1. transparent Astronomers are blessed to live in a largely transparent world While taking that for granted, we’re continually struggling to penetrate the remaining realms of darkness Southern Summer Nightsky

2. Earth’s atmosphere is remarkably transparent to visible light, with a few critical exceptions: 1)Clouds (suspended droplets) can easily block our vision of space.  Nice  to have desert mountain ranges,  Lucky  we’re not on our sister planet, Venus 0) Scattered light swamps much fainter signal from stars and galaxies: Don’t forget to turn out the lights!

3. Wish I were HERE Not THERE! Venera lander on gloomy surface of Venus

4. Earth’s atmosphere is remarkably transparent to visible light, with a few critical exceptions: 2)Ozone absorbs (lethal) Ultraviolet radiation (too little transparency would be fatal)

5. Transparency, The ratio of the red to the black curves, is 80% for most visible light UV, wiped out by O 3 IR, Absorbed by H 2 0, C0 2, CH 4 Sunlight, hitting Sunlight B G Y R

6. Earth’s atmosphere is remarkably transparent to visible light, with a few critical exceptions: 3) Greenhouse molecules absorb Infrared radiation escaping the Earth, keeping our surface above freezing

7. Nearly every aspect of scientific revolutions, from Galileo to Newton, to Einstein, needed transparent views of the sky, particularly our Solar System In recent decades, astronomers have spent $Billions$ to get around those 3 obstructions, with special telescopes in special locations

8. Next breakthroughs in human perception of the cosmos came because we can see beyond our Solar System  Very Lucky  Most of our Milky Way galaxy is transparent to visible light Hercules Deep Field

9. Milky Way Was revealed by Galileo’s telescope to be a GALAXY of billions of Suns

10. But a serious problem is evident: Blue light cannot travel through the disk of our Galaxy Transparency  Transparency only holds for long wavelengths, Infrared and Radio Closer view of the plane of the Milky Way towards Sagitarius, Which systematically absorbs the blue light, But Passes the redder light This infrared picture (tilted to horizontal) gives much better view of Our Milky Way galaxy than any visible photograph

11. Less than 1% of galactic mass is in SOLID form, “dust” grains the size of the wavelength of visible light Same effect is seen at sunset

12.  Strong motivation to improve infrared and radio telescopes Dusty regions of space are where all stars (and planets) were, and are formed

13. On outskirts of Milky Way, we have an unobstructed view out of our galactic plane  Lucky  Deep (intergalactic) Space is almost completely empty  Using large telescopes as time machines, Milky Way Star (foreground) In all directions, we see through the foreground of Milky Way stars, to discover an almost endless superposition of galaxies, all the way back to their birth

14. Hercules Deep Field, a 13-Billion Light-year Needle “drill core” across the history of the Universe, traces most of the evolution of galaxies, from origins to present day 2 minutes of arc (1/30 th of a degree) = about 1 million light years across, at typical galaxy distance Several thousand galaxies are detectable in this little picture, hundred billion over entire sky

15. Ultimate limit of Universe’s Transparency: 380,000 years after the Big Bang, all atoms were ionized  the resulting free electrons made earlier times in the Universe completely opaque Milky Way “Baby Picture” (scattering of light is just as bad as absorbing it)

16. We have seen that “free electron wall”, it is the Cosmic Microwave Background Radiation NASA’s WMAP Contrast of microwave brightness extremely enhanced, to reveal the 1 in 100,000 density excesses that were to evolve into…Us

17. Unlike the births of stars, planets or galaxies, the earliest epoch of the Universe will never be visible, at any wavelength of light

18. In principle, we will someday observe the earliest moments of the Big Bang not by detecting its photons, but instead its neutrinos and gravity waves They pass transparently through darn near anything

19. Nearly every aspect of scientific revolutions, from Galileo to Newton, to Einstein, needed transparent views of the sky, particularly our Solar System