We study stellar-halo formation using six Milky Way-mass galaxies in FIRE-2 cosmological zoom simulations .
We find that 5 - 40 \% of the outer ( 50 - 300 kpc ) stellar halo in each system consists of in-situ stars that were born in outflows from the main galaxy .
Outflow stars originate from gas accelerated by super-bubble winds , which can be compressed , cool , and form co-moving stars .
The majority of these stars remain bound to the halo and fall back with orbital properties similar to the rest of the stellar halo at z = 0 .
In the outer halo , outflow stars are more spatially homogeneous , metal rich , and alpha-element-enhanced than the accreted stellar halo .
At the solar location , up to \sim 10 \% of our kinematically-identified halo stars were born in outflows ; the fraction rises to as high as \sim 40 \% for the most metal-rich local halo stars ( [ Fe/H ] > -0.5 ) .
We conclude that the Milky Way stellar halo could contain local counterparts to stars that are observed to form in molecular outflows in distant galaxies .
Searches for such a population may provide a new , near-field approach to constraining feedback and outflow physics .
A stellar halo contribution from outflows is a phase-reversal of the classic halo formation scenario of Eggen , Lynden-Bell & Sandange , who suggested that halo stars formed in rapidly infalling gas clouds .
Stellar outflows may be observable in direct imaging of external galaxies and could provide a source for metal-rich , extreme velocity stars in the Milky Way .