Chapter 14 Our Galaxy

1. Chapter 14Our Galaxy

2. 14.1 The Milky Way Revealed • Our Goals for Learning • What does our galaxy look like? • How do stars orbit in our galaxy? Stars and planets are part of galactic ecosystem, within which the stars and planets are born, and recycled.

3. What does our galaxy look like?

4. The Milky Way galaxy appears in our sky as a faint band of light – to old Greeks it resembled a flowing ribbon of milk (galactos).

5. Dusty gas clouds obscure our view because they absorb visible light This is the interstellar medium that makes new star systems The dusty smoglike interstellar medium hides most of our Galaxy from us.

6. All-Sky View

7. We see our galaxy edge-on. Primary features: disk, bulge, halo, globular clusters (more than a million of stars densely packed in about 100 ly across).

8. How it would look like if would observe our galaxy from above?

9. Milky Way is a spiral Galaxy: if we could view it from above the disk, we would see its spiral arms. We know this thank to the edge-on observations and observations of other Galaxies.

10. Milky way has also 4 smaller galaxy companions: Large and small magellanic cloud are two small galaxies which orbit Milky way. They are visible only from the southern hemisphere. Sagittarius Dwarf and Canis Major Dwarf are each in a process of colliding with the Milky Way disk, and will be ultimately ripped apart by our own Galaxies strong gravitational field.

11. How do stars orbit in our galaxy?

12. It takes about tens of millions of years for each up and down bob. For stars at sun’s distance it takes about 200 million years to finish one orbit. Stars in the disk all orbit in the same direction with a little up-and-down motion. Like on a merry go round.

13. Thought Question Why do orbits of bulge stars bob up and down? A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk

14. Thought Question Why do orbits of bulge stars bob up and down? A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk

15. There is one important difference between a merry go round and the rotation within a galaxy: the stars in a galaxy all travel with the same speeds, no matter how far from the center they are (on a merry go round horses near the edge travel much faster). This is connected to the issue of Dark Matter.

16. Orbits of stars in the bulge and halo have random orientations

18. Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: 1.0 x 1011Msun This is essentially using Newton’s version of Kepler’s third law. What is this law? Why it gives us the mass of the Galaxy only within the Sun’s orbit?

19. What have we learned? • What does our galaxy look like? • The Milky Way Galaxy consists of a thin disk about 100,000 light-years in diameter with a central bulge and a spherical region called the halo that surrounds the entire disk. The disk contains the gas and dust of the interstellar medium, while the halo contains very little gas.

20. What have we learned? • How do stars orbit in our galaxy? • Stars in the disk all orbit the galactic center in about the same plane and in the same direction. Halo and bulge stars also orbit the center of the galaxy, but their orbits are randomly inclined to the disk of the galaxy.

21. 14.2 Galactic Recycling • Our Goals for Learning • How does our galaxy recycle gas into stars? • Where do stars tend to form in our galaxy? Generations of stars continually recycle the same galactic material through their cores, gradually raising the overall abundance of elements made by fusion.

22. How does our galaxy recycle gas into stars?

23. Star-gas-star cycle Recycles gas from old stars into new star systems 3) They return much of their content into interstellar medium through stellar winds and when they explode… 1) Stars are born in molecular clouds… 2) They shine and produce heavier elements in nuclear fusion…

24. High-mass stars have strong stellar winds that blow bubbles of hot gas. It glows where gas pikes up as the bubble sweeps through the interstellar medium. How do high mass stars end their lives?

25. Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae. Stellar winds are less strong in low mass stars. Example: by the time sun dies and forms planetary nebula, it will loose about half its mass in stellar winds.

26. The bubbles created by supernova have one more effect on interstellar medium, they produce shock waves – waves of pressure that move faster than speed of sound. They create a wall of fast moving gas on its leading edge (blue region on the picture, 20 million degree gas). Supernova remnants are aftermaths of these shock waves. X-rays from hot gas in supernova remnants.

