When I get a spectrum from SOHO, I have to decide what elements the emission lines come from. In other words I need to know whose fingerprints they are. After a while you get to know the most common wavelengths. You recognise them like old friends.
Here are some fingerprints from iron, silicon and magnesium.
The symbol Fe+15, means that an atom of iron (Fe), which normally has 26 electrons orbiting the nucleus, has been heated so hot that fifteen of those electrons have been stripped off. That leaves the iron ion with a positive charge of 15, hence the +15.
If I see a peak from Fe+15 in the spectrum, this means that somewhere the gas is really hot - two million degrees HOT!
At this temperature, you would not recognise the iron because it becomes a gas (or plasma).
Here is a plot of the temperatures you need to strip off different numbers of electrons from oxygen.
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