The key...
Superman leaves the key to his arctic
fortress sitting out in the open on the
doorstep, but this doesn’t make it much
of a security risk as it is apparently made
of dwarf star material and weighs
500,000 Tons.
Star life cycles depend on the size of the star.
As an average size star will start off fusing hydrogen atoms to make helium, eventually the hydrogen fuel runs out and the start starts to fuse helium to make larger atoms like carbon. As this happens the star expands to become a red giant. Once the helium runs out the star expands further into a planetary nebula before the remaining core becomes a white dwarf.
A dwarf star has a density of
1,000,000 g/cm^3. So according to
Superman’s statement the key must
have a volume of 0.5 m^3, which is really
a bit large for a key. In some comics it is,
however, shown to be a massive key.
A normal key has a volume of less than
2 cm3 (0.000002 m^3) which if made of
a white dwarf would make the key weigh
2,000,000 g (2,000 kg).
Alternatively the material Superman used might have a higher density than a white dwarf. To match Superman’s statement and the size of the key its density would be 2.5 x 1014 kg/m^3 (250,000,000,000 g/cm^3). It is possible, although not from a white dwarf. A neutron star has a density of between 3.7 x 1017 and 5.9 x 1017 kg/m^3 (370,000,000,000,000 g/cm^3).
