|Why is space black?
|Question Date: 1999-05-10|
Who says it is? Totally black means that in the
direction that you're looking (and each cell at
the back of your eye adds up the light from a tiny
cone of the sky) there's nothing at all shining.
Of course there is stuff there, it just isn't very
bright. Every little bit of dust in the solar
system reflects sunlight (just not much of it), as
do asteroids, comets, moons, and other planets.
Also, there are billion of stars inside the cup of
the big dipper. It's just that they're so far away
you can't see them without a telescope.
Why can a telescope see what you can't?
Because the telescope has a much bigger eye than
you, and lets in more light from the small angle
in which it's looking. But even a telescope can't
see if there's too much light pollution from the
nearest city... in that case the sky is too "grey"
to see the distant stars.
So here's a
question for you to investigate: How dark is
"black"? Ever notice that after a few minutes of
stargazing you see stars where you thought it was
completely dark? Your eyes adjust, right? So how
faint does a star have to be so that you can't see
it no matter how long your eyes adjust?
In order to see something, it either has to shine
like a star or reflect light like a planet.If you
don't see either things happening, you see
black--the absence of light.
I think the way to think about this is to consider
how our eyes work. We see things because
"visible" light is reflected by or emitted from
objects, enters our eyes and is interpreted by our
brain. What do we see if there is an absence of
Well, actually the absence of light causes it to
be dark. Light is produced in stars; this light
can reflect off objects. It is bright at the
surface of the earth because the sunlight bounces
off the rocks, trees and water and scatters the
light. But in deep space where there are no large
objects, there is nothing for the light to bounce
off of. The overall density of stars in space is
quite low; most space is empty like a vacuum.
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