A planetary nebula is a shell of gas usually with a fairly high degree of symmetry, surrounding a star and not associated with interstellar matter. Spectroscopic measurements show that the shell is expanding with a velocity on the order of 50 km/sec (30 mi/sec). The central star is highly evolved; its hydrogen has been converted to heavier elements, and it is contracting and cooling to become a white dwarf star, leaving behind the expanding shell of gas. Most single stars of approximately one solar mass probably become planetary nebulae at the end of their active lives; the Sun will do so in about 5 billion years.Planetary nebulae are unrelated to planets, except that because of their small angular sizes they might be confused with planets if observed through a very small telescope. Planetary nebulae have emission-line spectra, generally similar to the spectra of galactic nebulae, except that in many cases the planetary nebulae exhibit a higher degree of ionization. While in a typical galactic nebula the singly ionized oxygen (O§) emission line is stronger than the doubly ionized oxygen (O§§) lines, in most planetary nebulae doubly ionized oxygen is stronger, and ionized neon (Ney§ and Ne§§ is often seen. This is because the central stars in planetary nebulae often have quite high temperatures, up to 250,000 K, and emit high-energy ultraviolet radiation that can photoionize these atoms.