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 2005bc of the supernova at around −18.6 at maximum, rather faint for a Type Ia event. As the supernova was of Type Ia, its progenitor star was a White Dwarf star which exceeded the Chandrasekhar limit. Below is an image of SN 2005bc and NGC 5698 taken just over a week after peak brightness. SN 2005bc SN 2005bc was a Type Ia supernova occurring in the Barred spiral galaxy NGC 5698 in the constellation of Boötes. SN 2005bc was discovered on 2 April 2005 by LOSS and independently by Tim Puckett and L. Cox. The supernova was at magnitude 16.4 at