NGC 7635, an intriguing bubble in Cassiopeia

 
NGC 7635, an intriguing bubble in Cassiopeia

NGC 7635, also known as the Bubble Nebula and Sharpless 162 (Sh2-162) is an emission nebula and H II region, with a diameter of 10 light-years, surrounded by a giant molecular cloud. It is located 7100 to 11000 light-years away (sources differ) in the constellation Cassiopeia.

It is created by the fierce stellar wind (4 million miles per hour or 7 million kilometers per hour) and intense radiation from a massive hot, young, blue central star, BD+60 2522 (also known as SAO 20575), which is thought to have a mass of 10-40 Solar masses and causing the “bubble” to glow.

The molecular cloud is able to limit the expansion of gas in the bubble, but is torn to pieces by the scorching radiation from the central star of the Bubble. That radiation heats denser areas within the cloud and makes them glow as well.

The surface of the bubble is not uniform because as the shell expands outward it encounters regions of cold gas within the molecular cloud, which are of different density and therefore arrest the expansion by differing amounts, resulting in the rippled appearance.

It is this gradient of background material that the wind is encountering that places the central star off center in the bubble. There is more material to the northeast of the nebula than to the southwest, so that the wind progresses less in that direction, offsetting the central star from the geometric center of the bubble.

To the right of the central star is a ridge of much denser gas. The lower left portion of this ridge is closest to the star and so is brightest. The ridge forms a V-shape, with two segments that are aligned at the brightest edge.

The region between the star and ridge reveals several loops and arcs. The origin of this bubble-within-a-bubble” is unknown. It may be due to a collision of two distinct winds: the stellar wind colliding with material streaming off the ridge.

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