Chandra X-ray Image of W3 Main
W3 Main is a region of massive star formation about 6,000 light years from Earth in the Perseus arm of the Milky Way. The bright, point-like X-ray sources represent an extensive population of several hundred young stars, many of which were not found in earlier studies. Because its X-ray sources are all at the same distance, yet span a range of masses, ages, and other properties, W3 is an ideal laboratory for understanding recent and ongoing star formation in one of the Milky Way's spiral arms.
Scale: Image is 14 arcmin across
(Credit: NASA/CXC/Penn State/L.Townsley et al.)

Full-field Optical Image of W3
W3 is a region where many massive stars are forming in a string of stellar clusters, located about 6,000 light years from Earth in the Perseus arm of the Milky Way galaxy. W3 is part of a vast molecular cloud complex that also contains the W4 superbubble (not seen in this image). Scientists believe that the incredible amount of star formation in W3 has possibly been influenced by neighboring W4, an inflating bubble of gas over 100 light years across. W4 may directly trigger the birth of W3's massive stellar clusters as it expands and sweeps up molecular gas into a high-density layer at its edge, within which stars can form. Another possible scenario is that W4's expansion has caused a domino effect of star formation, forming the cluster IC 1795 which in turn triggered formation of the young, massive clusters in W3.
Scale: Image is 31 arcmin across
(Credit: Pal Obs. DSS)

Illustration of the Milky Way's Arms
This side-view schematic of the Milky Way galaxy, shows the prominent spiral arms, the central galactic bulge, and the location of the Sun.
Scale: Image is 31 arcmin across
(Illustration: NASA/CXC/M.Weiss)