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The Bubble Nebula (NGC 7635), found in the constellation of Cassiopeia, is an emission nebula discovered in 1787 by Friedrich Wilhelm Herschel. It resembles a "giant bubble" in outer space – and one might wonder what created this huge and impressive formation. The answer is a massive star at the center, constantly emitting a fierce stellar wind of ionized gas that stimulates the inner layers of the nebula cloud causing them to glow.
NGC 7635 is part of the larger complex of three bubbles around the massive star and also belongs to a larger network known as Sharpless 162 or S162. S162 lies near the galactic plane in the Perseus arm at a distance of about 3.6 kpc and has been created with the help of other stars as well. Some of its main characteristics are:
Apparent Magnitude: ~10
Distance: 7,100 - 11,000 ly
Radius: 3 - 5 ly
Expansion rate: 4 million miles per hour (7 million kilometers per hour).
Type: Emission Nebula
What is an emission nebula? It consists of high temperature gas that emits radiation of different colors depending on its composition (e.g. hydrogen or helium). These nebulae are usually the site of recent and ongoing star formation and their colors can be easily observed with an 8-inch telescope and a long-exposure photograph.
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What About the Blue Star?
The core of the nebula is an 8.7 magnitude blue star known as SAO 20575 or BD+60 2522. It is almost 40 times more massive than our Sun, emitting a wind so hot and intense that it pushes any surrounding sparse gas into a shell. The energetic radiation emitted from the star ionizes this shell, causing it to glow.
More specifically, the star—depicted at the lower center of this image—is so energetic that it is constantly shedding material into space at an incredibly fast pace. This material is being shaped into a bubble due to the resistance of the hot and dense gas that surrounds the stellar core. Its irregular shape is a result of encounters with gases of different thickness in different directions. The yellow clouds to the right of the star consist of much denser gas. The lower portion of these clouds appears brighter because it is closer to the stellar core and therefore heated and eroded at a much faster pace. More detailed analysis has shown the existence of loops and arcs between the yellow cloud region and the star. These are probably the result of material collision due to the star's intense wind as well.
Scientists claim that this is actually a rare case of a star located within the gas cloud, whose outward wind forms such an expanding bubble-shaped formation.
General characteristics of the central star SAO 20575:
Declination: +61o 11' 40.6''
Distance: ~ 3.6 kpc
Spectral type: O6.5
Apparent magnitude: 8.7
Absolute magnitude: 9.0
Terminal wind velocity: ~ 1800 km/s
Rate of stellar mass loss (M⊙yr−1): 10 -5.67
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Observing the Distant Gas Cloud in the Sky
There has been extensive observation of the nebula in a large portion of the electromagnetic spectrum; optical, infrared and radio band. Several techniques have been used to determine its structure, shape, composition and gas velocities. For example, researchers have used photographic and interferogram techniques to estimate an age of 3-4×105 years for the Bubble. Hα and NII emission line studies indicated a value of 35 km/s for the shell velocity and a dynamical lifetime of only 5×104 years for this high velocity shell—a rather short-lived formation. It has also been found that the gas cloud is younger, by a factor of 100, than the stellar core.
How can an amateur astronomer observe the NGC 7635? An 8 to 10-inch (250mm) telescope is enough to distinguish the nebula shell around the star. Since it appears quite faint, it is easier to observe it using averted or peripheral vision (not looking directly at the object). A 16 to 18-inch (460mm) telescope will reveal more details regarding the shape of the nebula. In the image here, we can also see the neighboring open cluster Messier 52 lying close to the Bubble.
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If you liked this article, you might also want to learn more about the Helix or "Eye of God" Nebula.
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- Anand M.Y., Kagali B.A. & Jayant Murthy, "Study of Bubble Nebula using IUE high resolution Spectra"
- "An Expanding Bubble in Space", Hubblesite.org
- "Nebulae", burro.astr.cwru.edu
- "Chapter3: Bubble Nebula", Tauvex.iiap.res.in