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STARS
& their life cycles
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!
STAR EVOLUTION
LOW MASS = Sun
HIGH MASS = Several times the mass of the Sun
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!
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.
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.
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.
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.
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!!!
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!
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!
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.
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.
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’……!?!
SUMMARY
THE END
PHYSAROONIE!!!
A Mr H. production