Chemical element isotopes: specifically the elements without any stable isotopes are technetium (atomic number 43), promethium (atomic number 61), and all observed elements with atomic numbers greater than 82. Of the 80 elements with at least one stable isotope, 26 have only one single stable isotope. The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element. The largest number of stable isotopes that occur for a single element is 10 (for tin, element 50). The mass number of an element, "A", is the number of nucleons (protons and neutrons) in the atomic nucleus. Different isotopes
SN 2003gd SN 2003gd, and which is believed to be the supernova's progenitor star. This progenitor star was a red supergiant, consistent with the expectations of existing single-star stellar evolution models. It is the first progenitor of a normal type II-P supernova to have ever been detected. SN 2003gd SN 2003gd was a type II-P supernova occurring in the spiral galaxy Messier 74 in the constellation Pisces. SN 2003gd was discovered on 12 June 2003 by Robert Evans, using a 0.31m reflector, and its discovery was confirmed on 13 June 2003 by R. H. McNaught using the 1.0m telescope at the Siding