A curious ring in star factory NGC 7538
The billowing clouds portrayed in this image from ESA's Herschel observatory are part of NGC 7538, a stellar nursery for massive stars. Located around 9000 light-years away, this is one of the few regions of massive-star formation that are relatively close to us, allowing astronomers to investigate this process in great detail.
Star factories like NGC 7538 consist mainly of hydrogen gas, but they also contain small amounts of cosmic dust. It was through this minor – but crucial – component that Herschel could image these star-forming regions, because dust shines brightly at the far-infrared wavelengths that were probed by the observatory.
With a total mass of almost 400 000 Suns, NGC 7538 is an active factory where stars come to life – especially huge ones that are over eight times more massive than the Sun. Hundreds of seeds of future stellar generations nestle in the mixture of surrounding gas and dust scattered across the image. Once they reach a critical mass, they will ignite as stars. Thirteen of these proto-stars have masses greater than 40 Suns, and are also extremely cold, with temperatures only a few tens of degrees above absolute zero.
One group of stellar seeds seem to trace a ring-like structure, visible right above the centre of the image. The ring consists of cool dust and its mass amounts to about 500 solar masses. With an oval shape, the ring spans about 35 light-years along its long axis and about 25 light-years along its short axis. It may be the edge of a bubble carved by stellar winds or supernova explosions from a previous population of very hot and massive stars. But no remnant of such a star is apparent within the centre of the ring, so astronomers are still investigating the origin of this peculiar arrangement.
The observations were taken as part of the Herschel OB Young Stellar objects (HOBYS) Key Programme, which used ESA's Herschel observatory to study the most massive kinds of stars in the Universe: O-type and B-type.
The image is a composite of the wavelengths of 70 microns (blue), 160 microns (green) and 250 microns (red). It spans about 50 arc minutes in height; north is to the right and east is up.