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Often overshadowed by the more famous Bubble Nebula which lies nearby, NGC 7538 is an exciting emission and reflection nebula located in Cepheus. While it is often overlooked by amateur astronomers, professionals looking to study stellar formation find it an exciting target as it is the host to ongoing star formation, including the largest known protostar.
Because of the dusty nature of this region, studies targeting the nebula are frequently conducted in longer wavelengths, ranging from the infrared to the radio. Previous studies have put the age of the forming stars at around ~1-4 million years and at a distance of ~2.8 kiloparsecs. Within it, several individual sub groups of star formation seem to have occurred. Among some of the more interesting individual forming stars are NGC 7538S and MM 1.
Observations from earlier this year targeted NGC 7538S. This protostar is embedded in a collapsing core of approximately 85 – 115 solar masses and hosts a rotating accretion disc as well as large outflows of material. Although the star has not finished forming, the conditions are right for it to form into a high mass B star and is undergoing accretion at an unusually high rate of 1/1000th of a solar mass per year.
More recently another paper explores several other forming stars in the region including the massive MM 1. This star is already estimated to have accumulated 20-30 solar masses and be well on the way to forming an O class star. But it’s not done yet. Radial velocity measurements of molecules in the protostar’s vicinity indicate it’s still undergoing large amounts of accretion, mostly from its equatorial plane. Numerous studies have shown that this massive star is creating powerful jets.
In addition, this new study identifies an additional eight cores forming into young stars near MM 1. These cores are interesting because they exist in regions where the density and temperature were not expected to be sufficiently high to induce star formation. This suggests that their formation was not uniquely due to a self induced collapse, but rather, triggered by shock waves or magnetic fields. Although no studies have searched for the signs of magnetic fields in the region, there are indications that numerous shock waves exist. Additionally, four of these cores have mass available to them similar to that of MM 1 which may allow them to form into a grouping of high mass stars similar to the famous Trapezium in Orion. These stars all exist in a narrowly confined region of about 1 light year, which is also similar to the separation of the Trapezium. Many of the newly discovered cores have large outflows and maser emission as well.
Further studies on this region will certainly uncover new protostars and assist astronomers in understanding how clusters of stars form. Already, astronomers have used it to help probe the Initial Mass Function which describes the number of stars forming for various masses. Additionally, with small clusters of stars like the Trapezium being common, catching one in the act of forming may help astronomers determine just how they form.
