1. ASTR100 (Spring 2008) Introduction to Astronomy Star Birth Prof. D.C. Richardson Sections 0101-0106

2. How do stars form?

3. Star-forming Clouds Stars form in dark clouds of dusty gas in interstellar space. The gas between the stars is called the interstellar medium. The clouds are called molecular clouds because they are cold enough for H 2 molecules to form.

4. Stars form in places where gravity can overcome thermal pressure in a cloud.

5. Gravity Versus Pressure Gravity can create stars only if it can overcome the force of thermal pressure in a cloud. Gravity within a contracting gas cloud becomes stronger as the gas becomes denser.

6. Mass of a Star-forming Cloud A typical molecular cloud must contain a few hundred solar masses for gravity to overcome pressure initially. Collapse continues so long as most of the thermal energy from contraction is radiated away.

7. Fragmentation of a Cloud This simulation begins with a turbulent cloud containing 50 solar masses of gas.

8. Fragmentation of a Cloud The random motions of different sections of the cloud cause it to become lumpy.

9. Fragmentation of a Cloud Each lump of the cloud in which gravity can overcome pressure can go on to become a star. A large cloud can make a whole cluster of stars.

10. Glowing Dust Grains As stars begin to form, dust grains that absorb visible light heat up and emit infrared light. View in IR View in visible

11. Glowing Dust Grains Long-wavelength infrared light is brightest from regions where many stars are currently forming.

12. Thought Question What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A.It would continue contracting, but its temperature would not change. B.Its mass would increase. C.Its internal pressure would increase.

13. Thought Question What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A.It would continue contracting, but its temperature would not change. B.Its mass would increase. C.Its internal pressure would increase.

14. Solar-system formation is a good example of star birth.

15. Cloud heats up as gravity causes it to contract. Conservation of energy. Contraction can continue if thermal energy is radiated away.

16. As gravity forces a cloud to become smaller, it begins to spin faster and faster. Conservation of angular momentum. Gas settles into a spinning disk because collisions force orderly motions among the gas particles.

17. Angular momentum leads to: Rotation of protostar. Disk formation. … and sometimes … Jets from protostar. Fragmentation into binary.

18. Protostar jets (flared disk seen edge-on).

20. Thought Question What would happen to a protostar that formed without any rotation at all? A.Its jets would go in multiple directions. B.It would not have planets. C.It would be very bright in the infrared. D.It would not be round.

21. Thought Question What would happen to a protostar that formed without any rotation at all? A.Its jets would go in multiple directions. B.It would not have planets. C.It would be very bright in the infrared. D.It would not be round.

22. Protostar to Main Sequence A protostar contracts and heats up until the core temperature is sufficient for hydrogen fusion. Contraction ends when gravitational equilibrium is established. Takes 50 million years for star like the Sun (less time for more massive stars).

23. Summary of Star Birth 1.Gravity causes gas cloud to shrink and fragment. 2.Core of shrinking cloud heats up. 3.When core gets hot enough, fusion begins and stops the shrinking. 4.New star achieves long- lasting state of balance.

24. How massive are newborn stars?

25. A cluster of many stars can form out of a single cloud.

26. Temperature Luminosity Very massive stars are rare. Low-mass stars are common.

27. Temperature Luminosity Stars more massive than 150 M Sun would blow apart. Stars less massive than 0.08 M Sun can’t sustain fusion.

28. Upper Limit on a Star’s Mass Photons exert a slight amount of pressure when they strike matter. Very massive stars are so luminous that the collective pressure of photons drives their matter into space.

29. Upper Limit on a Star’s Mass Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart. Observations have not found stars more massive than about 150 M Sun.

30. Lower Limit on a Star’s Mass Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 10 7 K. Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation. Is there another form of pressure that can stop contraction?

31. Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying the same state in the same place.

32. Thermal Pressure: Depends on heat content. The main form of pressure in most stars. Degeneracy Pressure: Particles can’t be in same state in same place. Doesn’t depend on heat content.

33. Brown Dwarfs Degeneracy pressure halts the contraction of objects with mass < 0.08 M Sun before the core temperature becomes hot enough for fusion. Star-like objects not massive enough to start fusion are brown dwarfs.

34. Brown Dwarfs A brown dwarf emits infrared light because of heat left over from contraction. Its luminosity gradually declines with time as it loses thermal energy.

35. Brown Dwarfs in Orion Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous.

37. MIDTERM #2 REVIEW Chapters 7–11

38. Chapter 7: Earth and the Terrestrial Worlds A.Earth as a Planet Geological activity, surface processes, atmosphere (greenhouse effect). B.The Moon and Mercury Geologically dead. C.Mars Once had surface water, but no more. D.Venus Runaway greenhouse effect. E.Earth as a Living Planet Carbon dioxide cycle, global warming.

39. Chapter 8: Jovian Planet Systems A.A Different Kind of Planet Composition, interiors, weather. B.A Wealth of Worlds Moon sizes, geological activity. C.Jovian Planet Rings Structure, origin.

40. Chapter 9: Asteroids, Comets, and Dwarf Planets A.Asteroids and Meteorites Asteroid belt, connection to meteorites. B.Comets Structure, origin. C.Pluto (and other dwarf planets) Structure, connection to comets. D.Cosmic Collisions Past impacts, dinosaurs, threat, role of other planets.

41. Chapter 10: Our Star A.A Closer Look at the Sun Gravitational equilibrium, structure. B.Nuclear Fusion in the Sun Proton-proton chain, energy transport, neutrinos. C.The Sun-Earth Connection Solar activity, sunspots, flares, prominences, coronal mass ejections.

42. Chapter 11: Surveying the Stars A.Properties of Stars Apparent brightness, luminosity, spectral type, binary stars. B.Patterns Among Stars Hertzsprung-Russell diagram, main sequence, giants/supergiants, white dwarfs. C.Star Clusters Open & globular clusters, main-sequence turnoff.

43. Midterm Information When: Tuesday April 15, 9:30 am Where: here! Bring pencil, student ID No notes, no calculators, no mobiles! Review: Monday April 14, 5–7 pm Where: here! Bring your questions and textbook!

44. Good Luck!