# Mass of Ionized Hydrogen

Trebor0808
Could anyone please explain why in the book The Physics of Stars it states that the mass of ionized Hydrogen is 0.5AMU, I cannot figure out why it isn't 1.0078AMU minus the mass of an electron?

Vagn
Good question, the mass of ionised hydrogen is just that of a proton (with corrections to account for deuterium and tritium). Can you provide the context?

Trebor0808
It's with regards to the pressure inside the sun, P=((density)/(avg mass of a partice))*kT where the avg mass of a particle in this case is 0.5 AMU. Assuming that the sun is made up entirely of Hydrogen in this case (comes out to be 0.61AMU when taking into account helium and other heavier elements).

Vagn
It's with regards to the pressure inside the sun, P=((density)/(avg mass of a partice))*kT where the avg mass of a particle in this case is 0.5 AMU. Assuming that the sun is made up entirely of Hydrogen in this case (comes out to be 0.61AMU when taking into account helium and other heavier elements).
In that case you are averaging over the free electrons and the hydrogen nuclei, I would guess. The mass is negligible in comparison to the hydrogen nuclei, so for the same number of electrons and nuclei, the average is half that of the hydrogen nuclei.

Trebor0808
Trebor0808
Oh okay that does make more sense now, thank you for your help.

ZealScience
In calculating the pressure, you need to consider what particles are exerting the pressure. When dilute enough, the plasma inside the star is a mixture of ion gas, free electron gas, and photon gas. But the photon pressure is usually orders smaller. So you only need to consider electron and ion gases.

*In fact, I think in some of the stars the quantum density is such that electrons are already degenerate. This means you have to use Fermi gas pressure for the contribution from electrons.

Trebor0808