Formation
of the Heavier and Heaviest Elements
(derived from Science
Learning, http://sciencelearn.org.nz)
Our
world is made of elements and combinations of elements called compounds. An
element is a pure substance made of atoms that are all of the same type. At
present, 118 elements are known, and only about 90 of these occur naturally.
Elements and the ‘Big Bang’ theory
We
already know, through our previous lesson, that during the formation of the
universe some 14 billion years ago in the so-called ‘Big Bang’, only the
lightest elements were formed – hydrogen and helium along with trace amounts of
lithium and beryllium. As the cloud of cosmic dust and gases from the Big Bang
cooled, stars formed, and these then grouped together to form galaxies.
The
other 86 elements found in nature were created in nuclear reactions in these
stars and in huge stellar explosions known as supernovae.
Elements and our Sun
For
most of their lives, stars fuse elemental hydrogen into helium in their cores.
Two atoms of hydrogen are combined in a series of steps to create helium-4.
These reactions called nuclear fusion reactions account for 85% of the Sun’s
energy. The remaining 15% comes from reactions that produce the elements
beryllium and lithium.
The
energy from these nuclear reactions is emitted in various forms of radiation
such as ultraviolet light, X-rays, visible light, infrared rays, microwaves and
radio waves. In addition, energized particles such as neutrinos and protons are
released, and it is these that make up the solar wind.
Earth
is in the path of this energy stream, which warms the planet, drives weather
and provides energy for life. The Earth’s atmosphere is able to screen out most
of the harmful radiation, and the Earth’s magnetic field can deflect the
harmful effects of the solar wind.
Dying stars
The
eventual end result of a star's life is determined by the mass of the star. The
larger the mass of the star, the more luminous they are. The more luminous they
are, the more reactions are taking place in their cores. And the more reactions
are taking place in its core, the faster it consumes its fuel, which is
Hydrogen.
When
a star’s core runs out of hydrogen, the star begins to die out. The dying star
expands into a red giant, and this now begins to manufacture carbon and
nitrogen atoms through a series of nuclear reactions using helium, beryllium
and lithium.
More
massive stars turn into Supergiants and are capable of taking the nuclear
reactions further to produce elements that range from oxygen through to iron.
Eventually,
stars will run out of fuel elements and explode. During a supernova, a star
releases very large amounts of energy as well as neutrons, which allows
elements heavier than iron, such as uranium and gold, to be produced. In a
supernova explosion, all of these elements are expelled out into space.
What is the Big Bang theory?
Our
world is literally made up of elements formed deep within the cores of stars
now long dead. As Britain’s Astronomer Royal Sir Martin Rees said, “We are
literally the ashes of long dead stars.” When you buy a party balloon that
floats in air, it is filled with helium gas – most of which was created when
the universe was only 3 minutes old! Examples of element-making (nucleogenesis)
in helium burning reactions:
• 3 helium atoms fusing to
give a carbon atom: 3 @ 4He → 12C
• carbon atom + helium
atom fusing to give an oxygen atom: 12C + 4He → 16O
• oxygen atom + helium
atom fusing to give a neon atom: 16O + 4He → 20Ne
• neon atom + helium atom
fusing to give a magnesium atom: 20Ne + 4He → 24Mg
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