Life Cycle of a Star

1. Life Cycle of a Star Astronomy Unit Earth/Environmental

2. Nebula Stage • Nebula – dark, cool clouds composed of dust and gases are the birthplace of stars • In the Milky Way nebulas are: 92 % hydrogen, 7 % helium and less than 1% of heavier elements • Nebula becomes dense enough to begin to contract, gravity squeezes particles which pulls everything towards the center

3. Protostar Stage • The temperature of this gaseous body slowly rises until it is hot enough to radiate energy • Protostar – is a developing star not yet hot enough to engage in nuclear fusion (occurs when less massive nuclei combine into more massive nuclei, releasing energy) • When the core of a protostar has reached about 10 million K(Kelvin), pressure within is so great that nuclear fusion of hydrogen begins, and a star is born

4. Main-Sequence Stage • Stable main-sequence star is balanced • Gravity, which is trying to squeeze it into a smaller sphere • Gas pressure, which is trying to expand it • Generally hydrogen fusion continues for a few billion years • Average star spends 90% of its life in Main-Sequence stage

5. Red Giant Stage • Occurs because the zone of hydrogen fusion continually moves outward • Once all of the star’s fuel has been used the core begins to contract • As the core contract it grows hotter by converting gravitational energy into heat energy, increasing the hydrogen fusion in the star’s outer layer • Result is a giant body hundreds to thousands of times larger • As the star expand, its surface cools

6. Burnout and Death • Stars eventually run out of fuel and collapse due to gravity • Low-Mass Stars • Small, cool red stars NEVER evolve into Red Giants • After they have used all of their fuel they collapse into a White Dwarf • Medium-Mass Stars(similar mass to our Sun) • Massive Stars

7. Burnout and Death Medium-Mass Stars(similar mass to our Sun) • Red Giants  White Dwarfs - cast off their large outer layer, creating an expanding cloud ring • The remaining hot, central White Dwarf heats the gas cloud causing it to glow

8. Planetary Nebula

9. Burnout and Death • Massive Stars(3 times that of the Sun) • End their lives in a brilliant explosion called a supernova • Supernova becomes million times brighter than its prenova stage • None have been observed in our galaxy since the invention of the telescope

10. Crab Nebula

11. Hertzsprung-Russell Diagram • Hertzsprung- Russell (H-R Diagram) represents the color and absolute magnitude of the star at various stages in its evolution

12. H-R Diagram

13. White Dwarfs • Remains of low-mass and medium- mass stars • High densities • Electrons have been displaced and are closer to the atom’s nucleus • Eventually they turn into a black dwarf • Smallest White Dwarfs are the most massive and the largest are the least massive

14. Neutron Stars • Are the remnants of a Supernova event • The electrons are forced to combine with protons to produce neutrons • Can be thought of as large atomic nuclei • Overall all small in size • Strong magnetic field

15. Veil Nebula

16. Supernovae • The outer layer of the star is ejected, while the core collapses into a very hot neutron star about 20 KM in diameter • Pulsar- small spinning star that radiates short bursts or pules of radio energy(remains of the supernova of 1054)

17. Black Hole • During a supernova event, remnants of stars 3 times more massive than the sun apparently collapse into objects even smaller and denser than a neutron star • Very hot, their gravity is so strong that not even light will escape