The Herschel Space Telescope has photographed the star formation region NGC 7538. As anticipated, it revealed images of clumps that are expected to become some of the largest stars known - but it also turned up another, so far unexplained feature.
NGC 7538 is a cloud of gas and dust from which stars are forming, located 8,800 light years away in Cepheus. While the nearby Bubble Nebula is more attractive to amateur astronomers, star formation regions are of great interest to researchers, as there is so much we don't know about the process of making stars.
With an estimated mass 400,000 times that of the sun, NGC 7538 attracts plenty of study, even more so since it has yielded such gems as the youngest high-mass star observed, NGC 7538 S with a mass at least 40 times that of the sun, and a surrounding disk at least twice the size. The same nebular also contains MM1, a slightly smaller, but also more advanced massive star.
"We have looked at NGC 7538 with Herschel and identified 13 massive, dense clumps where colossal stars could form in the future," said Professor Cassandra Fallscheer, of Whitman College in Walla Walla, and lead author of a paper reporting the findings in The Astrophysical Journal. The paper notes, “These 13 clumps have masses in excess of 40 solar masses and temperatures below 15K. They range in size from 0.4 pc to 2.5 pc (1.3-9 light years) and have densities between 3×103cm–3 and 4×104cm–3.” As such they represent prime targets for future study. Star formation is far too slow for us to see it happening, but by identifying enough similar regions at different stages of the process we can test our ideas of how it occurs.
Cold gas clouds contract under the influence of gravity until the pressure is large enough to force nuclei together to initiate fusion, causing stars to shine. Unsurprisingly, it takes a dense cloud to produce the large stars capable of ending their lives as supernovae. Until fusion begins the clouds are too cold to be seen in visible light unless lit up by nearby stars. Instead they need to be studied in the infrared and submillimeter wavelengths in which the European Space Agency's Herschel Observatory operates.
However, what was not expected was the oval shaped ring, 35 light years on the long axis and 25 on the short. It is made up of cool dusty material that Fallscheer estimates weighs 500 times the mass of the sun.
Apparently similar rings have been seen before, but these are associated with massive O-type stars, and are thought to be caused by the exceptional solar winds these giants produce, or by the forces unleashed by a supernova. However, in this case there is evidence of neither an O-type star, nor a supernova remnant that could be creating this shape.
"Further research to determine the mechanism responsible for creating the ring structure is necessary," said Fallscheer. One tentative theory is that an O-type star shaped the ring, before moving on, perhaps leaving a supernovae remnant somewhere else. Although follow-up observations were taken with the James Clerk Maxwell Telescope in Hawaii, deeper research is impeded since the Herschel ran out of liquid coolant last year. Analysis of the images taken while it operated continue however.
The paper notes that NGC's filamentary structure had not been seen before in submillimeter surveys.