= Solution
A <planetary nebula> combines a hot post-asymptotic-giant-branch stellar core, its ionized expelled envelope, neutral/molecular material and <interstellar dust>. Its ultraviolet-to-infrared <spectral energy distribution> is therefore not a single-temperature blackbody. The principal components are:
* \b[Central-star continuum.] A hot stellar photosphere emits strongly in the ultraviolet, especially the ionizing extreme ultraviolet, with its longer-wavelength tail extending through the optical and near infrared. Stellar-atmosphere absorption features and wind lines can be superposed. A companion can add a distinct cool stellar continuum.
* \b[Nebular continuum.] Thermal <bremsstrahlung>, <nebular free-bound continuum> emission and hydrogen two-photon emission produce a broad continuum. Free-bound edges, especially the Balmer discontinuity at 364.6 nm and the Paschen discontinuity near 820 nm, can be apparent. Hydrogen two-photon decay redistributes the Ly-alpha transition <energy> into a continuum of pairs rather than one 121.6-nm <photon>.
* \b[Gas emission lines.] Hydrogen and helium <recombination lines> include H-alpha, H-beta and He II 468.6 nm. Collisionally excited lines of metal ions can dominate individual bands: examples include optical \[O III\] 495.9/500.7 nm and \[N II\] 654.8/658.3 nm, ultraviolet C III\] near 190.9 nm, and infrared fine-structure transitions such as \[Ne II\] 12.8 micrometres. A broad-band photometric peak may therefore be a line rather than continuum.
* \b[Dust emission and solid-state structure.] Grains absorb stellar/nebular radiation and reradiate it as a broad infrared continuum, often from a distribution of <temperatures> of order tens to hundreds of kelvin; compact hot dust can add near-infrared emission. Carbon-rich material may show aromatic bands around 3.3, 6.2, 7.7, 8.6 and 11.3 micrometres, while oxygen-rich material may show silicate features near 9.7 and 18 micrometres. These are compositional alternatives, not mandatory features of every nebula.
* \b[Neutral and molecular emission.] Surviving molecular material in dense clumps or a photodissociation region can contribute infrared rotational/rovibrational lines, notably molecular-hydrogen emission such as the 2.12-micrometre line. Shocks as well as ultraviolet pumping can excite it. Colder dust continues into the far infrared, and low-lying CO rotational lines extend the overall spectrum into the submillimetre/millimetre range.
\Image[/past-exam-of-the-astrophysics-course-of-the-university-of-oxford/2015-planetary-nebula-spectral-components.png]
{title=Schematic stellar, nebular and dust components of a planetary-nebula spectrum; arbitrary relative normalizations}
The observed mixture also contains attenuation by internal and foreground <interstellar extinction>. A spectrum containing all these components need not spatially coincide: stellar light, ionized gas, dusty clumps and the molecular envelope sample different regions.
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