If space is a vacuum, how is the temperature in space on average °2.7 Kelvin and not °0 Kelvin?
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Space is not a perfect vacuum there are atoms everywhere, even in intergalactic space we talk about 10 atoms per cubic meter. Vaccum often just mean significantly lower pressure then atmospheric pressure on earth.
There is no degrees in Kelvin so it is 2.7 Kelvin, not °2.7 Kelvin. It refers to one single thing the Cosmic microwave background. That is the temperature of an object that emits the same spectrum of black body radiation as the Cosmic microwave background. So if you place an object between galaxies that it the temperature it will reach. The temperature drops until the incoming radiation is equal to the emitted radation.
Space does not need to be cold, it can be very hot. The warmest part of earth atmosphere is the Thermosphere which is 80-600km up. The pressure is so low we call it vacuum this is where ISS orbits earth. The temperature of the gas here can reach 2500C. But up the number of atoms and therefore the amount of energy per unit of volume is very low.
So incoming energy from the sun and radiate energy out to space is the major factor in determining the temperature of an object, the very thin atmosphere have a minuscule effect. If you are in direct sunlight you will be warmed then if you are on the surface of the earth, the sunlight is stronger because there is no atmosphere to reduce the intensity, there is also not air around you that can cool you down. If you are in shadow it is quite cols. So for ISS the sunny side can be 120C and the shades side -157C at the same time.
The solar corona which is a thing plasma around the sun can have a temperature in excess of 1 000 000 kelvin. That is at pressure we consider vacuum.
So there is not any perfect vacuum the 2.7K is the black body temperature of the Cosmic microwave background. The temperature you have in space will primarily be determined by where you are relative to other objects like the sun. The temperature of the thin gas around and the Cosmic microwave background do not have a huge effect if you are as close to the sun as Earth is.
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