Transcript The Life Cycle of a Star

The Life Cycle of a Star
Upper Fifth Physics
In The Beginning…
• Lots of gas and dust
• All this stuff gravitationally attracted
• Click picture for full news story
War between gravity and pressure
• If the mass of gas is large enough, then
the gravitational attraction of the
particles will overcome the pressure and
the cloud will contract
Ignition!
• As the cloud contracts, GPE is converted
into KE.
• Temperature of cloud is related to the
average KE of the molecules.
• Increase in KE ‘means’ an increase in
temperature.
• If temperature exceeds 15 million K,
hydrogen fuses into helium and we have
ignition!
Just keep glowing…
• Star continues to glow as long as
there is sufficient hydrogen.
• Battle between forces due to gravity
and pressure continues but is stable.
• Each second, the Sun converts 1
million kg of hydrogen in helium
Running low on gas
• Helium production continues until
hydrogen runs out.
• Helium fuses into other nuclei, but
the process isn’t as efficient.
• What happens next depends on the
original mass of the star (click on a
button)
Great Links
Little Star
Big Star
Little Star
• Hydrogen cloud around hot central
core drifts off
• As last of hydrogen drifts off, big
wind comes from surface of core
• Wind distorts hydrogen cloud.
• We have a ‘planetary nebula’
surrounding a white dwarf
• White dwarf cools and fades away
Return to ‘Running Low on Gas’
Big Star
• Star burns helium to other elements, which
in turn can be fuses into heavier elements
still.
Large pop!
• Iron doesn’t burn so eventually the fusion
stops!
• There is nothing to stop the gravitational
pull of the nuclei on each other.
• Iron core collapses in a second.
• Resulting explosion is a supernova, visible
between galaxies.
• During this explosion, heavier elements are
made (all elements in universe heavier than
iron originally came from supernovae –
including those inside you!
What’s left?
• If the star’s core was ‘small’ then a
small spinning lump of neutrons called
a neutron star is left.
• If the star’s core was ‘large’ then a
black hole is left.
Back to ‘Running out of Gas’
Great Links
• NASA – for a more in depth
description
• More pretty pictures of
– Stellar births
– Stellar deaths
• The science of black holes (odd stuff
happens there)