Star Formation

1. Star Formation • Our Sun formed billions of years ago • We see evidence that star formation is a constant process

2. Star Formation • Star formation starts with Dark Dust Clouds • Relatively Dense, but very cool • Cloud starts to contract under its own gravity • Contraction causes heating • Nuclear Reactions begin • Many things oppose the gravitational contraction • Heat • Rotation • Magnetism

3. Stage 1: Interstellar Cloud • DDC contracts • Very large (tens of parsecs) • Very massive (thousands of Sun masses) • Very cool (~10K) • Internal cloud pressure initially balances against gravity • Something happens for it to become unstable • Star explosion, formation • Cloud begins to fragment

4. Stage 2: Collapsing Cloud • The fragmented cloud is about 100x larger than our solar system • It is still very cool because it radiates away its heat out into space • The very center is the warmest part ~100 K • Eventually the cloud will contract enough (becoming dense enough) where it will trap most of the heat generated

5. Stage 3: Fragmentation Ceases • At this point the cloud is the size of our solar system • The gas at the center is very dense • Radiation cannot escape • Temperature increases to ~10,000 K • This region is called a Protostar • Protostar is still contracting • Internal pressure cannot yet counteract gravity

6. Stage 4: A Protostar • Protostar’s temperature increases with contraction • Reaches ~1,000,000 K • (10,000,000 K needed for nuclear ignition) • Has a luminosity 1000 times greater than our Sun • No nuclear reactions • Very Large • Releasing gravitational potential energy as it shrinks • Can plot it on H-R diagram

7. Stage 4: A Protostar • Protostar’s temperature increases with contraction • Reaches ~1,000,000 K • (10,000,000 K needed for nuclear ignition) • Has a luminosity 1000 times greater than our Sun • No nuclear reactions • Very Large • Releasing gravitational potential energy as it shrinks • Can plot it on H-R diagram

8. Stage 5: Protostar Evolution • The Protostar is very hot – the gas is at high pressure • But it still isn’t enough to counteract gravity • The Protostar continues to shrink • This is called the “T Tauri” phase

9. Stage 5: Protostar Evolution • The Protostar shrinks to about 10X the size of the Sun • Central Temp = 5,000,000 K • Not enough for fusion • Contraction slows • Harder for gravity to compress hotter gas

10. Stages 1 – 5

11. Stage 6: Newborn Star • 10 million years have passed • The Protostar has shrunk down to almost 1 solar radius • Its central temperature reaches 10,000,000 K • Fusion of Hydrogen begins • It is still not on the main sequence

12. Stage 7: The Main Sequence • For 30 – 50 more million years, the new star continues to contract • Eventually gravity is exactly balanced by the outward pressure of the Nuclear reactions • The star also exactly balances the energy it produces by fusion and the energy it radiates out • The core is at 15,000,000 K and the surface is at 6000 K. • It will stay like this for 10 billion years • This takes long? Full grown in 4 months!

13. Other Mass Stars • How do stars of other masses evolve? • The basic steps are the same, but they will settle on the main sequence at different points. • But then why is main sequence so broad? • Stars have slightly different compositions

14. Failed Stars • Some of the gas clouds that could form stars don’t. Why not? • They contract under gravity • They get hot • Where does it go wrong? • The gravity crushes the cloud, but not enough to ignite thermonuclear fusion • There just isn’t enough mass to create a large enough gravitational force

15. Failed Stars • These failed stars are called Brown Dwarfs • Brown dwarfs are less than 0.08 solar masses, • (80 times the mass of Jupiter) • The lower limit for Brown Dwarfs is 12 Jupiter masses • Anything less massive is just called a planet • These objects eventually become cold balls of gas • We estimate there are 100 billion of these in our Galaxy

16. Very High Mass Clouds • So the lower limit for stars is ~ 0.08 Solar Masses • Is there an upper limit? • We thought it was about 100 solar masses • There is so much gravitational force that the nuclear fusion begins quickly • Then there is so much nuclear fusion that it overcomes the force of gravity holding the star together • These stars blow themselves up early in their lifetime

17. Very High Mass Clouds • In 2010 Astronomers announced that they found a star with a birth weight of about 320 solar masses • Much larger than models predict could be stable • Current mass is about 265 solar masses

18. Main Sequence Stars • How do main sequence stars know how big to be? • What properties dictate temperature? • Will the star always remain this way?

