Popular Science USA – July-August 2019

IF YOU LOOK UP ON A CLEAR NIGHT,

you’ll likely see a few bright celestial

objects—and a lot of dark space between

them. But if there are hundreds of billions of

stars in our galaxy and hundreds of billions

of galaxies in the universe, the whole thing

should light up like Times Square. So why is

the cosmos pitch-black at night?

It isn’t. We just can’t see most of these inter-

stellar billboards. Space is full of light energy

on a spectrum from high-energy gamma rays

to low-energy radio waves, says Mark Ham-

mergren, an astronomer at Chicago’s Adler

Planetarium. Our eyes can see only a small

portion of that gamut, which scientists have

aptly dubbed “visible light.” Ultrashort gamma

waves can’t penetrate Earth’s atmosphere, for

example, and our retinas simply can’t register

the super-low-energy infrared waves.

When there’s nothing for our peepers to

pick up on, we see that absence of light as

an absence of color too. Coupled with other

astronomical realities—the enormity of the

universe and the fact that as it expands, stars

race away from us—space can seem pretty

empty. In reality, what looks like a blank back-

ground is a tapestry of distant constellations,

each one shining bright.

EYE ON THE SKY

by Alex Schwartz

ANSWER KEY

HEAD TRIP

[1] The Himalayas

[2] Grand Canyon

[3] Nile River

Delta and

Suez Canal

[4] Australian

coast and

the Great

Barrier Reef

[5] Niagara Falls

[6] Sahara

Desert

why

is space

black?

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