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Spectroscopic Atlas for Amateur Astronomers 111
eral Theory of Relativity, becomes relevant even for amateurs [30]. The required work for
the light (photons) to escape the gravitational field of a White Dwarf, causes at Hα a red
shift of slightly <1Å, corresponding to a radial velocity of about 20–40 km/s. This star-
specifically different value must therefore inevitably be accounted for measurements of the
Doppler shift. The log g values of white dwarfs are in the range of approximately 7–9.
24.6 Classification System by McCook & Sion
The first letter of the classification is D, which means "Degenerate". The second letter indi-
cates the primary spectroscopic characteristics [2], [261]. Besides the type DA the physical
background of most other subclasses is not yet fully understood.
DA – Only Balmer lines; no He I or metals
DB – He I lines; no H or metals present
DC – Continuous spectrum, no lines deeper than 5% in any part of the spectrum
DO – He II strong; He I or H present
DZ – Metal lines only; no H- or He lines
DQ – Carbon features, either atomic or molecular in any part of the spectrum
Additional letters and symbols may specify further effects and features:
P – Magnetic white dwarfs with detectable polarisation
H – Magnetic white dwarfs without detectable polarisation
X – Peculiar or unclassifiable spectrum
E – Emission lines are present
? – Uncertain assigned classification
V – Optional symbol to denote variability
d – Circumstellar dust
C I, C II, O I, O II – atomic species in spectra of hot DQ-Dwarfs
The classification digit, following this letter combination, specifies the effective tempera-
ture ܶ according to the formula:
ܶ = 50ᇱ400/ݏݏ݈ܽܥ. ݀݅݃݅ݐ
For rarely occurring temperatures above 50'400K the classification digit becomes less than
1.0 and is expressed for example as .9 .8 .7 etc.
Finally sometimes also the log g value of the gravitational acceleration is added. So the
classification eg DA 2.5_7.8 means a spectrum with hydrogen lines, ܶ of about 20'000K
and a gravitational acceleration at the surface of log ݃ = 7.8.
24.7 Commented Spectra
Table 65: shows a montage with broadband spectral profiles (200L/mm) of three differ-
ently classified White Dwarfs, all normalised on the same continuum section. Depending on
the source consulted, here [260], the indicated spectral class may vary considerably.
WD 0644 +375: GJ 246 Spectral class DA 2.3 Distance 50 ly [262]
J2000 RA: 06h 47' 38" Dec: +37° 30' 57" mV = 12.1m
Besides the highly broadened hydrogen lines no further spectral features are visible here.
The equivalent width of the Hβ line is here with 65Å about 2.4x as large as by the already
impressive absorptions of the main sequence star A1Vm Sirius (27 Å). The classification
digit (2.3) suggests here an effective temperature of about 22'000K. The spectrum was re-
corded with the 50μm slit and the 200L grating. Exposure time: C8/DADOS/Atik 314L+:
1x1800 sec, 2x2 Binning Mode, –20°C.
