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)