"It must be a strange world
not being a scientist, going
through life not knowing - or
maybe not caring - about
where the air came from, and
where the stars at night came
from, or how far they are
from us. I want to know."
Ancient astronomers believed the
universe was made up of about
3000 stars, the moon and the sun
and these objects revolved around
the earth. We now know that the
earth is not the centre of the
universe and that the sun is only
one of millions and millions of
Claudius Ptolemy’s earth-centered model of the universe
Many stars that we can see form recognisable
patterns. The stars in a constellation keep the
same shape but may only appear on summer or
winter nights due to the Earth’s revolution.
The distances to the stars are enormous with
vast spaces between them. Our sun is the
closest star being 150 000 000 km away. The
next closest star is 41 000 000 000 000 km away
or 270 000 times the distance of the sun.
The distances to the stars are far too large to
measure in kilometres. Instead we use the
astronomical unit of distance called the light-
year. This is the distance light travels in one
year. Light travels at about 300 000 000 metres
per second (3×108 m/s). So in one year light
travels about 9 500 000 000 000 km.
• A parsec (pc) is another commonly used
astronomical unit of length – it is equivalent of
• The parsec is based on the phenomenon
known as parallax.
When you look at the stars you are actually
looking back in time. The closest star in the
Southern Cross is 220 light years away. This
means the light left this star 220 years ago (i.e.
as the star was in 1788).
Science World 10
The stars in the sky have been given names
according to their brightness. The brightest star
is called the alpha star (α-star), the second
brightest the beta star (β-star), then gamma (γ-
star), delta (δ-star) and so on.
In the Southern Cross, the brightest star is called
the α-Crucis. The Pointers are located nearby
and this constellation is known as Centaurus.
The brighter of the 2 stars is known as α-
Centauri (closest star to Earth apart from the
Sun) and the second star is known as β-Centauri.
Movement of Stars
The stars appear to move across the sky. This
movement is caused by the west to east rotation
of the earth.
Science World 10
Stars are made of …?
Stars are gaseous objects in space that give off
light and heat.
Our sun is composed of: 80% H
1% trace elements
The enormous size of a star creates huge
gravitational forces which squeeze the atoms of
the gases together and creates immense
pressure and heat. At this temperature,
electrons are stripped from the atoms, leaving
positively charged nuclei which are in constant
Plasma a state of matter, different from solids,
liquids and gases, which exist in the extreme
heat in the interior of stars (conducts electricity
and generates a magnetic field).
Nuclear Fusion is when H nuclei fuse together to
form He and release large amounts of energy
(occurs in the core of the sun).
In the fusion reaction in the sun, 655 million
tons of H is converted to 650 million tons of He
every second. The 5 million tons of H used up
releases 4.5 x 1026 J/s. This is the same amount
of energy that all power stations in Australia
could produce in 7 billion years.
Light From Stars
• Brightness and colour of a star are important
in determining its size and life expectancy.
• Astronomers refer to a stars brightness as its
• The colour of a star is due to its temperature.
Brightness and Colour of Stars
• The brightness of a star when viewed from
• As the brightness of a star decreases the
apparent magnitude becomes more positive.
i.e. Stars just visible with the naked eye have a
magnitude of about 6. Sirius, the brightest star
in the sky, has a magnitude of – 1.4.
Apparent magnitude is not a measure of the
star’s actual brightness because a very bright
star could be so far away from earth that it
• Measures the actual brightness of a star by
comparing the amount of light given off by
that star if it was a set distance from earth.
• The amount of light given off is determined by
its size (amount of matter).
• Hottest stars (e.g. Rigel) – white to blue light
• Coolest stars – red light
• Very hot stars (Rigel) give off large amounts of
UV (ultraviolet) light which our eyes cannot
detect. Appears as blue.
• Astronomers are able to calculate the surface
temperature of stars by measuring the
wavelength (λ) of light given off.
Relationship – Brightness and Colour
• If two stars are the same colour, they must have
the same surface temperature.
• If the absolute brightness is larger, then star is
• If a red and a blue star are the same size they will
have the same brightness, but the blue star will
radiate more energy/second because it is hotter.
• Two stars that revolve around each other.
• There have been nearly 700 000 binaries
observed by astronomers.
• Two famous binaries in the Southern
Hemisphere are Alpha-Centauri (brightest star
of the Pointers) and Alpha-Crucis (star at the
foot of the Cross).
Star Life Cycles
• Stars are born in clouds of gas (mainly hydrogen)
and dust that occur throughout the universe.
Occasionally one of these clouds collapses on
itself, becoming hotter and denser as the
gravitational force increases. This is the
embryonic stage in the life of a star – Protostar.
