The calm before the storm: lurking within a mysterious dark cloud in the Milky Way lie hundreds of stellar cores on the cusps of igniting into massive stars.

The wispy dark cloud, known as M17 SWex, was first discovered in the Sagittarius constellation over 30 years ago, but it took the keen infrared eyes of NASA's Spitzer Space Telescope to seek out the smoldering stellar nursery secluded within. "We believe we've managed to observe this dark cloud in a very early phase of star formation before its most massive stars have ignited," says Penn State astronomer Matthew Povich.

It turns out that this concealed birthplace is actually one of the most intense star-forming regions within the Milky Way – Spitzer identified 488 newly forming stars, 200 of which are destined to become larger and hotter than our Sun. "Most of the stars we've detected are relatively bright," says Povich. "So we predict the actual number of stars forming in M17 SWex is over 10,000, since the fainter stars cannot be detected with the current observations."

Neighbouring nebula M17 is much more conspicuous, burning brightly with hot, massive class O stars. Such behemoths are absent from M17 SWex, but there is still time – such gigantic stars might need an extra boost to launch their formation. Such a trigger might come from a shock wave generated by the burst of nearby star births. Evidence for this process is found in the far left of the image, where giant O class stars, aged between two and five million years, have carved out a bubble, which has impinged on the left side of M17. The burning core of M17 is lit up by one million year old stars, while within the dark cloud of M17 SWex, many stars are still cocooned inside shells of dust and gas.

The time sequence across these three regions can be explained by the architecture of the Milky Way, says Povich. "The time-sequence of star formation proceeds in the same direction that a spiral arm crosses the M17 cloud complex." The M17 region is currently on a million year long passage though the Milky Way's Sagittarius spiral arm; and the greater concentration of gas and dust there forces material together that triggers waves of massive star formation through the nebula.

"The M17 region brings to mind images of other spiral galaxies where the leading edges of the arms appear blue, with young O stars, but the trailing edges are still dark, with obscuring dust like in M17 SWex," adds Povich.

The new data, gleaned from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) and the Multi-band Imaging Photometer for Spitzer Galactic Plane Survey (MIPSGAL), will provide information on whether massive star formation relies on the extra effect of an expanding shock wave to jolt them into life.

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