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Physics for Secondary Schools
100 000 light- years
FOR ONLINE READING ONLY
(a) Size the of Milky Way Galaxy (b) Milky Way Galaxy as viewed in a dark clear night sky
Figure 6.3: Milky Way Galaxy
energy which is radiated away in the form
Exercise 6.1 of light.The twinkling of stars is due to the
light variation as it undergoes refraction
1. Explain the term astronomy and state when travelling through atmospheric layers
its importance. of different refractive indices.
2. Briefly explain the composition of
the universe. The Sun is the closest star to the Earth
3. (a) Mention and describe briefly at a mean distance of about 150 million
the galaxy to which our solar kilometres. This distance is known as the
system belongs. Astronomical Unit, abbreviated as AU.
(b) What is the shape of the galaxy It is used to measure distances across the
described in part (a)? solar system.
Note: 1 AU = 150 million kilometres.
Stars Star formation
A star is a large celestial body made up Stars are formed from a collection of
of hot luminous ionized gases known as interstellar nebula (gas and dust) that has
plasma, held together by gravity. Plasma collapsed down from a giant molecular
is an ionised gas that contains certain cloud (star forming region). A young
proportion of electrons that are free, not star that gathers nebula from its parent
bound to an atom or molecule. The main molecular cloud before it collapses to
gases composing a star are hydrogen and form a new star is called a protostar
helium. Stars shine as they burn hydrogen (Figure 6.4). There are many stars in the
into helium in their cores, through a process universe, but of our particular interest is
of nuclear fusion. The nuclear fusion the Sun which gives energy to our solar
system.
process generates enormous amount of
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