101 Must-See Cosmic Objects: The Bubble Nebula

William Herschel first observed NGC 7635 in 1787 as a glow around the magnitude 8.7 star SAO 20575 (earlier cataloged as BD+60°2522). In a telescope, the Bubble Nebula looks more like a comma because the entire extent of its spherical shell is so faint that it has only been discovered by photography. This emission nebula, composed of ionized hydrogen fueled by an O-type star, is part of a larger complex of glowing gases.

Observing the Bubble Nebula requires a sky free of light pollution. A Hydrogen-II filter or UHC filter is useful for bringing out detail. If you observe under extremely dark skies, this nebula can be seen in a 6-inch telescope. This is one of those objects that benefits from increased openness.

Finding NGC 7635 is easy – it lies about 0.5° from the Cepheus to Cassiopeia boundary. If you can find the bright open cluster M52, sweep less than a degree to the southwest and you might find the bubble’s faint hydrogen glow. It is equidistant from the HII NGC 7538 region, located northwest of the Bubble. The Bubble Nebula measures 15 feet by 8 feet with a visual magnitude of about 10, although this light is spread out making it appear much dimmer. A magnitude 7 star in the region can interfere with an observer’s ability to fully adapt to darkness.

The star SAO 20575 is the creator of the Bubble Nebula and its weight is estimated at 45 solar masses. Its peculiar spectral classification of O6.5 indicates that its surface temperature is 67,000 degrees Fahrenheit (37,200 degrees Celsius). Astronomers also estimate that this star is losing mass at the rate of one solar mass every million years. The Bubble Nebula itself marks the edge of a shock wave interacting with hydrogen atoms in the interstellar medium.

The Bubble Nebula is part of the Cassiopeia OB2 stellar association – a group of hot, young stars that formed together and live fast and die hard. The nebulous and stellar combination lies about 7,000 to 8,000 light-years away from us, in the arm of Perseus.



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