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)