M17 is one of the youngest and most massive nearby star-formation regions in the Galaxy .
It features a bright H ii region erupting as a blister from the side of a giant molecular cloud ( GMC ) .
Combining photometry from the Spitzer  Galactic Legacy Infrared Mid-Plane Survey Extraordinaire ( GLIMPSE ) with complementary infrared ( IR ) surveys , we identify candidate young stellar objects ( YSOs ) throughout a 1.5 ^ { \circ } \times 1 ^ { \circ } field that includes the M17 complex .
The long sightline through the Galaxy behind M17 creates significant contamination in our YSO sample from unassociated sources with similar IR colors .
Removing contaminants , we produce a highly-reliable catalog of 96 candidate YSOs with a high probability of association with the M17 complex .
We fit model spectral energy distributions to these sources and constrain their physical properties .
Extrapolating the mass function of 62 intermediate-mass YSOs ( M _ { \star } > 3 Â M _ { \sun } ) , we estimate that { > } 1000 stars are in the process of forming in the extended outer regions of M17 .
The remaining 34 candidate YSOs are found in a 0.17 deg ^ { 2 } field containing the well-studied M17 H ii region and photodissociation region , where bright diffuse mid-IR emission drastically reduces the sensitivity of the GLIMPSE point-source detections .

By inspecting IR survey images from IRAS Â and GLIMPSE , we find that M17 lies on the rim of a large shell structure { \sim } 0.5 ^ { \circ } in diameter ( { \sim } 20 pc at 2.1 kpc ) .
We present maps of ^ { 12 } CO and ^ { 13 } CO ( J = 2 \rightarrow 1 ) emission observed with the Heinrich Hertz Telescope .
The CO emission shows that the shell is a coherent , kinematic structure associated with M17 , centered at v = 19 km s ^ { -1 } .
The shell is an extended bubble outlining the photodissociation region of a faint , diffuse H ii region several Myr old .
We identify a group of candidate ionizing stars within the bubble .
YSOs in our catalog are concentrated around the bubble rim , providing evidence that massive star formation has been triggered by the expansion of the bubble .
The formation of the massive cluster ionizing the M17 H ii region itself may have been similarly triggered .
We conclude that the star formation history in the extended environment of M17 has been punctuated by successive waves of massive star formation propagating through a GMC complex .