Volume = a x a x a
V = 2 cm x 3 cm x 4 cm => 24 cm³
Density = 19.3 g/cm³
Mass = ?
Therefore:
m = D x V
m = 19.3 x 24
m = 463.2 g
J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."
We know that it is a certainty based on things such as the size, mass, brightness, etc. They can determine to within a relative level of accuracy the age. Of course they can be off by millions of years, but that’s pretty precise for a 10–20 billion year time period for the life of a star. BUT we absolutely cannot predict exactly when it will explode and we see supernovas in other galaxies, but the last one in the Milky Way that we observed was in the 1600s.