Astronomers are blessed to live in a largely transparenttransparent world

While taking that for granted, we’re continually struggling to penetrate the

remaining realms of darknessSouthern Summer Nightsky

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!

Wish I were HERENot THERE!

Venera lander on gloomy surface of Venus

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)

Transparency,

The ratio of the red to the black curves,

is 80% for most visible light

UV, wiped out by O3 IR, Absorbed by H20, C02, CH4

Sunlight, hitting

Sunlight

B G Y R

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

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

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

Milky Way

Was revealed by Galileo’s

telescope to be a

GALAXY of billions

of Suns

But a serious problem is evident: Blue light cannot travel through the

disk of our Galaxy TransparencyTransparency 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 ofOur Milky Way galaxy than any visible photograph

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

Strong motivation to improve infrared and

radio telescopes

Dusty regions of space are where all stars (and planets) were, and are

formed

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

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 (1/30th 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

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)

We have seen that “free electron wall”, it is the Cosmic Microwave

Background Radiation

NASA’sWMAP

Contrast of microwave brightness extremely enhanced, to reveal the 1 in 100,000 density excesses that were to evolve into…Us

Unlike the births of stars, planets or galaxies, the earliest epoch of the Universe will never be visible, at

any wavelength of light

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

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