Ultraproduct $\prod_p \mathbb{F}_p/\sim$ and $\mathbb{Q}^*$ Is the non-principal ultraproduct of finite fields $\prod_p  \mathbb{F}_p/\sim$ a nonstandard model of the rationals $\mathbb{Q}$?
EDIT: Can we realize $\mathbb{Q}^*$ as an ultraproduct?
 A: It is easy to see that at least one of $-1,2,-2$ is a square in that field: the set of primes where neither $-1$ nor $2$ is a quadratic residue is contained in the set of primes where $-2$ is a quadratic residue.
A: A (non principal) ultraproduct  of finite fields of unbounded cardinalities is
a pseudo-finite field : 


*

*It is perfect (if its characteristic is a prime number $p$, then any element
is a $p$th power) ;

*For any positive integer $n$, it has exactly one extension of degree $n$
(equivalently, its Galois group is the profinite completion of the
integers $\mathbf Z$);

*It is pseudo-algebraically closed (any geometrically irreducible algebraic
variety has a rational point).
Indeed, these are first-order properties, hence are inherited by ultraproducts :
the first two ones hold for finite fields, while, it follows from Lang-Weil estimates that the last one is satisfied if the cardinality of the field is large enough, depending on the degrees of the equations.
Observe that the field $\mathbf Q$ of rational numbers does not satisfy the last two properties.
In fact, it is a theorem of Ax (1968, The elementary theory of finite fields, Ann. Math. 88 (1968) 239-271) that conversely, pseudo-finite fields are elementary equivalent to ultraproduct of finite fields.
A: Regarding your edit, of course any ultrapower $\Pi\mathbb{Q}/U$ of $\mathbb{Q}$ itself is a nonstandard model of the theory of $\mathbb{Q}$ (in whatever language you choose). So this version of $\mathbb{Q}^\ast$ is an ultraroduct.  Indeed, ultraproducts are one of the principal methods of constructing nonstandard models. 
