This is an expanded version of a Twitter thread from 2017. Spectral lines are listed in order of increasing wavelength. If the Einstein coefficients vary across a multiplet, I usually give the largest one. Citations are included purely for fun; i.e., they're subjective, highly weighted toward people that I know, and not meant to be representative of the scientific literature.
| λ (Å) | Source | A21 (s-1) | Description |
| 0.0243 | e- e+ | --- | Maximum possible wavelength of gamma rays that result from electron-positron annihilation. |
| 1.94 | Fe K-shell | --- | Hard X-ray "line," made by multiple ionization stages of iron in very hot plasmas. In quasars, its shape can be morphed by general relativity. |
| 13.4, 13.5, 13.6 | Ne IX | 4.18e9 | Helium-like X-ray triplet observed in stars by Chandra; used to probe variability in astrophysical shocks, and can also be used to measure both electron density and electron temperature. |
| 21.6, 21.8, 22.1 | O VII | 3.40e12 | X-ray triplet observed in stars by Chandra; used by Brickhouse et al. (2010) to discover weird turbulent hot-spots on T Tauri star TW Hya. |
| 131 | Fe XXI | 1.17e10 | SDO/AIA images the Sun in this "hot" emission line, which is sensitive to temperatures up to about 13 million K. Thus, it's a good line to use for flares, coronal mass ejections, and reconnection current sheets. |
| 171 | Fe IX and X | --- | Narrow bandpass in which lines from several ionization stages overlap, all emitting at temperatures around 1 million K. On the Sun, it highlights coronal loops quite nicely. This makes it a good target for fancy off-limb image processing (see, e.g., Gilly and Cranmer 2025). |
| 195 | Fe XII | 8.67e10 | The solar corona shines brightly in this emission line. In 1996 and 1997, SOHO people at Goddard would crowd around monitors to kibbitz about each newly discovered feature in these images. |
| 284 | Fe XV | 2.29e10 | The Sun emits this EUV coronal line, and EIT on SOHO produced lots of nice images. Unfortunately, AIA/SDO decided not to use it. :-( |
| 304 | He II | 1.003e10 | Strong resonance line of hydrogen-like helium. Seen by SDO/AIA on the Sun, it highlights gorgeous loops and prominences, but its atomic physics still is a bit of a mystery. |
| 584 | He I | 1.80e9 | Emission from this line is bright in one particular spot on the sky. Why? Some interstellar helium atoms fly near the Sun and become "focused" by its gravity (Michels et al. 2002). |
| 610, 625 | Mg X | 7.73e8 | UVCS/SOHO was barely able to detect emission lines from this ion in faint coronal holes (Kohl et al. 1999). I think it's a holy grail for next-generation spectroscopy missions to target! |
| 974 | [Fe XVIII] | 1.94e4 | I don't think it's common to see forbidden lines in the EUV like this, but UVCS/SOHO saw it flash in a thin "current sheet" connecting a solar coronal mass ejection with loops on the Sun's surface (Ciaravella et al. 2002). |
| 977 | C III | 1.77e9 | This line is seen in many hot plasmas, but it was surprising when Povich et al. (2003) observed it in a cold comet. Such double ionization was unexpected! |
| 1032, 1038 | O VI | 4.17e8 | Strong resonance doublet in the Sun's spectrum. UVCS/SOHO used it to diagnose 100 million K ion temperatures in the corona (Kohl et al. 1997). |
| 1036, 1037 | C II | 1.53e9 | Weak doublet due to ionized carbon. In the solar spectrum, it's not that useful on its own, but it lets us exploit an exotic "Doppler pumping" effect with O VI to detect non-Maxwellian particle distributions (see above). |
| 1160-1180 | S I | --- | "Autoionization" causes a number of weird features in the Sun's spectrum. Their origin was a mystery until Avrett, Kurucz, and Loeser (2006) figured it out. |
| 1216 | H I | 6.26e8 | The mighty Lyman alpha line, brightest emission line in the Sun's spectrum. Used to diagnose properties of the solar wind (see, e.g., Cranmer 2020), interstellar medium, and an intergalactic "forest" of gas clouds. |
| 1394, 1403 | Si IV | 9.06e8 | Resonance doublet, often exhibiting "discrete absorption components" (DACs), that we once used Hubble to observe -- and diagnose the properties of spiral-shaped gas streams -- around Be-star gamma Cas (Cranmer et al. 2000). |
| 1548, 1550 | C IV | 2.76e8 | Resonance doublet, first observed by the Copernicus satellite (Morton 1976), that's sensitive to stellar-wind outflows from the most massive stars. |
| 1909 | C III] | 1.04e2 | Semi-forbidden UV line of doubly ionized carbon. When redshifted into the visible, it helps diagnose quasar morphology. |
| 2796, 2803 | Mg II | 4.69e8 | Resonance doublet called "h" and "k" via analogy to Fraunhofer's H and K (see below). NASA's IRIS mission uses them to detect "cool" (30,000 K) spicules and jets in the Sun's chromosphere. |
| 3388 | [Fe XIII] | 7.37e1 | Weak forbidden line, sometimes seen in the solar corona during eclipses. Rare to spot on other stars, but it was seen from Proxima Centauri (Fuhrmeister et al. 2011)! |
| 3869, 3967 | [Ne III] | 1.65e-1 | Together with [O II] 3727, useful lines to diagnose the properties of H II regions. A class project at OSU led to one of my first papers! |
