The eigenvalues of the damped harmonic oscillator

The differential equation 
x'' = - x - a x'

is a damped harmonic oscillator. The left hand side is the acceleration, the right hand
side a sum of the spring force and a damping term. This system can be written as 
a system of first order differential equations: 
x' = y
y' = - x - a y

which is X' = A X for the matrix 
A = | 0   1 |
    | -1 -a |

For a=0, the harmonic oscillator case,
the eigenvalues of A are -i,i For positive a, the real part of the eigenvalues becomes negative and
the eigenvalues move to the left in the complex plane. 
There is the moment, when the eigenvalues are both -1. This is the critically
damped situation. For larger a, the eigenvalues are different. It is the overdamped situation.