• Eventually the gas becomes hot enough to start
nuclear fusion reactions and the star begins to
Nebula – massive cloud of gas and dust.
Supernova – a spectacular explosion which ends
the life of a star.
Birth and Death of Stars
• The life cycle of a star depends entirely on its
• A protostar with a mass less than 0.1 of the
mass of the sun will continue to shrink but will
never get hot enough for nuclear reactions to
begin. It will fade to form a small red star
before turning cold and dying.
A star about the size of our sun initially glows
very brightly. It then settles down to a long
stable middle-life period of about 10 billion
years. As the star ages and its hydrogen is used
up, its surface temperature decreases. Finally,
when little hydrogen remains, the stars core
shrinks, the outer layers expand and cool and
the star forms a red giant.
Gases in the outer regions then drift into space
and the remaining gases collapse into a small,
very dense object known as a white dwarf.
These stars are very small. Eventually the white
dwarf cools down and fades away.
Stars 2-6 times the size of the sun have much
shorter but spectacular lives. These stars only live
for about 1 million years. The mass in large stars
create enormous gravitational forces in the core of
the star. The nuclear reactions use fuel very rapidly,
creating very bright stars (blue). When the fuel runs
out there is a tremendous outburst of energy –
Supernova. The star’s matter is blown into space,
leaving a nebula.
When this explosion occurs the brightness of the
star increases a billion times.
One of the most useful and powerful plots in
astrophysics is the Hertzsprung-Russell diagram
(H-R diagram). It originated in 1911 when the
Danish astronomer, Ejnar Hertzsprung, plotted
the absolute magnitude of stars against their
colour (hence effective temperature).
Nebulas and Neutron Stars
• A supernova occurs about once every 75 years in our
• Some nebulas emit their own light and glow like stars.
They can glow pink, blue green or yellow depending on
the type of gas in the cloud. E.g. Great Nebula of Orion.
• Other nebulas do not glow and black out the light from
the stars behind them. E.g. The Horsehead Nebula. This
is a dark nebula which can be seen against the glow of
stars in the background.
A Neutron star is an incredibly dense star less
than 20km in diameter formed during a
supernova explosion. The remainder of the
star’s core is pulled inwards by immense
gravitational forces. They are so small that they
do not glow very brightly but send out pulsating
radio signals. They can be called pulsars.
• Invisible objects in space that emit X-rays and
that are thought to form when the most massive
• After the supernova explosion the core collapses
on itself, and unlike a neutron star, these massive
stars (10 times size of sun) keep on collapsing.
• The gravitational force that is created is so strong
it will not even allow light to escape.
• The Milky Way is just one of billions of galaxies
in the observable universe.
• It is estimated to contain between 200 and
400 billion star.
Astronomer Edwin Hubble revolutionised the
field of astrophysics. His research helped prove
that the universe is expanding, and he created a
classification system for galaxies that has been
used for several decades. http://www.biography.com/people/edwin-hubble
Origin of the Universe
• Cosmology is the study of the origin and
structure of the universe.
• Two models describing the origin of the
The Steady State Model
• This model proposes that the universe has always
existed. The universe is infinitely old, it has no
birth date and will never end. The universe is
endless in time and space. This model accounts
for the fact that stars and galaxies die and re-
• This model avoids the difficult question of how
the universe was created. There are few
supporters of this model because recent
astronomical evidence does not fit it at all.
The Big Bang Model
This model suggests that the universe began as a massive
explosion; it is based on evidence that the universe is
About 10 billion years ago all matter in the universe was
contained in a hot, dense ball of radiation and sub-atomic
particles. An explosion took place and the matter
expanded. As it expanded it cooled and electrons,
protons and neutrons formed. On further expansion and
cooling, small gaseous atoms like hydrogen and helium
formed, then larger ones. Where the exploded matter
was denser, gravitational forces squeezed particles of
matter together to from galaxies containing stars.
Astronomers have found that the wavelength of
the light (measured with a spectrometer)
coming from most stars has shifted towards the
red end of the spectrum. This occurs when stars
are moving rapidly away from earth. This is
considered evidence that the universe is
The Future of the Universe
• What will happen to the expanding universe?
There are two theories.
The universe will go on expanding forever. The
universe will eventually die as the stars and
galaxies are reduced to clouds of gas and dust.
The ‘Big Crunch’ suggests that the universe will
expand to a certain point and then collapse back
on itself in a reversal of the Big Bang. This will
once again form a hot, dense ball of matter that
will start a second Big Bang and a new universe
will be born containing all the matter that was in
the previous universe.
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