1. Life Cycle of a Star
Star Life Cycle: Stars are like humans. They are born, live and then die.

2. What exists BEFORE stars form?
Interstellar Medium – the total mass of the gas and dust between the stars.

3. What is a Nebula?
Nebula – a dense cloud of gas and dust

4. What is a Protostar?
The nebula begins to contract due to gravity and the pressure and temperature increase and becomes a protostar

5. What is a Main Sequence Star?
When the temperature gets hot enough, nuclear fusion begins and a main sequence star is born.
Inward force of gravity = Outward force of nuclear fusion
90% of all stars

6. Quick Check
The fusion reaction that fuels a main- sequence star converts hydrogen into:
Carbon
Iron
Neon
Helium

7. What is a Red Giant?
A star loses mass during fusion as energy is released. This decreases the star’s gravity.
A star will expand, becoming a Red Giant when the outward force of fusion is greater than the inward force of gravity

8. What is a White Dwarf?
As the fuel runs out in a star, fusion slows down.
When the outward force of fusion is less than the inward force of gravity, the star will shrink in size, becoming a white dwarf.
White Dwarf
Pulsar – a rotating white dwarf emitting radio waves.

9. Quick Check
There are many sizes and colors of stars. Which of the following best describes the category in which the Sun would be placed?
A. blue supergiant stars
B. red giant stars
C. yellow main sequence stars
D. white dwarf stars

10. What is a Supernova?
Supernova – an explosion that marks the end of a very massive star’s life.
When it occurs, the exploding star can outshine all of the other stars in the galaxy in total for several days and may leave behind only a crushed core.

11. How does a star’s life end?
The life cycle of stars depends on their mass.
Small and medium stars become black dwarves once they die.
Larger stars become novae and die as: neutron stars
A neutron star is an imploded core of an exploded star made up almost entirely of neutrons. A teaspoon of their material would weigh more than all of automobiles in the U.S. together

12. How does a star’s life end?
The life cycle of stars depends on their mass.
The most massive stars become supernovae and die as a black hole
A black hole is an extremely massive remnant from which light can not escape

13. Quick Check All stars end up as a supernova and black hole A. True
B. False

15. Why do larger/hotter stars burn their fuel faster and live shorter lives than the Sun.
Large, more massive stars have much more gravity than the sun. This greater internal pressure causes fusion reactions to occur quickly. This causes the largest stars to burn their fuel, and eventually run out, much more quickly
Larger stars live shorter lives.
Bigger stars are brighter and hotter due to the rapid rate of fusion.

16. Where do we get the elements??
All stars spend the majority of their lives fusing hydrogen into helium: the main sequence.
When all of the helium in the central regions is converted to helium, the star will begin to “burn” helium into carbon.
Stars heavier than about 5 times the mass of the sun can do this with no problem: they burn hydrogen and then helium and then carbon, oxygen, silicon, and so on…until Iron.

17. Iron is the lightest element that doesn’t release energy when you attempt to fuse it together. You actually end up with less energy than you started with! So instead of generating pressure to hold up the outer layers, the iron fusion actually takes it out of the core. Thus, there is nothing left to combat gravity from the outer layers. The result: collapse!
This implosion (supernova) happens very, very quickly: about 15 seconds. During the collapse, the nuclei in the outer parts of the star are pushed together, so close that elements heavier than Iron are formed.

18. The Hertzsprung-Russell Diagram
The H-R diagram plots the luminosity (brightness) or absolute magnitude of stars against their surface temperatures.
Most stars fall into 4 distinct groups in the H-R diagram, because the groups represent stages in the life cycles of the stars.

19. The Hertzsprung-Russell Diagram
The stars in the universe are at different stages in their life cycles.
Some stars are young and hot; others are older and colder.
The Hertzsprung-Russell Diagram (or H-R Diagram) gives us a picture of a star’s life.
Named for astronomers Ejnar Hertzsprung (of Denmark) and Henry Norris Russell (of the United States).