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3. A new view of the Universe VII Fred Watson, AAO April 2005

4. In the Universe, things aren’t always what they seem… …in fact, they hardly ever are!

5. Building-blocks of illusion… Natural (stereoscopic) depth perception is limited. Understanding Solar System dynamics extends our perception of depth to more than 10 billion km. Stellar parallax extends depth perception to a thousand or so light-years. A 2-dimensional view of the sky

6. Stellar parallax Earth Earth 6 months later Star appears to move against background Sun

7. Building-blocks of illusion… A fixed vantage-point You are here

8. Building-blocks of illusion… Natural sensitivity only to visible light

9. Building-blocks of illusion… Lookback times: Moon… 1.3 seconds Sun… 8 minutes Jupiter (this evening)… 35 minutes Nearest star… 4.2 years Large Magellanic Cloud… 165,000 years Andromeda Galaxy… 2.2 million years Finite speed of light

10. The Moon Illusion

11. Spot the Moon… What’s that beside your head, Liam? Only my love for you, my sweet. Oh, Yukkk!!!

12. Our perception of the sky—not a hemisphere, but an inverted dish

16. Are we at the centre of our Galaxy?

17. The Milky Way— relatively uniform.

18. Are star-counts telling the truth? No—they ignore the presence of dust

19. Globular clusters About 150 known in our Galaxy Most are in one part of the sky (Sagittarius) Many are above or below the plane of the Galaxy—no dust In 1919, Shapley measured their distances from the Sun

20. Deductions: we are in the galactic suburbs— and the Galaxy is much bigger than anyone thought The Galaxy’s halo of globular clusters We have a clearer view of the halo than the disk

21. Supernovae

22. Vela supernova remnant

23. Supernova 1987a photographed 6 Feb 1989

24. Supernova Light Echo Earth Dust cloud Supernova D a D = 0.34 a 2 / T

25. Supernova Light Echo Earth Supernova Dust sheets

26. What puts the spirals into spiral galaxies?

27. What are the spiral arms? Just strings of stars? No, because they would be tightly wound up. Suppose the age of the galaxy is 10 billion years. Its inner regions rotate once in 200 million years… Therefore, we’d expect about 50 turns. The galaxy would look like a clock spring.

28. This density wave triggers star-formation, producing masses of hot bright stars that reveal its position. The spiral arms are an illusion. They trace the passage of a ‘sound wave’ through the disk of the galaxy

29. Faster than light?

30. Many radio-objects have expanding lobes This is a double-lobed radio galaxy, observed with the ATNF, super- imposed on a visible- light photograph.

31. Twin jets of material from a binary star in our Galaxy The expansion took place during the course of a few days in late-1997.

32. Faster than light…? Jets moving outwards at very nearly the speed of light To Earth By the time the jet reaches HERE light from the outburst is only HERE giving the appear- ance from Earth of material moving at superluminal velocity.

33. Gravity’s lens

34. The discovery of gravitational lenses In the late 1970s, astronomers started to discover ‘double quasars’ Shortly afterwards, strange arcs of light were discovered, often near faint galaxies.

35. Gravitational lensing

36. An Einstein Ring… B0047-2808: z lens = 0.485; z source = 3.595

37. Gravitational lens in Abell 2218

38. Is there a Cosmic Illusionist?

39. The laws of physics create the illusions… but they also allow us to see behind them!

40. The End