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THE LIFE CYCLES OF STARS
In a group, create a theory that explains:
(a) The origin of stars
• Where do they come from?
(b) The death of stars
• Why do stars die?
• What happens to a star when it dies?
WHAT IS A STAR?
• Stars are massive, glowing balls of hot gases, mostly hydrogen and helium.
• Some stars are relatively close, and others are far away, so far that by the time the light reaches Earth, the star may already be dead!
• Stars come in all sizes, brightness, temperatures, and colour.
HOW ARE STARS FORMED
• The Solar Nebula Theory is the best proven theory of how stars (and planets) are formed.
Nebula: a vast cloud of dust and gas.
THE BIRTH OF STARS• Stars start from nebulas• A Nebula provides the gas and dust from which a star
will form.
• Stars begin with slow accumulation of gas and dust.
• Gravitational attraction of clumps attracts more material.
• Contraction causes temperature and pressure to slowly increase.
COLLAPSE TO PROTOSTAR
NUCLEAR FUSION!
• When the temperature in the centre of the protostar reaches 15 million degrees Celsius, fusion ignites!
4(1H) 4He
• Where does the energy come from?
Mass of four 1H > Mass of one 4He
• E = mc2
A BALANCING ACT• The increased pressure and energy production stops the
gravitational collapse of the star. The star becomes stable and begins to shine brightly.
• On a Hertzsprung-Russell diagram, the star is now located on the main sequence.
HERTZSPRUNG-RUSSELL DIAGRAM
MAIN SEQUENCE• This is the stage that stars live the majority of their
lives in. • Once nuclear fusion is achieved, stars begin to radiate
and the star shines.
Our Sun is a main sequence
star.
• The stars that are born in the nebula are either:1. Low mass stars (red dwarfs)
2. Main sequence stars with an intermediate mass
(red giant) ex. our sun!
3. High mass stars (Supergiant)
THE DEATH OF STARS:A.) THE BEGINNING OF THE END: RED GIANTS
• After hydrogen is exhausted in the core…
• The core collapses• Kinetic energy of collapse converted into heat.• This expands the outer layers.
• Meanwhile, as core collapses,• Increasing temperature and pressure…
MORE FUSION!• At 100 million degrees Celsius, Helium fuses:
3 (4He) 12 C + energy
• Energy sustains the expanded outer layers of the red giant.
THE END OF RED GIANTS…
• After Helium is exhausted, outer layers of star expelled and form a planetary nebula.
WHITE DWARFS• At the center of a Planetary Nebula lies a White Dwarf.
• Inward force of gravity balanced by the repulsive force of electrons.
The white dwarf, called SDSS 1228+1040, sits about 463 light-years from our solar system.
B.) FATE OF HIGH MASS STARS (SUPERGIANTS)
• After Helium exhausted, core collapses again until it becomes hot enough to fuse Carbon into Magnesium or Oxygen.
12C + 12C --- 24Mg
OR 12C + 4H 16O
• Through a combination of processes, successively heavier elements are formed and burned.
C.) THE END OF THE LINE FOR MASSIVE STARS
• Massive stars burn a succession of elements.
• Iron is the most stable element and cannot be fused further.
• Instead of releasing energy, it uses energy.
• When a massive star’s fuel is exhausted, the core collapses, releasing a huge amount of energy. This causes a massive blast called a supernova.
WHAT’S LEFT AFTER THE SUPERNOVA?
• Neutron Star (if mass of core < 5x Sun)
• Under collapse, protons and electrons combine to form neutrons.
• 10 km across
• Black Hole (If mass of core > 5 x Sun)