## 5 Possible Connections with Some Processes Occurring in Space  
There has also been evidence that NV centers in NDs could be at the very origin of a long-lasting, puzzling, visible (red) emission observed in particular classes of nebulae, in particular, carbon-rich planetary nebulae. This is the so-called extended red emission (ERE) that has been a literature topic since the 1980s [128; 129; 130]. In Figure 20 we report the spectrum of the extended red emission (ERE) from the so called red rectangle (right panel), from [129], collected at different observation angles and compared with a typical spectrum of crushed CVD NDs with an average tail \(\approx\)40 nm. As can be seen, the ERE emission from the red rectangle (which is 25 times more intense than the other ERE nebular emissions, probably explaining why in all other lower signal strength spectra, they have not been resolved) is compatible with the photoluminescence spectrum of commercial uncoated Bikanta [131] 40 nm-tailed NDs, if the VIS (532 nm) pumping redshift with respect to far-UV pumping is taken into account, as was done in [131]. In particular, it is evident how the zero-phonon-line (ZPL) of the NV\({}^{-}\) emission is present both in the ERE as well as in the crushed commercial NDs, thus suggesting a significant VIS pumping versus the far-UV pumping of the NDs causing the ERE emission from the red rectangle [128; 132].  
Nanometer-tailed fluorescent NDs produced via PLA techniques and the proposed thermodynamic model for their formation can contribute to interpretation of ERE space emission because in the PLA technique, high pressure, high temperature, and fast cooling conditions are controllable quantities that permit us to infer the circumstellar environmental conditions. Figure 21 gives an idea of the relevance of this type of cross-analysis.  
Figure 20: Comparison between a typical photoluminescence spectrum of 40 nm crushed NDs obtained under 532 nm optical pumping and the extended red emission (ERE) emission from the red rectangle. The peak at 637/638 nm is strongly reproducible and was first observed in [128]. Its compatibility with the zero phonon line of the NV\({}^{-}\) center in NDs is evidenced with a vertical line. A better, longer wavelength, PL-tail compatibility is expected under far-UV irradiation, as from Reference [132]. The peak at around 580 nm could be correlated with the NV\({}^{0}\) zero phonon line. Data of ERE (blue and red curve) are reproduced from [129].  
Moreover, in a recently published paper regarding the liquid state of carbon [134], it was pointed out that while the temperature-pressure conditions to have liquid carbon are hardly ever found on earth, this is not the case in stellar and planetary interiors [135, 136], where the existence of the liquid state is quite probable. Indeed, as pointed out by [PERSON] [136], the thermodynamic conditions in the liquid central layers of, for example, Uranus and Neptune planets, favor the pyrolysis of methane gas into a mixture of elemental carbon and hydrogen. In the PLA synthesis of NDs [41, 64, 65] the conditions to generate liquid carbon from which NDs are formed are investigated fairly well in controlled experimental conditions. These studies then allow us to generate laboratory conditions that help to formulate plausible hypotheses in relation to some astrophysical processes.  
Finally, a comprehensive list of papers on regarding the liquid state of carbon in stellar and planetary interiors, and possible nanodiamond-related origin of ERE can be found in References [135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145].