| As clouds of gas and dust come
together to form a star, there is an energy change similar to the
energy change as a ball falls to the ground. The temperature in
the clouds of dust and gas becomes so high that nuclear fusion
occurs. In one such reaction, two isotopes of hydrogen 12H
and 31H
fuse together.
(a) Explain why the gas and dust come together. [1] (b) State the energy change that causes the rise in temperature as the gas and dust come together. [2] (c) Explain why the high temperatures are needed for nuclear fusion to occur. [2] (d) Describe the structure of a nucleus of 31H. [2] (e) State which element is created by the fusion of two hydrogen nuclei. [1] (f) State one effect of nuclear fusion on a star. [1] |
Now, you mark your answer:
(a) The gas and dust come together due to gravitational attraction. [1]
(b) As the gas and dust come together, gravitational potential energy of the particles changes to kinetic energy. [1] The increase in kinetic energy of the particles is the cause for the rise in temperature of the gas and dust. [1]
(c) The positively charged nuclei repel one another when they are close together. [1] High temperature is needed for the nuclei to have high speeds to overcome the electrostatic repulsion, come closer to one another and collide for fusion to occur. [1]
(d) The nucleus of 31H contains 1 proton [1] and 2 neutrons [1].
(e) Nuclear fusion in the star makes it emit large amounts of heat and radiation. [1]
After having tried the question which focuses on the birth of the Sun, you may be interested to read a little more about the Sun's life cycle. This is for your interest because it's not in your syllabus.
Life cycle of the
Sun:
Nebula --> main
sequence star --> red giant --> planetary nebula --> white
dwarf --> black dwarf
Nebula: The gas and dust mentioned in the above question has a name called the nebula. The gas is mainly hydrogen. All stars are formed inside nabulae. The gas and dust in the nabula collapses due to gravitational attraction. This forms a rotating gas globule called a protostar.
Main sequence star: The very high temperature in the core of the star is hot enough for nuclear fusion to occur. The protostar then becomes a main sequence star which is the longest and most stable stage. Our Sun is in this stage. (For a primary science article on why the Sun doesn't need oxygen to "burn", click here.)
Nuclear fusion which powers the main sequence star involves two stages:
Stage 1: proton (H nucleus) --> neutron + positron + neutrino
Stage 2: 2 protons + 2 neutrons --> He nucleus + energy (heat, light, radiation)
Where does the energy come from? The sum of the masses of the 2 protons + 2 neutrons is slightly lower than the mass of the helium (He) nucleus. This small mass deficit is converted to energy given by E = mc2.
If you're wondering why the star doesn't continue to collapse due to gravity, it shows you're thinking. The nuclear fusion in the core creates an outward pressure which balances the inward pressure that's due to gravity.
The Sun hasn't stopped “burning” because there's a huge number of H nuclei which haven't been converted to He nuclei. It has been shinning for about 5 billion years and will continue to shine for another 5 billion years.
Red giant: What happens if the hydrogen in the Sun's core is used up? Fusion in the core stops, and there's no outward pressure to balance the inward pressure that's due to gravity. The outer gas collapses to the core due to gravity. The core contracts and heats up. Since the core becomes much hotter, more fusion can occur to form more elements such as carbon, oxygen, nitrogen up to iron. The outer gas then expands many times to become a red giant. The expansion takes in energy and so the surface cools and reddens.
Planetary nebula, white dwarf:The core of the red giant will become the white dwarf while the outer hydrogen gas becomes the planetary nebula. No nuclear fusion occurs and the white dwarf radiates its heat into outer space.
Black dwarf: The white dwarf cools and fades away to a cold black dwarf (dead star).
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