| 3934, 3968 | Ca II | 1.39e8 | Strong violet doublet (Fraunhofer's H and K lines) in the Sun's spectrum. Best way to see shocks and bright magnetized "plage" in stellar chromospheres. |
| 4032-4297 | Tc I | Lines from the rare element technetium were discovered by Merrill (1956) in red giants prior to others identifying it in natural sources on Earth. | |
| 4144 | Fe I | Humble iron absorption line (one out of thousands) that happens to be used as a spectral-type indicator for G-type (Sun-like) stars. | |
| 4295-4315 | CH | --- | Fraunhofer's G-band, dominated by hundreds of methylidyne molecular lines. Great place to spot tiny intergranular bright points in the Sun's photosphere. |
| 4471 | He I | 1.88e7 | Strong "indigo" absorption line in B-type stars. Line shapes probe how fast stars are spinning; some so fast they flatten at the poles (Cranmer 2005)! |
| 4686 | He II | 2.21e8 | Used as a luminosity-type indicator for O and B type stars. Also probes mass transfer in extremely-close binaries. |
| 4862 | H I | 2.06e7 | Balmer H-beta line (Fraunhofer's cyan F line). Useful for measuring gas dynamics on the Sun (fibrils and filaments), stars, and galaxies. |
| 4959, 5007 | [O III] | 2.05e-2 | Pale green lines emitted by colorful astrophysical nebulae. In the 19th century, some thought this was due to an undiscovered element ("nebulium"), but Bowen (1927) figured out why they were so bright. |
| 5167, 5173, 5184 | Mg I | 5.6e7 | Magnesium triplet (Fraunhofer's "b"); its scattering polarization is used for magnetic activity diagnostics in the solar chromosphere (Li and Qu 2015). |
| 5303 | [Fe XIV] | 5.52e1 | Brightest "green line" seen in total eclipses. In the 1800s, Lockyer thought its emission was due to a new element ("coronium"). SOHO's LASCO/C1 instrument observed it in detail (Schwenn et al. 1997). |
| 5461 | Hg I | Yellow-green line of the mercury vapor lamp. Useful for lab calibration; biologists illuminate cells with it. | |
| 5577 | [O I] | 1.34e0 | Forbidden neutral oxygen line; previously thought to be due to an uknown element ("aurorium"). It's the main reason aurora borealis is bright green. |
| 5694 | [Ca XV] | 9.56e1 | Yellow "coronium" line from solar flaring/active regions. When NSO sees it, they issue a "coronalert!" |
| 5876 | He I | 5.29e7 | Janssen and Lockyer discovered helium (from "Helios") in solar eclipse spectra, 27 years prior to anyone identifying it on Earth. Informally called the "D3" line; see below. |
| 5890, 5896 | Na I | 6.16e7 | Fraunhofer's D lines (also called D1 and D2). Causes yellow-orange glow of sodium vapor lamps, and the deepest lines in the brightest part of the Sun's spectrum. |
| 6173 | Fe I | Solar absorption line used by SDO/HMI to measure high-quality maps of the Sun's magnetic field ("magnetograms"). | |
| 6200-6820 | SrOH | --- | Strontium salts have many transitions at these wavelengths; thus they're traditionally used in red fireworks. Victorian-era magicians used them for spooky crimson flames. |
| 6300 | [O I] | 6.3e-3 | Forbidden line, causes some red aurora borealis ("aurorium?"). In T Tauri stars, blueshifts in this line detect polar winds/jets (Hartigan et al. 1995). |
| 6374 | [Fe X] | 6.94e1 | Coronal "red line" seen during eclipses. |
| 6563 | H I | 6.46e7 | Balmer H-alpha line (Fraunhofer's C line). It's why the Sun's CHROMOsphere is so colorful. Many uses in astronomy. Google "Coronado telescopes" for a whole product line (now defunct?) based around this transition. |
| 6702 | [Ni XV] | 5.68e1 | Forbidden emission line seen in the solar corona during eclipses. Mimics its brighter cousins, but it's just cool to see nickel in space! |
| 6708 | Li I | Red doublet used for determining stellar ages. As stars get older, primoridial lithium sinks down and the line gets weaker. | |
| 6768 | Ni I | Solar absorption line used by NSO's GONG network to measure high-quality maps of the Sun's magnetic field ("magnetograms") and Doppler-based helioseismology. | |
| 6867 | O2 | --- | Fraunhofer's B line. "Telluric" molecular absorption caused by Earth's atmosphere. |
| 7000-7500 | C55H72MgN4O5 | --- | Not a single line, but chlorophyll makes a "red edge" that may be useful to search for life on exoplanets (Seager et al. 2005)! |
| 7136 | [Ar III] | 3.24e-1 | Combine this forbidden line with [S III] 9069, and you can measure temperatures of galaxies. |
| 7594 | O2 | --- | Fraunhofer's deep-red A line. "Telluric" molecular absorption band, caused by Earth's atmosphere. |
| 7892 | [Fe XI] | 4.39e1 | Deep red coronal forbidden line observed during total eclipses. Habbal et al. (2007) found it to be extra sensitive to ion "pileup" in helmet streamers. |
| 8498, 8542, 8662 | Ca II | 9.40e6 | Calcium infrared triplet; sensitive to chromospheric activity, sunspot magnetism, and energetic flares on other stars. |
| 9069, 9532 | [S III] | 5.16e-2 | Near-IR doublet; used when scanning planetary nebulae and Seyfert galaxies for heavy elements. |
| 10747, 10798 | [Fe XIII] | 1.40e1 | Forbidden density-sensitive ratio. Emission observed above solar limb by CoMP and DKIST/Cryo-NIRSP (Molnar et al. 2026). |
| 10830 | He I | 1.15e7 | Metastable triplet. Detects wind/accretion flows on other stars (Dupree et al. 2014); coronal holes on Sun. Strange atomic physics sensitive to UV/X-ray irradiation. |