  Sr isotopes are located in the mass region $A\approx 100$, where a very quick
onset of nuclear deformation exists, being other notable examples of this area
Yb, Zr, and Nb nuclei. The presence of the proton subshell closure $Z=40$
allows the existence of particle-hole excitations that produces low-lying
intruder bands.
  Purpose: The goal of this work is the study of the nuclear structure of the
even-even $^{92-102}$Sr isotopes through the accurate description of excitation
energies, $B(E2)$ transition rates, nuclear radii and two-neutron separation
energies.
  Method: The interacting boson model with configuration mixing will be the
framework to calculate all the observables of the Sr isotopes. Only two types
of configurations will be considered, namely, 0particle-0hole and
2particle-2hole excitations. The parameters of the model are determined using a
least-squares procedure for the excitation energies and the $B(E2)$ transition
rates.
  Results: For the whole chain of isotopes, the value of excitation energies,
$B(E2)$'s, two-neutron separation energies, nuclear radii, and isotope shifts
have been obtained, with a good agreement between theory and experiment. Also,
a detailed analysis of the wave functions have been performed and, finally, the
mean-field energy surfaces and the value of the nuclear deformation, $\beta$,
have been obtained.
  Conclusions: The presence of low-lying intruder states in even-even Sr
isotopes have been confirmed and its connection with the onset of deformation
has been clarified. Lightest Sr isotopes present a spherical structure while
the heaviest ones are clearly deformed. The rapid onset of deformation at
neutron number $60$ is due to the crossing of the regular and intruder
configurations and, moreover, both families of states present an increase of
deformation with the neutron number.
