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[sci.astro] Astrophysics (Astronomy Frequently Asked Questions) (4/9)
Section - D.12 What is the temperature in space?

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Empty space itself cannot have a temperature, unless you mean some
abstruse question about quantum vacuums.

However, if you put a physical object into space, it will reach a
temperature that depends on how efficiently it absorbs and emits
radiation and on what heating sources are nearby.  For example, an
object that both absorbs and emits perfectly, put at the Earth's
distance from the Sun, will reach a temperature of about 280 K or 7 C.
If it is shielded from the Sun but exposed to interplanetary and
interstellar radiation, it reaches about 5 K.  If it were far from all
stars and galaxies, it would come into equilibrium with the microwave
background at about 2.7 K.

Spacecraft (and spacewalking astronauts) often run a bit hotter than
280 K because they generate internal energy.  Arranging for them to
run at the desired temperature is an important aspect of design.

Some people also consider the "temperature" of high energy particles
like the solar wind or cosmic rays or the outer parts of the Earth's
atmosphere.  These particles are not in thermal equilibrium, so it's
not correct to speak of a single temperature for them, but their
energies correspond to temperatures of thousands of kelvins or higher.
Generally speaking, these particles are too tenuous to affect the
temperature of macroscopic objects.  There simply aren't enough
particles around to transfer much energy.  (It's the same on the
ground.  There are cosmic rays going through your body all the time,
but there aren't enough to keep you warm if the air is cold.  The air
at the Earth's surface is dense enough to transfer plenty of heat to
or from your body.)

User Contributions:

1
Keith Phemister
Sep 13, 2024 @ 11:23 pm
Copied from above: If the Universe were infinitely old, infinite in extent, and filled
with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP

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