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Does planet Earth receive any heat at all from the millions of other stars in our galaxy?

Does the light that brings the heat perhaps cool down on the long journey through space to planet Earth, explaining why no significant amount of heat arrives here?

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  • $\begingroup$ @DavidHammen is correct. A couple of other thoughts: 1) light/heat from stars radiates in all directions. The fraction of that light that hits the Earth is miniscule. 2) As far as we know, heat caused by radiation (electromagnetic waves such as light) does not cool down while traveling. $\endgroup$
    – user21
    Feb 19, 2015 at 4:26
  • $\begingroup$ How could it not be cooling down during it's journey through Space because that would mean that the Sun's rays reaching the Earth would be 1 1/2 million degrees which would fry the Earth wouldn't it ? What am I missing here ? $\endgroup$
    – Peter U
    Feb 21, 2015 at 22:48
  • $\begingroup$ @barrycarter If the Sun's ray's did not cool down during it's trip to Earth wouldn't it fry the Earth then. It is 1 1/2 million degrees Fahrenheit when it leaves for the Earth. Is this not correct ? $\endgroup$
    – Peter U
    Feb 24, 2015 at 21:32
  • $\begingroup$ Actually, I did see your earlier comment, and am trying to figure out how to best answer it. Light radiation doesn't really have temperature itself, it can only warm up other things. What matters is the amount of light something receives, but there's no such thing as temperature of light. However, I don't think I'm explaining well, and asking other experts to help. [note: fluorescent bulbs talk about "warm light" and "cold light", and even mention temperatures, but that's not quite the same thing] $\endgroup$
    – user21
    Feb 25, 2015 at 13:48
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    $\begingroup$ About 15 years ago, Dame Jocelyn Bell, the one who discovered pulsars, gave a talk in my hometown and each attendee received a small paper saying: “The energy you spent unfolding this piece of paper is more than the combined energy we receive from all known pulsars over a year.” So yeah, we do receive energy/heat, but not for the worth of it! $\endgroup$ Feb 27 at 9:43

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Does the Earth receive any heat at all from the millions of other Stars in our Galaxy ?

Effectively, no. Stars are too few and far between.

Qualifying that "effectively, no": From http://stjarnhimlen.se/comp/radfaq.html#10, the stellar magnitude from total starlight is -5. Compare that to the -26.7 magnitude of the Sun as viewed from the Earth. That difference of 21.7 means that starlight is responsible for one part in 1022 of the heating of the Earth. Another way to express one part in 1022 is "effectively none".

Another way to look at it: The Earth would eventually cool to 2.7 kelvins if the Sun and stars magically turned off. If it was only the Sun magically turned off, the Earth would cool to 3 kelvins. Compare that to the nice balmy 287 kelvins we experience thanks to the Sun.

Is it light that is bringing the heat and perhaps it cools down on the long journey in Space getting to the Earth and that's why no significant amount of heat is getting here ?

The stars in our galaxy are extremely close to us in a cosmological sense. Even the Andromeda galaxy is extremely close. The light we see from stars in our galaxy is more or less the same as emitted.


Over very, very long distances (much, much longer than the distance to Andromeda), the cosmological expansion of space means that light is redshifted. How much light is redshifted offers a clue as to the distance to some remote object.

We do receive a minuscule amount of energy from the cosmic microwave background. That radiation was not emitted by stars. It marked the transition from the very early hot and opaque universe to a cooler and transparent universe. The universe transitioned from opaque to transparent when the temperature dropped below 3000 K or so. Now that the light has an effective temperature of only 2.725 K.

