Space - What is the Average Temperature?

Q.

What is the average temperature of space outside earth's gravitational influence. If we believe heat is transmitted by three means, how do heat or temperature exist in space without the medium being present there?

A.

The average temperature in space – in fact, ANY temperature in space – was not something that could be detected many years ago. With the understanding of what temperature is and the ability to measure it, along with some fancy equations, we can now answer this question.

The average temperature in space is just a bit above Absolute Zero, the temperature at which molecules stop moving. On the Kelvin scale, which is the best to measure space temperature because it starts at Absolute Zero, the average temperature of space is 2.725. (Keep in mind that 0 degrees Kelvin is Absolute Zero.)

On more commonly used temperature scales this equates to approximately -270 Celsius and -455 Fahrenheit.

Considering that the temperature of our sun can reach 10,000 Kelvin, it may seem incredible that space can get as cold as 2.725 Kelvin, until one factors in the vast distances there are in space. What we call temperature is simply a measurement of the activity of molecules, and deep, deep space is very, very empty and far from the radiation of stars, giving those few molecules the chance to slow down to near stillness.

Since there is no thermometer long enough to reach out past our solar system (which at its outer rim has an average temperature much higher than deep space, about 40 Kelvin) how can we measure the temperature of space? By measuring radiation using something called Plank’s black body radiation law. “Black body” here refers to a perfect absorber, and deep, empty space is the most perfect black body there is.

In 1894 physicist Max Plank was hired by an electric company to increase the output of light from a light bulb while decreasing energy, and his attempts lead to the discovery that every object radiates electromagnetic energy according to its temperature. It wasn’t until 1965, however, that this law was used to measure deep space.

It was then that Arno Penzias and Robert Wilson tuned some communications equipment to microwave wavelengths, seeking a reduction in “noise” – instead they discovered quite a bit. Eventually they tried pointing their antenna into space and tuned into what is called CMB, Cosmic Microwave Background – the background noise of the universe. It was in measuring this radiation – spawned noise that scientists were able to determine the average temperature of space.

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