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

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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 '15 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 '15 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 '15 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 '15 at 13:48
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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. $\endgroup$ – Wayfaring Stranger Feb 11 '15 at 14:38
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    $\begingroup$ @WayfaringStranger: There's also enough energy to trigger the cells in my retinas. $\endgroup$ – Keith Thompson Feb 11 '15 at 19:18
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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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