19. PVT Thermostat • Stars have P, V, T, , and  values •  is the rate of nuclear reactions • They have a self-regulating feature to balance the internal and external forces • If  then Q  T  P  V     

20. Stellar Evolution • Stars have only two significant regions during their evolution • Core – where the fusion is taking place • Envelope – an inert layer of gas • There is no convection between core and envelope • As Hydrogen gets turned into Helium, the He will be inert (not hot enough to fuse) • Later in the star’s life it runs low on Hydrogen

21. Stellar Evolution

22. The Active Sun • The Sun seems steady and predictable • The luminosity (energy output) is nearly constant • The surface activity of the Sun is changing and unpredictable • Doesn’t affect the evolution of the Sun • Greatly affects the Earth

23. Sunspots • Sunspots are dark areas on the Sun’s surface • Typically about 10,000 km across • Umbra – dark center • Penumbra – grey surroundings • Due to temperature change

25. Sunspots • Sunspots appear randomly, in different numbers • However, there is an 11 year cycle

26. Sunspots • Sunspots are caused by magnetic field lines blocking convection • The magnetic field is locally strong and breaks through the surface • If this happens so we see it at the side of the Sun, it is a prominence

27. The Sun and Earth • These factors affect the climate of the Earth • Luminosity actually decreases slightly when sunspots are absent • “Maunder minimum” was a period of solar inactivity from 1645 – 1715 • May be linked to the “Little Ice Age” in Europe • We haven’t been studying these things long enough to conclude the effects yet!

28. The Sun and Earth

29. The Sun and Earth

30. The Active Sun • Prominences eject hot solar gases • They follow magnetic fields • Gas cools and falls back into the Sun • Releases massive energy • 1025 Joules • 1 Billion years of Earth’s energy production

31. The Active Sun • Solar Flares are more energetic than prominences • Caused by rapid release of magnetic energy • Equivalent to millions of 100-megaton bombs • Gas breaks free of magnetic field lines and escapes into space

32. The Active Sun • Coronal Mass Ejections are a release of a “magnetic bubble” of gas • This gas interferes with the normal solar wind and can interact with the Earth’s magnetic field • Imparts a large amount of energy and disrupts communications

33. Nuclear Fusion • The Sun (and all other stars) convert mass directly into energy according to Einstein’s equation: E = mc2 Energy = Mass X (speed of light)2 • The mass of individual atoms is very small, and sub-atomic particles (protons, neutrons) is even less • Usually measured in atomic mass units (u) instead of kilograms

34. Nuclear Fusion • 1 atomic mass unit = 1.66053886 × 10-27 kilograms • Converting 1 amu directly to energy: • E = mc2 = (1.66x10-27 kg)(3x108 m/s)2 = 1.5 x10-10 J • Doesn’t seem like a lot, does it? • One kg of mass transformation equals 9x1016 J! • The United States uses approximately 4,000,000 Giga Watt hours per year. (1.5x1019 J) • That is only 167 kg of mass turning into energy! • Hiroshima bomb only turned 1 gram of mass into Energy

35. Nuclear Fusion • Similar electrical charges repel • Protons are both positively charged • However, if they have enough energy they can overcome the repulsive force

36. Nuclear Fusion • Protons fuse to Deuterons, Deuterons fuse with protons to form Helium-3, Helium-3 fuses to form Helium-4

37. Nuclear Fusion • Mass of a Hydrogen atom (proton): 1.008178u • Mass of Helium-4: 4.003976 • Does the mass of Helium-4 equal the sum of its parts (4 Hydrogen atoms)? 4.032712 = 4.003976 ? • No. The parts are more than the whole. Where did the mass go?

