<P> However, when Huggins looked at the Cat's Eye Nebula, he found a very different spectrum . Rather than a strong continuum with absorption lines superimposed, the Cat's Eye Nebula and other similar objects showed a number of emission lines . The brightest of these was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known element . </P> <P> At first, it was hypothesized that the line might be due to an unknown element, which was named nebulium . A similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868 . While helium was isolated on Earth soon after its discovery in the spectrum of the Sun, "nebulium" was not . In the early 20th century, Henry Norris Russell proposed that, rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions . </P> <P> Physicists showed in the 1920s that in gas at extremely low densities, electrons can occupy excited metastable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur at higher densities . Electron transitions from these levels in nitrogen and oxygen ions (O, O (a.k.a. O iii), and N) give rise to the 500.7 nm emission line and others . These spectral lines, which can only be seen in very low density gases, are called forbidden lines . Spectroscopic observations thus showed that nebulae were made of extremely rarefied gas . </P> <P> The central stars of planetary nebulae are very hot . Only when a star has exhausted most of its nuclear fuel can it collapse to such a small size . Planetary nebulae came to be understood as a final stage of stellar evolution . Spectroscopic observations show that all planetary nebulae are expanding . This led to the idea that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life . </P>

Where do atoms in nebulae get energy from
find me the text answering this question