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Spectroscopic Atlas for Amateur Astronomers 110
24 Post AGB Stars and White Dwarfs
24.1 Position of Post AGB Stars in the Stellar Evolution
This section explains the final stage for stars with less than about 8 solar masses. This is
too light for a final SN explosion of type II. Only as a member of a binary system a White
Dwarf can become a SN type Ia by accretion of matter and finally exceeding its critical
Chandrasekhar mass limit (sect. 25).
24.2 Post AGB Stars
After the final AGB-stage as a carbon star it begins to repel its envelope as a planetary neb-
ula. This stellar stage is called "post-AGB" and includes also the central stars of planetary
nebulae. Such a very early Post-AGB-Object (HD44179) is presented in sect. 28 as an excit-
ing source of the protoplanetary nebula Red Rectangle (Table 85).
24.3 Spectral Features at Post AGB Stars
During the repulsion of its shell the star, now increasingly becoming hotter, performs an
impressive loop in the upper part of the HRD and passes on this "farewell tour" almost all
spectral classes (see chart sect. 20). In extreme cases, its temperature may reach far be-
yond 100'000K and the dying star can generate for a very short time even a Wolf Rayet-like
spectrum WRPN (sect. 28.3).
24.4 White Dwarfs
By pushing off of a planetary nebula, the star loses mass, the thermonuclear fusion proc-
esses inside the star extinguish and the remaining rest is finally reduced to an earth-sized,
extremely dense object, with an enormously strong gravitational acceleration at its surface.
Most of the White Dwarfs are composed of a "degenerate" carbon-oxygen core, the prod-
ucts of the previous helium fusion, and a thin shell of hydrogen and helium. The spectral
class sinks in the HRD to its lowest area of the White Dwarfs. The absorption spectrum is
produced here just by the remaining residual heat of the very slowly cooling stellar corpse.
Their absolute luminosity is now so low, that for amateurs only a few objects in the imme-
diate solar neighbourhood are reachable, ie within a radius of about 50 light years. A corre-
sponding list can be found in [262]. The nearest and brightest With Dwarfs are the com-
panion stars Sirius B and Procyon B. Anyway, due to their close orbits around the A-
components they are spectroscopically inaccessible for amateurs. Easiest to observe, visu-
ally and somewhat limited also spectroscopically, is 40 Eridani B (mV = 9.5) as a component
of a triple system, identified as a white dwarf not until 1910. Visually, this object was de-
tected by William Herschel already 1783. The brightness of the remaining White Dwarfs
lies already within the range of magnitudes 12m – 13m.
24.5 Spectral Characteristics and Special Features of White Dwarfs
The exorbitantly high gravitational acceleration at the surface causes, especially due to the
Stark effect of the interatomic electric fields (Kuiper 1939), extremely broadened absorp-
tion lines. This effect forms here the spectral "brand" and affects mainly the hydrogen
Balmer series. In addition, also lines of helium and calcium may appear. Due to the low
brightness the display of the finer absorption lines remains here reserved to high-resolution
spectrographs at large professional telescopes.
In astrophysics, the gravitational acceleration g is expressed as a logarithm to the base 10,
however strangely not in ሾ݉/ݏଶ], but in [ܿ݉/ݏଶ]. For Sirius B this value is log g = 8.57 , cor-
responding to some 371ᇱ000ᇱ000 ܿ݉/ݏଶ or 3′710′000 ݉/ݏଶ. Compared to just 9.81 ݉/ݏଶ on
earth, this is so extremely high, that the gravitational redshift, predicted by Einstein's Gen-
