These spectra are dominated by the [C ii] line from the main isotope and show the weak hyperfine component of 13C+ near vLSR = 20 km s−1. This line is used to estimate the optical depth of the main isotope line and thus the mass of the emitting gas. The red spectrum corresponds to the area between the two large circles in Extended Data Fig. 8, but excluding Huijgens region in the small circle. The blue spectrum is an average over the bright parts in the eastern shell, in the declination range −5° 35′ to −5° 45′. The inset shows a close-up of the (faint) [13C ii] line in the average shell spectrum.