38. Nuclear Fusion • The missing amount of mass is called the Mass Defect • The Mass Defect is converted directly to energy • This energy represents the binding energy of the nucleus • For low atomic masses, a larger nucleus is more stable and therefore lower energy • Fusion releases energy • For large atomic masses a smaller nucleus is more stable • Fission releases energy

39. Nuclear Fusion • By looking at the luminosity of the Sun, we can calculate that it gives off 3.9x1026 W • The mass difference between He and 4 H is 0.028736 amu • This equals an energy of 4.3x10-12 J • At this rate 600 million tons of Hydrogen must be converted into Helium every second

40. Stellar Evolution • Because there is no convection between the core and envelope, there is no Hydrogen re-supply • The Helium builds up in the core • With less nuclear reactions: • The temperature goes down • The pressure goes down • Gravity starts to win vs. pressure • The star contracts

41. Stellar Evolution • As the star contracts, it will heat up • (Gravitational PE conversion) • Temp well above 10 Million K • Not hot enough for HE fusion • 100 Million K • Hydrogen shell of the core burns fast and furious • Increased energy causes star to “puff” outward

42. Stellar Evolution Stage 8 Star • The star’s surface temperature drops • Luminosity increases slightly • Radius increases by ~3x • On its way to becoming a Red Giant • Will take ~100 million years • Envelope expands due to increasing gas pressure while core is shrinking

43. Stellar Evolution Stage 9: The Red Giant Branch • The star grows to about 100x its main sequence size • The surface temperature stays constant • Cooler surface is becoming opaque to interior radiation • Luminosity is very high because of its size • While the outer shell has grown, the core continues to shrink • It is mostly non-burning Helium “ash” • No pressure to counteract gravity • Temperature increases to above 100 Million K

44. Stellar Evolution Stage 10: Helium Fusion • Two Helium atoms collide and create Beryllium-8 • Triple Alpha Reaction • Beryllium-8 is very unstable • Usually breaks apart in 10-12 s • Because of high density in core, Be-8 will fuse with a Helium nucleus to form Carbon-12

45. Stellar Evolution • The core is developing layers like an onion • Hydrogen is fusing in outer core shell • Helium is fusing in middle layer • Inert Carbon ash is building up in its core • Not hot enough to fuse into another element • Gravity is continuing to compress the core • The core is getting hotter

46. Stellar Evolution • The core will shrink until it reaches “electron degeneracy” • Ionized electrons are swimming around in the core • At the degeneracy point gravity has contracted the core so much that individual electrons “touch” • At this point gravity can’t contract it any more • The core is supported by this electron “pressure” • Temperature continues to increase

47. Stellar Evolution • The core temperature is rising • The PVT Thermostat should result in a pressure increase, increasing the volume of the star • However the pressure is fixed • Pressure of the electron degeneracy • This breaks the PVT Thermostat • Helium fusion is running rampant • Dumps a lot of energy into the core: Helium Flash

48. Stellar Evolution • Helium Flash is an explosion that expands the star’s core outwards • How can the explosion overcome gravity? • The Helium Flash creates a lot of photons • Results in a large “Radiation Pressure” • This pressure overcomes gravity • Core becomes “normal” again, pressure – gravity equilibrium is re-established

49. Stellar Evolution • Hydrogen and Helium continue fusing • As a result of the Helium flash the core cools • This reduces energy output • It moves on the H-R diagram • Where it goes depends on mass • 20-30% escapes

50. Stellar Evolution • The carbon core continues to get larger • Hydrogen and Helium burning gets more intense • Temperatures get hotter and hotter Stage 11 • Star expands back out to the Giant Branch