27. Supernova remnant cools and begins to emit visible light as it expands New elements made by supernova mix into interstellar medium. What kind of spectra is this?

28. It is not easy to stop the gas from exploding stars: after supernova explosion, ionized gas flies out at speeds of several thousands kilometers – much faster than the escape velocity from the galaxy. What is stopping it from leaving our Galaxy? Multiple supernovae create huge hot bubbles that can blow out of disk. Gas clouds cooling in the halo can rain back down on disk.

29. Cooling and cloud formation: Initially we have an ionized gas, … Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons. It has the usual composition: …

30. Cooling and cloud formation: Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons. It has the usual composition: … 70% H, 28% He, 2% heavy elements (half of this are dust grains, C and Si mineralswhich prevent us from seeing through the disk of Galaxy). Atomic hydrogen emits a spectral line with a wavelength of 21 cm. What part of spectra does it belong?

31. Cooling and cloud formation: Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons. It has the usual composition: … 70% H, 28% He, 2% heavy elements. Atomic hydrogen emits a spectral line with a wavelength of 21 cm. What part of spectra does it belong? Radio observation of the sky show that this 21 cm line comes from all directions: atomic H is distributed throughout the galactic disk. Gravity slowly draws blobs of this gas together, and the gas becomes cooler and denser.

32. Cooling and cloud formation: Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules. Molecular clouds are dense and heavy compared to the rest of interstellar gas and therefore they tend to settle in the central region of Milky Way’s disk (the dark lanes running through the luminous band of light, when we look at the Milky Way)

33. Molecular clouds in Orion • Composition: • Mostly H2 • About 28% He • About 1% CO • Many other • molecules: H2O, NH3, C2H5OH (ethyl alcohol)

34. Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle.

35. Radiation from newly formed stars is eroding these star-forming clouds. It ionizes and heats up molecular cloud – matter evaporates and joins hotter ionized gas encircling molecular clouds. Stars are formed only in dense knots which remain compact.

36. Summary of Galactic Recycling • Stars make new elements by fusion • Dying stars expel gas and new elements, producing hot bubbles (~106 K) • Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K) • Further cooling permits molecules to form, making molecular clouds (~30 K) • Gravity forms new stars (and planets) in molecular clouds Gas Cools

37. Thought Question Where will the gas be in 1 trillion years? A. Blown out of galaxy B. Still recycling just like now C. Locked into brown dwarfs and in stellar corpses (white dwarfs, neutron stars, black holes)

38. Thought Question Where will the gas be in 1 trillion years? A. Blown out of galaxy B. Still recycling just like now C. Locked into brown dwarfs and in stellar corpses (white dwarfs, neutron stars, black holes)

39. We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths of light

40. Infrared Visible Infrared light reveals stars whose visible light is blocked by gas clouds

41. X-rays X-rays are observed from hot gas above and below the Milky Way’s disk

42. Radio (21cm) 21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk

43. Radio (CO) Radio waves from carbon monoxide (CO) show locations of molecular clouds – concentrated more in the midplane of the disk

44. IR (dust) Long-wavelength infrared emission shows where young stars are heating dust grains –density of dust is very similar to that of the molecular clouds

45. Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds –follows locations of atomic and molecular clouds.

46. Where do stars tend to form in our galaxy?

47. Ionization nebulae are found around short-lived high-mass stars, signifying active star formation They look reddish because when electrons fall from 3 to 2nd energy level in H the emit red light! Transition in other elements produces different colors…

48. Reflection nebulae - starlight reflected from dust grains. Why do reflection nebulae look bluer than the nearby stars?

49. Reflection nebulae - starlight reflected from dust grains. Why do reflection nebulae look bluer than the nearby stars? For the same reason that our sky is blue! (interstellar dust grains scatter blue light much more readily than the red light)

50. What kinds of nebulae do you see?