Globular clusters are gravitationally bound stellar systems containing, on average, about 100,000 stars. With one significant exception (omega Centauri), it appears that all stars in any one cluster formed at the same time out of a parental gas cloud, and all the stars began their lives with the same chemical composition. Not all globular clusters are alike, however. Some have very low abundances of elements heavier than helium, so that Z, the heavy-element mass fraction, may be over 100 times lower than that found in the Sun's atmosphere (where Z is about 0.018). Other clusters have relatively high Z values, reaching almost as high as that of the Sun. Most of the globular clusters in the Milky Way Galaxy lie within a few kiloparsecs of the galactic center, but some lie as far out as 100 kiloparsecs (1 kiloparsec equals 3260 light-years, 3.1 × 10¹⁶ km, or 1.9 × 10¹⁹ mi; for reference, the Sun lies about 8 kiloparsecs from the center).