fact sheet
Life cycle of stars
Star clusters are groups of stars, formed from the same giant gas cloud, that are
held together by gravity. Because of their special properties, star clusters provide
astronomers with a unique environment to study the age and evolution of stars.
Star clusters are important to astronomers because their
stars formed at about the same time, so they are roughly the
same age; the same distance away from us; and are made of
the same materials. Despite this, stars within clusters vary in
brightness and colour.
Brightness is related to a star’s mass. Stars with greater mass
tend to be brighter than stars with lesser mass.
Star colours vary because more massive stars ‘burn up’
quicker. They go through different stages of their life cycle
sooner and emit different coloured light because of the
nuclear fuels they burn and their surface temperatures.
There are two types of star cluster:
Open clusters contain stars that can be observed
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individually through a telescope. These star clusters Massive globular cluster NGC 104
eventually break up due to gravitational interactions with (47 Tucanae) contains millions of stars.
other objects in the galaxy. Open clusters, such as the credit: SPIRIT image by Paul Luckas
Jewel Box, typically contain hundreds of younger stars.
Globular clusters may contain up to a million stars in a spherical array. They typically contain
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some of the oldest stars in a galaxy. Globular clusters contain little free gas or dust, so no new
stars are being formed in them.
Star colours STAR SURFACE
STAR COLOUR
TEMPERATURE (K)
Most stars appear white to the naked
eye. But some, such as Betelgeuse (a <3700 red
red supergiant star), are orange or 3700 – 5200 orange
reddish in appearance while others, 5200 – 6000 yellow
such as Rigel (a blue supergiant star),
are bluish-white. A star’s surface 6000 – 7500 yellow-white
temperature is the major factor in 7500 – 10 000 white
determining the colour of light emitted. 10 000 – 30 000 blue-white
>30 000 blue
surface temperature and colour of stars
ast0972 | Evolution of the Universe 4: Life cycle of stars (fact sheet) developed for the Department of Education WA
© The University of Western Australia 2014 for conditions of use see spice.wa.edu.au/usage
version 1.0 page 1 Licensed for NEALS
Why do stars change throughout their life cycles?
Stars change throughout their lives because of the fuels they ‘burn’ in their cores. When their core temperature
is high enough hydrogen atoms fuse together in nuclear reactions to make helium and release huge amounts of
energy. Our Sun has been doing this for about 5 billion years and is currently about halfway through this stage of
its life cycle that astronomers call ‘main sequence’.
When a star runs out of hydrogen it starts ‘burning’ helium to make heavier elements. But before this can
happen, gravity causes its core to collapse. As it does, outer layers of the star expand and cool and it grows to
form a red giant. Red giants can grow to hundreds of times a star’s original size.
The most massive stars in the Universe fuse heavier atoms together, releasing so much energy that the stars
explode in supernovae, blasting their outer layers into space. Matter and energy released in supernovae creates
all elements in the Universe heavier than iron.
The length of a star’s life depends on its mass. High-mass stars have more fuel, but they burn it faster so they
have short lives. Low-mass stars have less fuel, but use it at a slower rate, so they live longer lives.
THE END OF STARS
low-mass stars Their cores shrink to form white dwarf stars that are about the same size as
< 4 solar masses Earth. Their outer layers form gas clouds called planetary nebulae that spread
out in space and eventually fade from view.
medium-mass stars These explode in massive supernovae. Their cores shrink to form neutron
4 – 8 solar masses stars that may be only 10–20 km in diameter. Gravity is so strong in cores of
neutron stars that protons and electrons are forced together to form neutrons.
high-mass stars These release energy so rapidly that they explode in supernovae. Energy
> 8 solar masses released is sufficient to create all elements heavier than iron. If enough matter
is left in the star remnant, gravity may cause the core to collapse into a black
hole. Not even light can escape the intense gravity of black hole.
Dramatic end of a star
These two images show M 82 (the Cigar
Galaxy) on 10 December 2013 (upper image)
and 21 January 2014 (lower image). The
more recent image shows what appears to
be a new star.
This is a supernova, now named SN 2014J,
which is formed by a collapsing star.
Supernovae can be billions of times brighter
than the Sun.
SN 2014J is a type 1A supernova, believed to
be formed by the collapse of a double star
system.
credit: University College of London (Dr Steve
Fossey, University of London Observatory)
For more information on SN 2014J, discovered by
students during an astronomy class, see
http://www.ucl.ac.uk/maps-faculty/maps-news-
publication/maps1405
ast0972 | Evolution of the Universe 4: Life cycle of stars (fact sheet) developed for the Department of Education WA
© The University of Western Australia 2014 for conditions of use see spice.wa.edu.au/usage
version 1.0 page 2
stellar nursery
Stars form in a nebula from
collapsing clouds of interstellar
gas and dust.
Sun-like stars massive stars
(up to 4 times the (more than 4 times
mass of the Sun) the mass of the Sun)
red
red giant supergiant
planetary
nebula
supernova
remnant less than remnant more than
white dwarf 3 solar masses 3 solar masses
black dwarf neutron star black hole
credit: planetary nebula – NASA, ESA and the Hubble Heritage; stellar nursery and supernova – SPIRIT image by Paul Luckas
ast0972 | Evolution of the Universe 4: Life cycle of stars (fact sheet) developed for the Department of Education WA
© The University of Western Australia 2014 for conditions of use see spice.wa.edu.au/usage
version 1.0 page 3
