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Analysis and Interpretation of Astronomical Spectra 59

Luminosity compared to the Sun14.3 The Evolution of the Sun in the HRD

The following simplified short description is based on [1]. It demonstrates how the spectral
class allows the determination of the stellar state of development. In the diagram below,
the evolutionary path of the sun is shown. By the contraction of a gas and dust cloud, at
first a protostar is formed, which subsequently moves within some million years on the
Main Sequence. Here, it stabilises at first the luminosity by about 70% of the today's value.
Within the next 9-10 billion years the luminosity increases to over 180%, whereby the
spectral class G2, eg the photospheric, or effective temperature, remains more of less con-
stant [1]. During this period as a Dwarf- or Main Sequence star, hydrogen is fused into he-
lium.

Towards the end of this stage the hydrogen is burning in a growing shell around the helium
core. The sun becomes now unstable and expands to a Red Giant of the M-class (luminosity
class ca. II). It moves now on the Red Giant Branch (RGB) to the top right of the HRD, where
after 12 billion years it comes to the ignition of the helium nucleus, the so called Helium
Flash. The photosphere of the Red Giant expands now almost to the Earth's orbit. By this
huge expansion the gravity acceleration within the stellar photosphere is reduced dramati-
cally and the star loses therefore at this stage about 30% of its mass [1].

Subsequently the giant moves with merging helium core on the Horizontal Branch (HB) to
the intermediate stage of a Yellow Giant of the K-class (lasting about 110M years) and then
via the reverse loop of the Asymptotic Giant Branch (AGB) to the upper left edge of the HRD
(details see [33]). The experts seem still be divided whether the sun is big enough to push
off at this stage the remaining shell as a visible Planetary Nebula [1], [2]. Assured however
is the final shrinkage process to an extremely dense White Dwarf of about Earth's size. Af-
ter further cooling, and a down move on the branch of the White Dwarfs, the sun will finally
get invisible as a Black Dwarf, and disappear from the HRD. During its live the Sun will pass
through a large part of the spectral classes, but with very different luminosities.

106

Ia
105 Planetary Nebula Iab

104 Ib

II AGB

103

102 III Giant Branch
Sun
101 today Proto Star

100

White Dwarf

10-1

10-2 O5 B0 B5 A0 A5 F0 F5 G0 G5 K0 K5 M0 M5

Spectral Type