NGC 7538

NGC 7538 is an emission nebula in the constellation of Cepheus, discovered in 1787 by astronomer William Herschel using his 18.7-inch reflecting telescope and later recorded by Johan Dreyer in his New General Catalogue.

The nebula is about 9,100 light-years away, is located on the Perseus arm of the Milky Way, and is directly connected to the large Cassiopeia-OB2 association. Like the OB2 association, it is an H-II region, that is, a molecular cloud composed predominantly of hydrogen, which is excited to glow by stars formed within it. The total mass of NGC 7538 is about 400,000 solar masses.

In the emission nebula there are different phases of star formation. It is most advanced in the northwestern sector of the nebula. Two infrared surveys of the area in the 1970s show 11 bright sources, designated IRS1 to IRS11. Within them, IRS1, IRS2, and IRS3 form a small group of stars of spectral classes O and B at the southeastern edge of the nebula, of which IRS1, located in a small H-II region 180 AU in diameter, is the brightest at ¹⁰⁴ L☉; spectra of OH maser, water, formaldehyde, ammonia, and methanol are detectable in its vicinity. A diffuse H-II region around IRS4-6 is predominantly illuminated by IRS6, an O7V star. IRS7, IRS8, and probably IRS10 are stars in the foreground or background that do not belong to the nebula. IRS9 is a protostar in a similar environment to IRS1 and is only one arcminute away from it. IRS11 has an OH maser spectrum and is another protostar. Together with IRS9 it is one of the youngest objects of the nebula. A protostar, also very young and designated NGC7538 S, stands out as having one of the largest accretion disks found to date, measuring 14,000 AU, about 300 times the size of the solar system. More sensitive IR surveys of the nebula reveal thousands of infrared sources and suggest formed and nascent star clusters up to 4 billion years old, but also younger than 1 billion years, concentrated in the areas mentioned.