| Emission nebula | Lists of nebulae |
|---|---|
| Observation data (Epoch J2000) |
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| Type | Emission[1] |
| Right ascension | 23h 20m 48.3s[2] |
| Declination | +61° 12′ 06″[2] |
| Distance | 11 kly (3.4 kpc) (7100[3] to 11000[4][5] ly, sources differ) |
| Apparent magnitude (V) | ~10[6] |
| Apparent dimensions (V) | 15′ × 8′[1] |
| Constellation | Cassiopeia |
| Physical characteristics | |
| Radius | 5 ly (3[3] to 5,[5][7] sources differ) |
| Absolute magnitude (V) | Unknown |
| Notable features | Shell around SAO 20575[2] |
| Other designations | Bubble Nebula[2] Sharpless 162 (Sh2-162) Caldwell 11 |
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NGC 7635, also called the Bubble Nebula, Sharpless 162, or Caldwell 11, is a H II region[2] emission nebula in the constellation Cassiopeia. It lies close to the direction of the open cluster Messier 52. The "bubble" is created by the stellar wind from a massive hot, 8.7[2] magnitude young central star, the 15 ± 5 M☉[4] SAO 20575 (BD+60 2522).[7] The nebula is near a giant molecular cloud which contains the expansion of the bubble nebula while itself being excited by the hot central star, causing it to glow.[7] It was discovered in 1787 by Friedrich Wilhelm Herschel.[6] The star SAO 20575 or BD+602522 is thought to have a mass of 10-40 Solar masses.
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With an 8 or 10-inch (250 mm) telescope, the nebula is visible as an extremely faint and large shell around the star.[1][2] The nearby 7th magnitude star on the west hinders observation, but one can view the nebula using averted vision.[1] Using a 16 to 18-inch (460 mm) scope, one can see that the faint nebula is irregular, being elongated in the north south direction.[1]
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