1. The Stars Formation of a star Created September 2015 by Joshua Toebbe

2. Learning Goals The Stars: Formation of a star How are stars born? How do stars make energy? How do stars maintain stability? What evidence do we have? Created September 2015 by Joshua Toebbe

3. How are stars born? The Stars: Formation of a star Created September 2015 by Joshua Toebbe

4. How are stars born? The Stars: Formation of a star Space is not exactly empty It consists of a thin dispersion of gases among other things These gases slowly clump together through gravitational forces Creating giant molecular clouds These are what give birth to stars Created September 2015 by Joshua Toebbe

5. The Stars: Formation of a star How are stars born? Stars Diameter: 2 X 10 5 to 2.5 X 10 10 km Mass: 10 28 – 10 32 kg Density: 1 – 100,000 g/cm 3 Temp: 4,000 – 40,000 k Giant molecular clouds Diameter: 10,100 X 10 10 km Mass: 10 150 kg Density: 1 X 10 -20 g/cm 3 Temp: 5 - 10 K Created September 2015 by Joshua Toebbe

6. How are stars born? The Stars: Formation of a star Molecular clouds can not start forming stars spontaneously, four things stand in the way. 1.Thermal Energy Even at only 10 K a hydrogen atom can move at 800 mph. 2.The interstellar magnetic field Ions can not move freely through a magnetic field. 3.Rotation As a cloud contracts its increasing rotation can resist further contraction. 4.Turbulence Strong currents in the cloud can cause molecules to resist gravitational collapse. Created September 2015 by Joshua Toebbe

7. How are stars born? The Stars: Formation of a star For a molecular cloud to begin star formation something else must occur. The first trigger is a shockwave. These can be produced by supernovae. Created September 2015 by Joshua Toebbe

8. How are stars born? The Stars: Formation of a star Large stars can also ionize nearby gas and blow it outwards creating a pseudo shock wave. Two clouds could collide causing partial collapse. Or a cloud could get compressed as it passes through one of the galaxies spiral arms. In any case, the disturbance allows gravity to overcome the other barriers to collapse. Created September 2015 by Joshua Toebbe

9. How are stars born? The Stars: Formation of a star Even after the molecular cloud begins to collapse it must still increase its temperature by thousands of kelvin before it is hot enough to become a star. As the star starts to draw material to its center you can consider the molecules to be falling. As they fall they gain speed. This is known as Free fall contraction. As the particles approach the center the rate of collision increase converting the gravitational energy into thermal energy. Created September 2015 by Joshua Toebbe

10. How are stars born? The Stars: Formation of a star The cloud continues to collapse from the inside out developing a protostar at its center. A protostar is a relatively loose term but generally refers to a pre-stellar object that is hot enough to radiate infrared radiation, but not hot enough to sustain nuclear fusion. The outer layer of the cloud that has yet to collapse remains dark and cold, and are referred to as cocoons, which hide the protostar. (some astronomers think the Oort Cloud is the remant of the Suns cocoon) The cocoon absorbs all of the energy radiated by the star and re-emits it as infrared radiation. Created September 2015 by Joshua Toebbe

11. How are stars born? The Stars: Formation of a star As the contraction continues the cloud slowly flattens into a Protostellar disk. (much like the one astronomers think earth formed from) As the particles in the disk collide they lose angular momentum and sink into the protostar. When the protostar becomes hot enough it generates a stellar wind. In addition, photons can exert radiation pressure when they interact with other particles. The stellar wind and radiation pressure combine to blow away the protostars cocoon making it visible. Created September 2015 by Joshua Toebbe

12. What evidence do we have? The Stars: Formation of a star Created September 2015 by Joshua Toebbe

13. Evidence for star formation? The Stars: Formation of a star 1.T Tauri stars are very young, and still in the process of contracting (as confirmed by observations). These stars can be seen all over the night sky. 2.Visual and infrared observations reveal bok globules which are small dusty clouds. 3.New born stars have just the right temperature and luminosity that would be expected according to modern theories. 4.Jets of gas and waves of ionized gas can be seen compressing molecular clouds, producing star formation pillars. Created September 2015 by Joshua Toebbe

14. Evidence for star formation? The Stars: Formation of a star The entire process of star formation can almost be seen in the Orion nebula. The visible nebula is illuminated by very bright and hot stars but it is only a portion of a larger molecular cloud. The nebula is expanding ionized hydrogen which is compressing the nearby molecular cloud. Hidden within the molecular cloud are several protostars visible with infrared telescopes. Many of the stars in the nebula are still surrounded by a protostellar disk. Created September 2015 by Joshua Toebbe

15. How do stars make energy? The Stars: Formation of a star Created September 2015 by Joshua Toebbe

16. How do stars make energy? The Stars: Formation of a star Many people mistakenly think that stars burn. However, stars cannot burn as there is nowhere near enough oxygen in space. When something is burned energy is released by the breaking and forming of chemical bonds. Chemical bonds are a result of electron configuration, so we can say energy is a result of the electromagnetic force. The sun generates energy by breaking and forming bonds inside the atomic nuclei. Therefore the energy is generated as a result of the strong and weak nuclear forces which hold the nuclei together. Stars generate energy when their cores become hot enough to generate and sustain nuclear fussion. Created September 2015 by Joshua Toebbe

