A single massive star has blown a thin, glowing shell into the gas of Cassiopeia. NGC 7635, the Bubble Nebula, is an emission nebula — an H II region — about 7,100 light-years from Earth. The bubble itself is about 7 light-years across, roughly one and a half times the distance from the Sun to Alpha Centauri. On the sky it spans about 15 by 8 arcminutes and glows near magnitude 10, faint to the eye and rich in a long exposure.
At the heart of the shell is BD+60°2522 (SAO 20575), an O-type star of about 45 times the Sun’s mass, shining at magnitude 8.7. Gas leaving its surface races outward at more than 4 million miles per hour and sweeps the surrounding cloud into a bright rim. Denser gas along one side slows that expansion, so the star sits offset inside a shell that still looks nearly round. Cool pillars of hydrogen and dust stand in the cloud beyond the rim, lit by the same ultraviolet light.
William Herschel recorded the nebula in 1787. It is also catalogued as Caldwell 11 and Sharpless 162, and it lies close to the open cluster Messier 52. The central star is only about 4 million years old. In another 10 to 20 million years it is expected to end as a supernova.
Facts
- About 7,100 light-years away in Cassiopeia (some estimates reach about 11,000)
- Bubble about 7 light-years across; the wider nebula is 15′ × 8′ and about magnitude 10
- Central star BD+60°2522, an O-type star of about 45 solar masses and magnitude 8.7
- Stellar wind faster than 4 million miles per hour (about 2,000 km/s)
- Near the open cluster Messier 52; coordinates 23h 20m 48s, +61° 12′
- Also called Caldwell 11 and Sharpless 162; found by William Herschel in 1787
