2. Stellar Evolution The Birth & Death of Stars

3. Chapter 33 Section 33.2 and 33.3  Star Formation: Interstellar Medium & Protostars.  Stars & Their Properties.  Stellar Death: Supernovas, Neutron Stars & Black Holes.

4. Star Formation  The Interstellar Medium is the space between stars and is made up of trace amounts of gas(90% hydrogen, 10% helium) and dust.  Regions of this medium are denser than normal and are known as nebula for their cloud like appearance.  It is in these interstellar clouds that stars are born.

7. Star Formation  A shockwave from a supernova can hit one of these clouds.  This triggers a gravitational collapse, which pulls the gas and dust particles together.  As the cloud condenses, smaller regions break off into “globules”

8. Star Formation  These globules eventually form hot gaseous spheres known as protostars.  A protostar is not a true star in the sense that it has not started “burning hydrogen” through nuclear processes.  Gas and dust continue to accrete on the protostar and the temperature in the core rises.  Once the core reaches 10 million Kelvin, nuclear fusion begins.

9. A STAR IS BORN!!!

10. Nuclear Fusion  As gravity is pushing inward on the core, nuclear fusion is creating energy that pushes outward. These forces create an equilibrium that allow the star to sustain a definite size and shape.  The star is now in a state of hydrostatic equilibrium.

12. Nuclear Fusion  In stars with a core temperature of less than 15 million K, nuclear fusion occurs via the Proton- proton cycle.  First step - 2 protons (H nuclei) fuse together forming a deuterium nucleus, a neutrino, and a positron.  Next, the deuterium nucleus fuses with a proton and forms an isotope of He.  Finally, 2 He isotopes fuse and form a normal He atom and 2 protons (H nuclei).

14.  Overall, 4 H + nuclei are fused to form 1 He nucleus.  E=MC 2 demonstrates that mass and energy are interchangeable. This missing mass is released as energy.

15. Nuclear Fusion  Stars with a core temperature ranging from 15 million to 100 million K undergo the carbon cycle. The overall result is the same as the proton-proton cycle, but with different intermediates.  In stars with a core temperature above 100 million K, the dominant nuclear process is the triple alpha process. 2 alpha particles fuse to form Be, and a third alpha particle combines with the Be to produce 1 carbon nucleus.

16. Stellar Properties: Luminosity and Brightness  Luminosity (L) - total power radiated in watts.  Apparent Brightness (l) - the power crossing unit area at the Earth perpendicular to the path of the light.  l = L/4πd 2

18. Stellar Properties: Parallax  How do we measure the distance to stars outside of our solar system?  The method of parallax is used to measure the distance to nearby stars.

19.  One parsec (pc) is the distance to a star whose parallax angle is one second of arc(1″), where 1″=1/3600 °  1 pc=3.26 light-years  Distance to Star (in pc)=1/parallax(″)

20. Stellar Properties: H-R Diagram  A graph of temperature vs. luminosity with stars plotted as single dots.  When thousands of stars are plotted, they fall into definite regions, suggesting a relationship between a stars temperature and luminosity.  90% of stars fall into a band called the main sequence which runs from the upper left to lower right corners.

22. Stellar Death: Small Stars  Some models predict that the smallest red dwarf stars may stay on the main sequence for a few trillion years.  All of these small stars that have ever been born are still on the main sequence. We do not yet know what happens to them at the end of their lives.

23. Stellar Death: Medium Sized Stars  Medium sized stars, like our sun, begin to show their age as helium builds up in the core.  The helium core does not provide any energy and gravity causes the core to contract while hydrogen continues to fuse in a shell around the helium.  This gravitational collapse of the core causes releases tremendous amounts of heat that causes the outer hydrogen shell to expand.  At this point, the surface temperature drops and the star appears red. It is now known as a red giant.

25. Stellar Death: Medium Sized Stars  The core will continue to collapse due to gravity until the temperature reaches 100 million K.  At this point, helium begins to fuse into carbon. This signals the last stage in a medium stars life as there is not energy to fuse carbon into heavier elements.  The outer shell of the star has such a low density that it can drift off into space and form a planetary nebula.

27. Stellar Death: Medium Sized Stars  After the outer shell is gone, the helium envelope continues to burn around the carbon core.  Eventually the helium fuel will run out and gravity will once again compress the core.  With no source of energy to stop the gravitation collapse, the star will shrink until it reaches a point called electron degeneracy.  The star is now know as a white dwarf and will continue to radiate energy until all that remains is dead core of ash.

28. Stellar Death: Massive Stars  Massive stars follow the same evolutionary track as medium sized stars up to the point of the carbon core.  The core of these massive stars will eventually reach 600 million K and carbon will begin to fuse and produce oxygen, neon and magnesium.  Once the core reaches 1 billion K, oxygen ignites and produces silicon.

29. Stellar Death: Massive Stars  At 2 billion K the silicon ignites.  This process of producing new elements is known as nucleosynthesis and is the source of all the elements heavier than hydrogen and helium.  The human body is made up of 10% hydrogen mass with the remaining 90% made up of heavy elements. In other words, most of our body is made up of materials that were once inside the core of very massive stars.

30. Stellar Death: Massive Stars  This process of nucleosynthesis will continue until the core is made of iron.  Iron does not release energy in the fusion process but instead requires energy. As the core continues to heat up, the iron atoms will simply absorb this energy but will not fuse with each other.

32. Stellar Death: Super Nova  Once the iron core has absorbed the energy from the fusion taking place in the outer shells, gravity contracts the core together.  Eventually the individual particles will be packed so tightly they touch each other, at which point the collapse is stopped.  At this point the star explodes in what is called a type II supernova.  During these explosions, free neutrons may be captured by atoms to produce elements heavier than iron.  The debris from a supernova can create a nebula.

34. Stellar Death: Neutron Stars  After a supernova, an extremely dense core of neutrons may be left in what is called a neutron star.  These neutron stars are so dense that one teaspoon of material from a neutron star would weigh billions of tons.  All stars rotate and thus have angular momentum. When a star loses most of its mass in a supernova, the remaining neutron star rotates very quickly.  The fastest observed neutron star rotates at 716 revolutions per second.

35. Stellar Death: Black Holes  A supernova may explode so violently that the remaining core is compressed into an infinitely small, infinitely dense black hole.  Black hole’s have such a strong gravitational pull that even light can not escape if it gets any closer than the event horizon.  The radius, R, at which a body of mass M must be contracted to in order to form a black hole is given by R=2GM/c 2.  This radius is given a special name, the Schwarzchild radius.