The Life Cycles of Stars

1. The Life Cycles of Stars

2. In a group, create a theory that explains: • The origin of stars • Where do they come from? • The death of stars • Why do stars die? • What happens to a star when it dies?

3. 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.

4. 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.

5. The Birth of Stars • Stars start from nebulas • A Nebula provides the gas and dust from which a star will form.

6. Collapse to Protostar • Stars begin with slow accumulation of gas and dust. • Gravitational attraction of clumps attracts more material. • Contraction causes temperature and pressure to slowly increase.

7. 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

8. 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.

9. Hertzsprung-Russell diagram

10. 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.

11. The stars that are born in the nebula are either: • Low mass stars (red dwarfs) • Main sequence stars with an intermediate mass (red giant) ex. our sun! 3. High mass stars (Supergiant)

13. 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…

14. More Fusion! • At 100 million degrees Celsius, Helium fuses: 3 (4He)  12 C + energy • Energy sustains the expanded outer layers of the red giant.

15. The end of red giants… • After Helium is exhausted, outer layers of star expelled and form a planetary nebula.

16. 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.

17. 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.

18. 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.

19. 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)