The Milky Way

1. The Milky Way Dr Bryce 29:50

2. Class notices • Homework: We are moving towards the end of semester, it is vital that you maximise your grade by completing all your homework • CSP observing exercise • Exam behaviour

3. The Milky Way galaxy appears in our sky as a faint band of light

4. “All sky view” • The Milky Way in Visible light

5. Dusty gas clouds obscure our view because they absorb visible light This is the interstellar medium that makes new star systems

6. Interstellar Medium • Can both absorb and emit light • Most of the interstellar medium is gas and it is easiest to observe when it forms an emission cloud/nebula • Good examples of this include the Orion Nebula • Because the gas is predominantly hydrogen we see lines associated with atomic or ionized hydrogen

7. We see our galaxy edge-on Primary features: disk, bulge, halo, globular clusters

8. Globular clusters • We know from our H-R diagrams that globular clusters are old • One way to map the Milky Way is to consider the distribution of globular clusters

9. Mapping Globular clusters

10. If we could view the Milky Way from above the disk, we would see its spiral arms

11. Our interpretation of the Milky Way • Disk is thin and wide • Note spiral arms and bar

12. Stars in the disk all orbit in the same direction with a little up-and-down motion

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

15. Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: 1.0 x 1011MSun Sun is about 8kpc from the galactic centre

16. Orbital Velocity Law • The orbital speed (v) and distance from the galactic centre (d) of an object on a circular orbit around the galaxy tells us the mass (M) within that orbit

17. Star-gas-star cycle Recycles gas from old stars into new star systems

18. High-mass stars have strong stellar winds that blow bubbles of hot gas

19. HII regions • “H two” • Strong emission lines • A central hot star emits UV photons which ionize the hydrogen • When an electron is recaptured by a proton the HII line is emitted

20. HII regions • Require a hot star to have formed in a molecular cloud • The hotter the star the larger the HII region can be • HII regions tend to be red – see the Rosette Nebula

21. Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae

22. X-rays from hot gas in supernova remnants reveal newly-made heavy elements

23. The Milky Way at X-ray Wavelengths • X-ray emission is produced by hot gas bubbles and X-ray binaries

24. Supernova remnant cools and begins to emit visible light as it expands New elements made by supernova mix into interstellar medium

25. Radio emission in supernova remnants is from particles accelerated to near light speed Cosmic rays probably come from supernovae

26. Multiple supernovae create huge hot bubbles that can blow out of disk Gas clouds cooling in the halo can rain back down on disk

27. Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules

28. Molecular clouds in Orion • Composition: • Mostly H2 • About 28% He • About 1% CO • Many other • molecules

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

30. Radiation from newly formed stars is eroding these star-forming clouds

31. Gas 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

32. Interstellar gas temperature • Molecular clouds are dense and at low temperatures (~10K) • Interstellar gas is much less dense and much warmer (~10,000K) • We also see very hot (~1 million K) gas from Supernova shock waves, it is these regions that are responsible for the X-ray bubbles

33. The Milky Way at 21cm wavelength • Neutral hydrogen in confined to the plane of the Milky Way

34. 21cm line • Associated with the lowest energy level of Hydrogen • Doesn’t involve the hydrogen atom interacting with another photon so we can “see” this line anywhere in space

35. Dark Nebula • Associated with interstellar dust • Dust particles block the photons from the stars behind them • Dust will re-emit in the infra-red

36. The development of our Model • Galileo first observed that the Milky Way is made up of stars and many astronomers have tried to map it • For example Herschel used star counts, see below

37. Early models • Were incorrect as they didn’t include the effects of interstellar dust which will dim starlight (this effect is called extinction) and interstellar reddening • It is for these reasons that we actually find it easier to study other galaxies rather than the galaxy in which we live

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

39. Halo: No ionization nebulae, no blue stars  no star formation Disk: Ionization nebulae, blue stars  star formation

40. Halo Stars: 0.02-0.2% heavy elements (O, Fe, …), only old stars Halo stars formed first, then stopped Disk Stars: 2% heavy elements, stars of all ages Disk stars formed later, kept forming

41. Much of star formation in disk happens in spiral arms Whirlpool Galaxy

42. Spiral Structure • We can easily observe spiral arms in other galaxies but within the Milky Way our view is hindered by the effects of interstellar gas and dust

43. Spiral arms are waves of star formation • Gas clouds get squeezed as they move into spiral arms • Squeezing of clouds triggers star formation • Young stars flow out of spiral arms

44. Stars slow down in the spiral arms Density Waves

45. Our galaxy probably formed from a giant gas cloud

46. Halo stars formed first as gravity caused cloud to contract

47. Remaining gas settled into spinning disk

48. Stars continuously form in disk as galaxy grows older

49. Collisions cause the flattening of the disk Upwards or downwards motions tend to be cancelled out Cloud collisions

50. Rotation • Possible models for rotation • Wheel or Merry-go-round • Planetary or Keplerian • Milky Way doesn’t rotate like either of these models