Temperature Of Space

[rose]

According to a discussion I read, 'The temperature in space is close to "absolute zero" because any object there will radiate heat until it cools to that point.' But how to measure the temperature of Space? 

[CraigD]

... I read, 'The temperature in space is close to "absolute zero" because any object there will radiate heat until it cools to that point.'

Before we can sensibly ask “what it the temperature of space”, we need a useful definition of “temperature” and “space”. Unlike basic physics terms like mass, position, and velocity, temperature has multiple common definitions, while space can be ambiguous.

 

The most straightforward way I can think of define them is as follows:

• space – what’s between planets and stars – is a very low-density gas. A better name than “interstellar space” (let’s ignore the interplanetary region, since there so much less of it than the interstellar) is “the interstellar medium”
• temperature is the average kinetic energy of a collection of particles. For particles that aren’t moving at extreme speeds – which is the case in most of space - kinetic energy is well approximated as mass times speed squared (m v²).

So “the temperature of space” is just the average kinetic energy of the particles in the interstellar medium. Most of the particles in the interstellar medium are the components of hydrogen – protons and electrons, either bound together into neutral (rather than ionized) hydrogen molecules, or free, which is called ionized or plasma.

 

Definitions down, what’s the temperature of the interstellar medium? It varies.

• In areas where it’s most dense (about 100 to 1,000,000 times average), the “stellar nursery” clouds, it’s its coldest, 10 to 20 K (degees C about absolute zero).
• In the largest volumes near the galactic plane, it’s much higher, 6,000 to 10,000 K.
• In the lowest density (1/100 to 1/10,000 time average) region, the spherical corona surrounded the galactic disk, it’s highest, as high as 10,000,000 K, about the same as the surface of the Sun.

I’d be remiss if I didn’t mention the cosmic background radiation. As it’s made of photons, not bodies like protons and electrons it’s sensible to say have kinetic energy, it’s not sensible to say it has a temperature as defined above. However, we can precisely calculate the temperature a body would have to be to emit it: about 2.7 K. This is essentially the coldest a body in space can be, unless it’s specially shielded from background radiation.

 

But how to measure the temperature of Space?

Simply put, it can be measured by how the interstellar medium absorbs and emits photons, as simply as the spectral line its neutral hydrogen emits, or as complicated as how it delays lower frequency light from other objects. In detail, this is a complicated subject.

[smalfry]

Very interesting! I have to admit I have never really thought about the temperatue or space. Craig- that is a lot of information and I have to say that I totally confused me. I'm glad in the end you wrote that it is a complicated subject. I am more intriqued now about the whole question.

[PiSquare]

I have always been intrigued by the concept of space. One simple but powerful definition I found on the space.com website is the following: "...Space is a vacuum, meaning that sound cannot carry because molecules are not close enough together to transmit sound between them. That's not to say that space is empty, however ...".

 

This (partial) definition points out two important aspects. Firstly, space is a vacuum, yet space is not empty. My concept of a vacuum (as I have been taught in school) was that it contains absolutely nothing.

 

Secondly, sound cannot carry in space. It might be a matter of logic to scientists, but it was quite a revelation to me simply because I never thought of whether sound is able to propagate through space.