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Ultra-violet astronomy

Ultraviolet (UV) photons carry energies of a few to about 100 electron Volt (eV). They are thus somewhat more energetic than optical photons (around 1 eV), but less energetic than X-ray photons (above 100 eV).

UV continuum emission

There are two primary sources of UV continuum radiation. Active Galactic Nuclei (AGNs) emit nonthermal UV. This light is produced as part of the radiation from the material in the surroundings of a massive black hole. The second class of UV emitters, massive stars, emit thermal UV light. In their case the energy of the radiation is a measure of their surface temperature. An example of such stellar UV continuum radiation is displayed in the figure below, which shows a UV image of the starburst galaxy NGC 4666 with overlaid contours of X-ray emission.

Prominent UV lines

The UV continuum emitters mentioned above, namely massive stars and AGNs, also emit UV lines. So does hot interstellar gas. Some of the most important diagnostic lines in the UV are those of triple-ionised carbon, C3+ or [C IV], of [N V] (four-times ionised nitrogen, N4+) and [O VI], i.e. five-times ionised oxygen (O5+). These trace inoised gas (plasma) with temperatures in the range from 100000 to 300000 K.

Not only emission, but also absorption of UV photons is observed. All emission lines from highly-ionised species that we observe imply that these have been ionised by something in the first place. Which means that energy has been absorbed. The absorption of UV photons is one of the dominant processes of ionisation of interstellar gas. Quite logically, this process is called "photo-ionisation".

A selection of UV observatories is presented on a separate page.