17. How do stars make energy? The Stars: Formation of a star Created September 2015 by Joshua Toebbe

18. How do stars make energy? The Stars: Formation of a star Created September 2015 by Joshua Toebbe

19. How do stars make energy? The Stars: Formation of a star This is where it gets complicated… Created September 2015 by Joshua Toebbe

20. How do stars make energy? The Stars: Formation of a star This process is known as the proton-proton chain. Energy is emitted in the form of gamma rays, neutrinos, positrons and the kinetic motion of the particles. The positrons re-combine with free electrons and vanish producing a gamma ray Some of the neutrinos combine into photons, the rest race out of the star near the speed of light and carry off about 2% of the energy produced in the reaction. The rest is re-used to help sustain fusion. Created September 2015 by Joshua Toebbe

21. How do stars make energy? The Stars: Formation of a star Because protons carry a positive charge they repeal each other with an electrostatic force, which creates the Coulomb barrier. The temperature must be millions of kelvin before particle collisions are violent enough to penetrate the coulomb barrier and fuse together. Created September 2015 by Joshua Toebbe

22. How do stars make energy? The Stars: Formation of a star This is where it gets complicated… Again… Created September 2015 by Joshua Toebbe 12 C 13 N 13 C 14 N 15 O 15 N 12 C

23. How do stars make energy? The Stars: Formation of a star This process is known as the CNO cycle. This process is highly temperature sensitive, requiring an environment around 16 million kelvin. Carbon has more protons than hydrogen which means it has a higher positive charge. Therefore carbon has a higher Coulomb barrier and thus requires more violent collisions to penetrate that barrier. Created September 2015 by Joshua Toebbe

24. How do stars maintain stability? The Stars: Formation of a star Created September 2015 by Joshua Toebbe

25. How do stars maintain stability? The Stars: Formation of a star According to the first law of thermodynamics, energy will flow from hot to cold regions (without work input of course). The surface of the sun transfers energy into the cold dark of space just as the core transmits energy to the surface of the star. There are three ways this energy can be transmitted inside a star: 1.Conduction 2.Radiation 3.Convection Created September 2015 by Joshua Toebbe

26. How do stars maintain stability? The Stars: Formation of a star Conduction is the transfer of thermal energy from particle to particle. As particles vibrate faster and faster they begin to bump into each other. With each collision momentum is conserved and transferred from one particle to another. As the rate of collisions increase the neighboring particles begin to vibrate. Each collision transfers energy from one particle to another. Conduction only takes place in dense materials which only occurs in the most peculiar of stars. Created September 2015 by Joshua Toebbe

27. How do stars maintain stability? The Stars: Formation of a star Radiation is the transfer of energy through wave motion. When electrons excited by thermal energy drop higher energy levels to lower energy levels they must emit that energy as electromagnetic radiation in the form of photons (yay for quantum mechanics). Photons are essentially energy packets. Energy flow depends on how difficult it is for the photons to travel through the gas. The denser or more opaque the gas, the less radiation penetrates the gas. Created September 2015 by Joshua Toebbe

28. How do stars maintain stability? The Stars: Formation of a star The radiation passes through the low density areas of the stars easily. Due to the energy production at the core the gas is extremely hot and has relatively low density. As the energy being radiated reaches the cooler outer layers of the star it has trouble penetrating the gas and energy builds up. This energy begins to heat the gas causing it to expand and rise. The cooler more dense gas at the surface begins to sink according to Archimedes principle (the buoyant force on an object is equal to the fluid displaced by that object). This produces convection currents which transfer energy through the cycling of matter. Created September 2015 by Joshua Toebbe

29. How do stars maintain stability? The Stars: Formation of a star When combining this with our knowledge of nuclear fusion (proton-proton chain and CNO cycle). Large stars using the CNO cycle generate more than 50% of their energy in less than 2% of their mass. This massive energy generation prevents radiation from draining the energy away fast enough causing the core to churn like a convection oven. Farther out the energy density becomes less allowing the star to radiate energy outward. Created September 2015 by Joshua Toebbe

30. How do stars maintain stability? The Stars: Formation of a star Stars generating most of the energy through the proton-proton chain generate energy in a much larger area. The sun generates about 50% of its energy in around 11% of its mass. This allows energy to radiate away from the core. Closer to the surface the cooler more dense gas becomes opaque causing convection to occur. Created September 2015 by Joshua Toebbe

31. How do stars maintain stability? The Stars: Formation of a star But regardless of what order this takes place in the majority of the energy transfer happens through radiation. Only a small region near the surface of medium stars or the core of large stars undoes convection. But low mass stars do not generate enough heat to decrease the density of the gas and reduce its opaqueness, meaning the bulk of their mass undergoes convection, since radiation cannot occur. Created September 2015 by Joshua Toebbe

32. How do stars maintain stability? The Stars: Formation of a star The relationship between pressure and temperature is very important. If a star generates too much energy, the outward pressure exceeds gravity causing the star to expand. This expansion would lower the central temperature of the star reducing energy production. If the star does not generate enough energy gravitational compression exceeds its outward pressure The compression of the star raises the internal temperature increasing energy production. In this way a star can maintain its stability. Created September 2015 by Joshua Toebbe

33. Sources Cover image underlay: from genkkis.deviantart.comgenkkis.deviantart.com Seeds, M. A. [2008] Foundations of Astronomy: Tenth Edition. Thomson, Brooks/cole. Created September 2015 by Joshua Toebbe The Stars: Formation of a star