The red giant phase is particularly important for understanding the origin of elements needed for planets and living organisms. When hydrogen is depleted in the core of the main sequence star (that is, when the core is entirely helium), nuclear burning continues in a shell around the core. When the core reaches ~120 million degrees Kelvin, then helium is not longer stable and can react with itself to form heavier elements.
First, the helium flash initiates the reaction
Stars like the Sun will then burn hydrogen to helium in a shell around the
core where helium is burning to carbon. The star becomes very luminous but the
outer layers swell to huge radii and cool. The star thus moves upward and to
the right in the Hertzsprung-Russell diagram. The yellow curve in the H-R
diagram below is labeled “1 for one solar mass
(1 M
Hertzsprung-Russell diagram showing evolution of stars of different masses after the main sequence. Credit: Lithopsian
Cores of more massive stars (5-100 M
and similar reactions give
These rare high-mass stars become supergiants, the most luminous stars in the
Galaxy, during their brief red giant phases. Several nuclear reactions can
occur simultaneously in the core and shells around the core (like layers of an
onion; see diagram below). Each nuclear reaction releases more energy, so the
stars becomes more and more luminous. These are the massive supergiants with
luminosities L ~
Artist’s illustration of the core of a massive star just prior to a type II supernova explosion. The core is a series of nested spherical shells, with each shell fusing a different element from hydrogen to helium, to carbon, through the periodic table to iron.Source: Penn State Astronomy & Astrophysics.
Iron is the last element that can be made in this fashion because it is the most stable nucleus. That is, any nuclear reaction involving iron requires more energy than it emits. Iron is thus the end-product of normal stellar nucleosynthesis.
The synthesis of nuclei in red giants/supergiants explains the relative abundances of different elements in the Universe. The plot below shows that, after hydrogen & helium, the most abundant elements in the Sun are the even-numbered elements between 6 and 26 protons: C, O, Ne, Mg, … Fe. The odd-numbered elements are rarer. Note also the rarity of elements heavier than iron; these heavy elements (like lead, gold and uranium) are formed only in the rare supernova explosions of the most massive supergiants. Similar abundances of the elements are seen in other stars, the Galaxy’s interstellar medium, Jupiter and elsewhere. This universal pattern is powerful evidence for the theory of red giant stars, and implies that red giants are the main source of elements with 6-26 protons in the Universe.
Abundances of the chemical elements in the Solar System. Credit: MHz`as.
How do these atoms formed inside red giant stars get out into the Galaxy? Convection (like boiling) in red giants brings these heavier elements produced in the interior to the surface. There they condense into tiny solid particles of rock (typically Mg-Si-O mixtures) and carbon which are pushed outward into space by the starlight. These explain the interstellar dust particles, which scatter and redden light in the Milky Way. Later, the gas and dust ejected by red giant stars enter cold molecular clouds, where it undergoes gravitational collapse and forms new stars and planets.
Thus, the Earth, you and I are composed of C/N/O/Si/Fe/… atoms that were created in red giant stars before our solar system was born. The atoms were formed by nuclear reactions in the giant/supergiant phase, ejected into the interstellar medium in both gaseous and solid form, and later incorporated into the solar system, the biosphere and ourselves. The link between living organisms and red giant stars led Carl Sagan, the famous astronomer of the 1980-90s, to declare: