Helium Ions (He II) in the Sun's Atmosphere
All stars are made up mostly of hydrogen and helium. Since atoms in the Sun's atmosphere are extremely hot, they move around very, very quickly. The atoms often collide, and such collisions can knock electrons loose from an atom. Atoms with missing (or extra!) electrons are called ions. A helium (chemical element symbol He) atom that is missing one electron is called He II. Normal, neutral helium atoms that still have both electrons are called He I.
Under conditions (temperatures around 60,000 to 80,000 kelvins) that exist in the Sun's atmosphere, helium atoms are ionized to form He II ions. These ions emit extreme ultraviolet (EUV) radiation at a wavelength of 30.4 nm (304 Å). Radiation at this wavelength allows scientists to see structures and processes in the transition region - a special area in the Sun's atmosphere. The transition region is the boundary between the Sun's lower atmosphere (chromosphere) and its upper atmosphere (corona). These high energy UV emissions are (thankfully!) blocked by Earth's atmosphere, so scientists must use orbiting solar telescopes on satellites above the atmosphere to view the Sun at this wavelength.
Helium is a very rare element on Earth. Because it is lighter than air, helium is used to inflate balloons. Helium holds on to its electrons very strongly, making it extremely difficult to ionize. As a result of this, helium does not react easily with other chemicals.