Temperature required for ionisation What is the temperature required for an atom of helium so that atom can have enough energy to ionise? No more info is given.
 A: You need to know the electron density as well as the ionization energy X.  You might think you'd get ionization when kT ~ X, but actually you tend to get ionization when kT << X.  For example, you may have heard that the era of recombination of the universe was when T ~ 3000 K, but that means kT is about 1/60 of the X for hydrogen.
The reason for this has to do with what is known as the "statistical weight of the continuum,"  which basically means, how many free states are available to the ionized electron.  When bound, the electron has only the ground state (let's ignore the two spin states, as they are there for free electrons also so are not playing any important role), but when ionized, it has access to a truly a vast number of free states.  So that strongly favors that the electron should be ionized, so when T ~ 3000 K, the probability of a transition from the ground state to a single one of the ionized states is extremely tiny, since kT/X is very small, but there are so many such states, the transitions do happen.  The number of states available to the electron is inversely proportional to the electron density.