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Yes, the Earth does receive energy from the stars, but not much. The effective temperature of the night sky is about 3 Kelvin which is not much more that that of the cosmic microwave background at 2.7 Kelvin. The difference being due to the total energy of starts etc.(room temperature is ~295 kelvin)

The coldness of the night sky is interesting in itself see Olber's Paradox

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    $\begingroup$ I can take a picture of the stars with my digital camera, so there's enough energy coming in to trigger its CCD: en.wikipedia.org/wiki/Charge-coupled_device That energy is more than zero, and since it's absorbed, it'll be turned into heat. $\endgroup$ Feb 11, 2015 at 14:38
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    $\begingroup$ @WayfaringStranger: There's also enough energy to trigger the cells in my retinas. $\endgroup$ Feb 11, 2015 at 19:18
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The photons emitted from each star are spreading out on a sphere whose radius is growing in proportion to time --- literally, growing at the speed of light --- so the area of that sphere is growing in proportion to the square of time, so the density of photons per unit area on that sphere is shrinking as in proportion to the inverse square of time.

So, no photon is "cooling down". It's simply that the density of photons per unit of area that are hitting you, and therefore the total heat energy density per unit area that you are receiving from that star, is really, really tiny.

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  • $\begingroup$ This argument fails, because if the number density of stars is uniform, then the increasing number of stars at a given distance exactly cancels this out. $\endgroup$
    – ProfRob
    Mar 1 at 7:01
  • $\begingroup$ Would it not be more accurate to say that either my argument fails (which is derived from the simplest laws of physics, and so those arguments fail), or the assumption of uniform density of stars in the universe fails? $\endgroup$
    – Lee Mosher
    Mar 1 at 13:26
  • $\begingroup$ Or, perhaps, what this argument shows is that the density per unit area of stellar photons hitting Earth is proportional to the density of stars in the universe, which is really, really tiny. $\endgroup$
    – Lee Mosher
    Mar 1 at 14:15
  • $\begingroup$ Well the density only has to be uniform on large scales. Every sight line would end up at the surface of a star and the sky would be as bright as the Sun. Olber's paradox is well known. $\endgroup$
    – ProfRob
    Mar 1 at 20:21
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As others have said, the earth gets a tiny amount of heat from the stars, but it is almost nothing. But I just wanted to emphasize that it is not because the light is "cooler" - it's just because there is so little of it.

Light heats things up when it is absorbed. Think about a light bulb in a room. It's easily bright enough to see by, but it doesn't warm you up noticeably unless you get really close. This is true even though an old fashioned incandescent light bulb gives off both visible and infrared light.

A starlit night is so dim you can barely see. Just as the faraway stars are much much less bright to us than a nearby light bulb, they also give us much much less heat.

It is true that light can be "cooler" in the sense that cool things don't give off as much short-wavelength light. (Radio waves have long wavelengths, while the wavelengths for visible light are quite short, and gamma rays are the shortest of all.) But anything giving off visible light is already pretty hot.

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Of course the earth gets heat from other stars. It has to. Heat is, in actuality, kinetic energy from vibrating molecules that radiate heat waves and stars radiate that energy in all directions. Since it is kinetic energy, it is movement. That movement propagates through space. Law of inertia keeps that energy radiating forever. However, unlike a definite sized piece of mass that retains its constant velocity and it's mass, thus retains it's kinetic energy directed to a point, heat waves radiate through an ever increasing radius from the source, thus reducing the actual amount of kinetic energy that reaches a destination in space. Obviously, the further that destination is away, the larger the radius of the spherical heatwave and the more the kinetic energy is dispersed, so the further something is away, the less heat the destination will receive. But it won't ever reach a temperature of absolute zero. If you removed the sun, earth would not reach absolute zero. You would have some amount of Kelvin worth of thermal energy from other stars heating the earth. Obviously not NEAR as much as what comes from the sun, but a little bit. Given the nearest star is 4 light years away, it's not hard to see how little of that heat actually reaches earth. Definitely not zero, though.

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    $\begingroup$ Garbled wall of text that mixes up several physics concepts in a wrong way to arrive at a correct conclusion that others have already given... so -1. $\endgroup$ Feb 27 at 4:15

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