NGC7635 - BUBBLE NEBULA

The Bubble Nebula is a striking emission nebula in the constellation Cassiopeia, located approximately 7,100 light-years from Earth. Its defining feature is a nearly perfect spherical shell of gas roughly 7 light-years in diameter, blown by the fierce stellar wind of the massive O-type star BD+60 2522 (also known as SAO 20575). This star, estimated at 10 to 20 times the mass of the Sun, produces a wind traveling at over 2,000 kilometers per second that sweeps up the surrounding interstellar gas into a thin, glowing shell. The bubble appears off-center relative to its parent star because the density of the surrounding interstellar medium varies, with denser material to one side slowing the expansion and creating the asymmetric appearance. The Bubble Nebula sits on the edge of a giant molecular cloud, and the interaction between the expanding bubble and this dense cloud creates a bright rim along one side of the structure. This boundary region shows evidence of compression and heating as the bubble's shock wave encounters the molecular material. The nebula was famously imaged by the Hubble Space Telescope in 2016 to celebrate Hubble's 26th anniversary, producing one of the most widely circulated astronomical images of the decade. In that image, the delicate translucent shell of the bubble is visible against a backdrop of complex emission nebulosity, with pillars and knots of dense gas being sculpted by the central star's radiation. The Bubble Nebula is a challenging but rewarding target for amateur astronomers. Its relatively small apparent size of about 15 arcminutes means it benefits from moderate magnification, and narrowband filters greatly enhance the visibility of the bubble structure against the surrounding emission.

The bubble shell measures approximately 7 light-years in diameter, driven by stellar winds exceeding 2,000 km/s from the central O6.5 star, at a distance of roughly 7,100 light-years.