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This page is intentionally blank A new view of the Universe VII Fred Watson, AAO April 2005 In the Universe, things aren’t always what they seem… …in fact, they hardly ever are! Building-blocks of illusion… A 2-dimensional view of the sky • 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. Stellar parallax Earth Sun Earth 6 months later Star appears to move against background Building-blocks of illusion… A fixed vantage-point You are here Building-blocks of illusion… Natural sensitivity only to visible light Building-blocks of illusion… Finite speed of light 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 The Moon Illusion Only my love for you, my sweet. What’s that beside your head, Liam? Spot the Moon… Oh, Yukkk!!! Our perception of the sky—not a hemisphere, but an inverted dish Are we at the centre of our Galaxy? The Milky Way— relatively uniform. Are star-counts telling the truth? No—they ignore the presence of dust • • • • 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 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 Supernovae Vela supernova remnant Supernova 1987a photographed 6 Feb 1989 Supernova Light Echo Dust cloud a D Earth Supernova D = 0.34 a2 / T Supernova Light Echo Supernova Dust sheets Earth What puts the spirals into spiral galaxies? 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. The spiral arms are an illusion. They trace the passage of a ‘sound wave’ through the disk of the galaxy This density wave triggers star-formation, producing masses of hot bright stars that reveal its position. Faster than light? Many radio-objects have expanding lobes This is a double-lobed radio galaxy, observed with the ATNF, superimposed on a visiblelight photograph. 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. 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 appearance from Earth of material moving at superluminal velocity. Gravity’s lens 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. Gravitational lensing An Einstein Ring… B0047-2808: zlens = 0.485; zsource = 3.595 Gravitational lens in Abell 2218 Is there a Cosmic Illusionist? The laws of physics create the illusions… but they also allow us to see behind them! The End