  The experimental data on hadron yields and ratios in central lead-lead and
gold-gold collisions at 158 AGeV/$c$ (SPS) and $\sqrt{s} = 130$ AGeV (RHIC),
respectively, are analysed within a two-source statistical model of an ideal
hadron gas. A comparison with the standard thermal model is given. The two
sources, which can reach the chemical and thermal equilibrium separately and
may have different temperatures, particle and strangeness densities, and other
thermodynamic characteristics, represent the expanding system of colliding
heavy ions, where the hot central fireball is embedded in a larger but cooler
fireball. The volume of the central source increases with rising bombarding
energy. Results of the two-source model fit to RHIC experimental data at
midrapidity coincide with the results of the one-source thermal model fit,
indicating the formation of an extended fireball, which is three times larger
than the corresponding core at SPS.
