Re: How is the Temperature of outer space taken?

Hi Danielle,

First of all, let me refer you to the answer to another question, on temperatures in space. That posting talks a bit about some of the many different temperatures that exist in different conditions in space.

We don't actually measure a temperature of space itself. Temperature is associated with motions of atoms and molecules, and so it doesn't have any real meaning for the vacuum of space. We can, however, talk about the temperature of atoms in space, and the temperature of radiation (light) travelling through space.

Because we don't actually travel far into space, we must make temperature measurements using spectra--essentially, we pass light through a prism (or something similar), and see how much light is coming to us from different wavelengths.

Light from any hot, dense, glowing object (the Sun is a good example) follows what we call a continuous spectrum. That means that we see light at all wavelengths, the full rainbow. More light comes at some wavelengths than others, however, and the wavelength at which most of the light comes out tells us the temperature of the emitting object. The hotter the object, the bluer the light that it emits. If you were travelling in the Solar System, for example, you would be bathed in light coming from the Sun, which has a temperature of about 6000 K.

Another measure of the temperature in space is how quickly the atoms and molecules that fill space are moving around. To measure this, we also use spectra. The gas in space, however, doesn't emit a continuous spectrum, but instead emits emissions lines. That means that we see light at only certain, specific wavelengths, that depend upon what atoms are present, and how hot the gas is (you may also want to look at an earlier posting on spectra).

A simple way to think of an atom (actually oversimplified, but very useful) is to imagine electrons orbiting the nucleus, somewhat like the planets orbiting the Sun. Unlike the planets, however, the electrons can only have certain orbits. To move an electron from an orbit close to the nucleus to one further away takes energy, either from a collision with another atom, or by absorbing a photon. An electron in an orbit far from the nucleus, however, will want to "fall" to one that is closer. It emits energy as it does so, which comes out in the form of one or more photons (one photon for each "jump" the electron makes from one orbit to another).

If the difference in energy between two orbits is larger, then a more energetic (bluer) photon is emitted when the electron jumps from the higher to the lower orbit. Because the spacing of the orbits is different for different atoms, we can use the spectra to determine the chemical makeup of a gas that emits an emission-line spectrum.

We can also use the spectrum to measure the temperature of the gas. In a "typical" interstellar gas cloud, atoms are ionized (electrons are stripped from the atoms) when they absorb ultraviolet photons. The free electrons then collide with other atoms, heating the gas. Some electrons will collide with atoms that have lost one or more electrons, and will "recombine," that is they will become bound to the atom, emitting photons in the process. Other collisions will excite electrons in atoms to higher orbits, and they will emit photons as they "fall" back to lower orbits. The photons that are emitted by these processes are lost to the gas (a few end up in our telescopes), and help to cool the gas.

To measure the temperature of the gas, we could, for example, look at how many photons we are receiving from atoms of a given chemical element that have lost different numbers of electrons. More electrons being stripped means a hotter gas. We can also look at photons coming from transitions within the atoms. If we see more transitions that result from electrons that start at higher levels, then it means that the atoms are more highly "excited," that is, the gas is hotter, and so more electrons are being "kicked up" to higher energy levels by collisions.