literature numerous tables can be found, assigning the effective temperatures to
the individual spectral classes. In addition, for the luminosity class III and V, separate and
significantly different values are presented. In the region of early spectral classes, even be-
tween renowned sources, differences may arise up to >1000 K. Thus, especially for the
early types of the spectral type O, for the same star often significantly different classifica-
tions are published. This shows that this method, at least for amateurs, is limited to the
main-sequence stars, since the determination of the luminosity class is quite difficult. As an
example, follows here a table with data, taken from a lecture at the University of Northern
Iowa: http://www.uni.edu/. At the early spectral classes, some of these values significantly
differ from those, shown in the table in sect. 14.5, or in [33].

Spectral Main Sequence Giants (III) Super Giants (I)
Type (K)
(V) (K) (K)

O5 54,000 21,000
O6 45,000 16,000
O7 43,300 14,000
O8 40,600 12,800
O9 37,800 11,500
B0 29,200 11,000
B1 23,000 10,500
B2 21,000 10,000
B3 17,600 9,700
B5 15,200 9,400
B6 14,300 9,100
B7 13,500 8,900
B8 12,300
B9 11,400 8,300
A0 9,600
A1 9,330 7,500
A2 9,040 7,200
A3 8,750
A4 8,480
A5 8,310
A7 7,920
F0 7,350
F2 7,050