NGC 7538 - Bubble Nebula
The Bubble Nebula NGC 7635 is an example of an object formed by the strong stellar winds of a Wolf-Rayet (WR) star. The central star of the bubble-shaped nebula is 40 times more massive than the Sun. Due to the non-uniform distribution of gas and dust in the surrounding interstellar matter, the star is off-center of the nebula (at about the 8 o’clock position within the bubble). The bright structure west of the WR star seems to be located within the bubble but is not. It is just outside the shock front that is formed by the stellar wind – the front moves with a speed of about 2,000 kilometers per second – colliding with dense interstellar matter. The bubble’s diameter is just 6 light-years. The central star BD +60°2522 (SAO 20575) is a giant O-type with a surface temperature of about 35,000 K. It has reached the end of its life time and will probably explode as a Supernova within the next few 100,000 years. BD +60°2522 is also responsible for ionization of the whole surrounding gas cloud cataloged as Sharpless 162. The entire HII region is located in the Perseus arm of the Milky Way being about 7,100 light-years away.