Fusion in Stars
The Sun, an average star in the Milky Way Galaxy, is 4.3 light minutes
away from Earth.
The Alpha or Proxima Centauri is 4.3 light years from the Sun.
The Procyon is 8.6 and 11.4 light years from the Sun
The universe consists of millions of stars.
Fusion in the Sun
The mass of the Sun is 332,000 times that of the Earth.
The density at the centre of the Sun is 100 time that of water.
The temperature at the centre is at least 15 million K, and 6000 K at the surface.
The Sun is a ball of plasma, held together by gravity.
These conditions of high temperature and high density are ideal for
thermal nuclear fusion reaction. There are two detailed schemes
for the fusion reactions: the hydrogen cycle and the
The hydrogen cycle
H + H ® D + b+
(+ b-) + n
D + H ® 3He + g
3He + 3He ®
4He + 2 H
The overall reaction:
4 H ® 4He + 2 (b
+ (+ b-) + g + n)
The fact that carbon, nitrogen, and oxygen atoms are present in the Sun
and the high probability of the following reactions suggest another
scheme catalyzed by carbon for the fusion of hydrogen in the Sun. The
following scheme is called the carbon cycle
12C + H ® 13N + g
13N ® 13C + b+ (+ b-) + n
13C + H ® 14N + g
14N + H ® 15O + g
15O ® 15C + b+ (+ b-) + n
15N + H ® 12C + 4He + g
The overall reaction:
4 H ® 4He + 2 (b+ (+ b-) + g + n)
Fusion takes place mainly in the interior of the Sun, and energy released
becomes kinetic energy of particles. Energy is absorbed and re-radiated
by particles. Emission of energy from the surface of the Sun is
Lives of Stars
Depending on the masses, the lives of stars go through different stages.
The real massive ones explode into supernova after the white dwarf stage,
but the massive ones explode into a nova.
- Condensed mass - interstellar matter contraction under their own gravity
- Protostar - contraction causes temperature to rise to give a red glow
- Star - at T = 107 K, fusion starts and reaches a stable state
- Red Giant - hydrogen burned out, temperature drops, fusion of He starts
- White Dwarf - lowered surface temperature but expands outward to a planetary nebula
- Nova or Supernova - very massive star exploded after burned out
- Neutron Star or Black Hole- remaining core of supernova