recognising the fingerprints

Rollover this plot of a solar spectrum from the SOHO-CDS instrument to identify some emission lines.

The symbol Fe+15 means this wavelength is emitted by iron (Fe), which would normally have 26 electrons, but which has been heated up so much that 15 of its electrons have escaped. As there are still 26 protons in the nucleus, this ion of iron has a charge of +15, so we write it as Fe+15.

To get 15 electrons to break free from an iron atom means it has to be unimaginably energetic and hot: at least 2,000,000 °C. At such temperatures, no atoms of any element can exist as an ordinary gas and instead they become ionised. They form the fourth state of matter after solids, liquids and gases, which we call plasma.