Promethium is the second element after technetium, of which no stable isotopes exist. However, it is much more unstable than this, the most long-lived isotope, 145Pm, has a half-life of just under 18 years. The only natural isotope, 147Pm, has a half-life of only about two and a half years. This on Earth occurs only in traces, most of it is from nuclear waste. There are some few, very special applications for promethium, e.g. in radioisotope thermoelectric generators. Very few stars contain promethium, which they seem to produce themselves, however they may do that.