Bubble Nebula
| Polish version is here |
The Bubble Nebula, registered in various catalogs as NGC 7635, Sharpless 162, and Caldwell 11, is a striking emission nebula and H II region in the constellation Cassiopeia. It is sculpted by the hot, young O-type star SAO 20575 (BD+60°2522), which has an estimated mass of about 44 M☉. Powerful stellar winds propel plasma outward at over a thousand miles per second, where it collides with a nearby giant molecular cloud, compressing and heating the gas until it radiates in H-α. This interaction hollows out an almost spherical cavity fringed by irregular layers and filaments of interstellar matter. Since Sir William Herschel first recorded it in November 1787, the nebula has captivated observers with its soap-bubble appearance.
Observations
June 21, 2024, around 11:00 PM – Jaworzno, Poland
Conditions: urban sky, heavy light pollution
The nebula lies less than half a degree southwest of the open cluster Messier 52. The photograph below shows where to begin your search.
Among the surrounding constellations, note Ursa Minor. Though smaller than Ursa Major, it hosts Polaris (α Ursae Minoris), positioned near the north celestial pole and long used for navigation. Nearby stretches Draco, linked to the mythic Ladon (Λάδων) and containing about eighty naked-eye stars. Polish observers can follow it all year, most easily in summer. Camelopardalis occupies a broad but faint swath of sky that the ancient Greeks considered nearly starless. Cepheus, husband of Cassiopeia and father of Andromeda, is composed of moderately bright stars, while Cassiopeia itself forms a distinctive “W” (or “M”) nestled in rich Milky Way star fields. Southeast of these figures lies Lacerta, a small constellation introduced by Johannes Hevelius in the seventeenth century.
Observing the Bubble Nebula visually requires patience and sufficient aperture, as its surface brightness is about 10m. Long-exposure imaging, however, readily reveals its structure (Photo 2).
The image even captures the central star SAO 20575. Studies suggest that the star is approximately two million years old, while the surrounding nebula is only about forty thousand years old. The bubble likely formed where the star’s powerful wind collides with the surrounding interstellar medium, generating a supersonic shock front. This wind, flowing outward at 1 120–1 550 mi/s (1 800–2 500 km/s), causes the star to shed more than one-millionth of the Sun’s mass each year.
Photo 2 Parameters:
- Total exposure time: 90 minutes (stack of 180 RAW frames at 30s each, using an appropriate number of dark, bias, and flat frames)
- Canon EOS 60D
- ISO: 1500
- Maksutov-Cassegrain telescope (100/1400), prime focus exposure
- A filter was used to reduce the effects of artificial light pollution and atmospheric glow
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system.
Further readings:
- Caldwell-Moore P., Firefly Atlas of the Universe, Firefly Books Limited, 2003
- O’Meara S. J., The Caldwell Objects, 2nd ed., Cambridge University Press, Cambridge, 2016, pp. 59–62
- König M., Binnewies S., Bildatlas der Sternhaufen & Nebel, Kosmos, Stuttgart, 2023, p. 171
Marek Ples