# Mass fraction?

1. Nov 8, 2007

### malawi_glenn

Hi!

I am reading text in metal abundances, for example one of my books states that the metallicity of the milky way is:

z = 0.02

etc.

Is that mass or number fraction? I have searched in all my books and google a bit but can't seem to find an answer :S

Help

2. Nov 8, 2007

### Staff: Mentor

Metallicity is defined as the proportion of mass in an object that is not hydrogen or helium - could be carbon, iron, whatever else. Metals in this case are not the metallic elements as we think of the term. Carbon is a metal in this case, for example.

It is the $$metal mass / total mass$$

It's a fraction.

3. Nov 8, 2007

### malawi_glenn

Great thanx!

I know all the other stuff you wrote, was just unsure if z is number or mass ratio.