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20,000 light-years away, near the outer edge of the Sagittarius arm of our own Milky Way galaxy, lies what is believed to be the densest cluster of massive stars in the entire galaxy, surrounded by the Milky Way’s most expansive cloud of interstellar gas.
NGC 3603 was first discovered by Sir John Herschel in 1834. It is known to harbor a blue supergiant star called Sher 25 that astronomers believe is on the brink of exploding as a supernova.
Most of the bright stars in the image are hot blue stars whose ultraviolet radiation and violent winds have blown out an enormous cavity in the gas and dust enveloping the cluster. The largest individual stars are about 115 solar masses. The swirling gases comprise about 400,000 solar masses. Lurking within this vast cloud are a few Bok globules, dark clouds of dense dust and gas with about 10-50 solar masses. They resemble insect cocoons and are undergoing gravitational collapse on their way to forming new stars. Bok globules appear to be some of the coldest objects in the Universe.