# Are there any spectral lines with smaller frequencies than the hydrogen line?

The 21 centimeter hydrogen line originates from the hyperfine transition of neutral hydrogen. Are there any bigger wavelengths that originate from this method? Radio emissions from rotating stellar objects are not part of this category; clarifying in case this is brought up.

• Welcome to Astronomy SE. Are you asking about spectroscopic measurements only? Aug 19 '21 at 20:14
• Yes. I’m asking about the spectroscopic measurement and the origin of that spectra emission. Aug 19 '21 at 20:20

For example Peters et al. (2011) discuss "decametre" wavelength transitions, and some of these have been detected, mainly associated with carbon ions (e.g. the C631$$\alpha$$ radio recombination line at 26 MHz $$=$$ a wavelength of 11.5 m).