STARS

1
STARS
their life cycles
2
Like us, stars are born, grow older, become
middle aged and eventually die!!!
Unlike us, stars take billions of years to
complete their life cycles, but we can observe
different types of stars piece the bits
together!
3
STAR EVOLUTION
LOW MASS Sun HIGH MASS Several times the mass
of the Sun
4
STAGE 1a -
BIRTH
Stars are formed when clouds of gas - usually
hydrogen - collapse under the effect of gravity.
As the cloud contracts, over millions of years,
it gradually heats up and becomes denser. This is
called a PROTOSTAR.
When it reaches a temperature of a few million
degrees it ignites - thermonuclear fusion
reactions begin, and the star begins to shine!
5
STAGE 1b -
BIRTH
As the new stars begin to shine, they blow away
any traces of remaining gas and dust that have
not formed planets.
This pictures shows the Rosette Nebula - new,
young stars have blown away an area of the gas
cloud, about 12 light years across, allowing them
to be seen clearly. Stars make the gas glow.
6
STAGE 2a -
GROWING UP
The star now begins to fuse atoms of hydrogen
into helium at an amazing rate, giving out lots
of energy.
As it cools, it changes colour from white to blue
to yellow. By this time, the star has become
middle aged - we call it a MAIN SEQUENCE star.
7
STAGE 2b -
GROWING UP
The star is in equilibrium between 2 forces -
gravity trying to collapse it heat trying to
make it expand due to pressure. Our Sun is a good
example of this.
Most stars staysbalanced like this for several
billion years.
8
STAGE 3a -
GETTING OLD
As the star runs out of fuel the outer layers of
gas expand up to 100 times and it cools, becoming
redder. It is now called a RED GIANT.
Eventually, the star produces an unstable shell
of heavier elements which it blows off into
space. This is called a PLANETARY NEBULA. Energy
from the star causes it to glow.
9
STAGE 3b -
GETTING OLD
The core shrinks heats up, becoming a very
dense WHITE DWARF star. 1cm3 of one of these may
have a mass of about 10,000kg!
Finally, it fades away, radiating all its
remaining energy to become a BROWN DWARF - a cold
mass of gas in space, cooling down. This will
eventually happen to our Sun!!!
10
HIGH MASS STARS - a
These are formed from large gas clouds, giving
them masses several times that of the Sun, and
several hundred times the diameter! They undergo
the same early stages, but do so much faster, as
they use fuel a lot quicker.
They also undergo a much more drastic death!
11
HIGH MASS STARS - b
Near the end of the RED SUPERGIANT phase, high
mass stars become very unstable, and produce many
envelopes of different elements. Eventually, the
star can take no more and it explodes violently
as a SUPERNOVA.
As before, the core contracts, trying to become
a WHITE DWARF, but things do not stop there!
12
HIGH MASS STARS - c
The white dwarf may continue to acquire mass from
a nearby companion star, becoming so dense that
electrons and protons get squeezed together,
leaving only neutrons. The results is a NEUTRON
STAR - an object only 10-30km across, with a
density of 300,000 million g/cm3!
Periodic bursts of radiation may be emitted from
around the neutron star, if it is spinning, due
to its magnetic fields. This is a PULSAR.
13
HIGH MASS STARS - d
The neutron star may continue to gain mass from
nearby stars. At a critical moment, it becomes so
dense it collapses in on itself, becoming a
single point of zero size! Its gravity is so
strong that even light cannot escape from inside
a certain boundary - the EVENT HORIZON. The star
is now a BLACK HOLE !
Although we will never actually SEE a black hole,
we can see their effects. A number of objects do
show BLACK HOLE properties.
14
COSMIC RECYCLING
Supernovae explosions fill the space nearby with
gas and new heavy elements - in fact, elements
more massive than iron in the periodic table can
only be made this way.
These nebulae may then lead to the formation of a
new generation of stars - this has happened to
our Sun.
Some atoms in our bodies where formed this way!
We are all made from stardust..or all the
leftover crud!?!
15
SUMMARY
16
THE END
PHYSAROONIE!!!
A Mr H. production