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 2008ha SN 2008ha SN 2008ha was a type Ia supernova which was first observed around November 7, 2008 in the galaxy UGC 12682, which lies in the constellation Pegasus at a distance of about from Earth. SN 2008ha was unusual in several ways: with an absolute V band magnitude of −14.2 it is one of the faintest supernovae ever observed; its host galaxy type very rarely produces supernovae. Another unusual feature of SN 2008ha was its low expansion velocity of only ~2000 km/s at maximum brightness, which indicates a very small kinetic energy released in the explosion. For comparison, SN 2002cx