Imagine a star 40 times as massive and several hundred thousand times more luminous than our sun? Well, BD +60°2522 is such a star. Itsenormous energy output and powerful stellar winds have blown a titanic bubble of ionized gas measuring 6 light years in diameter. Popularly known as the Bubble Nebula, the strange symmetrically round nebula is the outcome of the prodigious energy output and fierce stellar winds of an unusually powerful star known as a Wolf-Rayet star. Named after the French astronomers Charles Wolf and Georges Rayet, who first described the unusual stars in 1867, less than 300 Wolf-Rayet stars (WR) have been identified in our own galaxy and some have even been identified in other galaxies. These extremely powerful stars mark the end stage of rare O type stars that begin their lives with at least 25 times the mass of our sun. Their hot surface temperatures range between 30,000 and 60,000 degrees Kelvin and their stellar winds can exceed 1500 kilometers per second, capable of rapidly depleting the stars outer layers. WR stars can lose two thirds of their mass during this final stage of their stellar life. A star entering the WR stage with 35 solar masses can end up as a 10 solar mass star before it explodes as a supernova.
Wind blown bubbles, concentric rings and filamentary shaped nebulae are common outcomes of Wolf Rayet driven winds on surrounding gas clouds. The peculiar shape of the Bubble nebula marks the leading edge of the Wolf-Rayet wind front as it plows into the denser stationary material of the interstellar medium. The prodigious winds of this WR star travel at 9 million kilometers per hour. The asymmetry of the bubble in relation to BD +60°2522 is believed due to subtle differences in the density of the surrounding gaseous material. Also the bright arcs and small condensations which characterize the bubble's surface are also related to density variations in the swept up material forming the bubble wall. The prominent inner bright knot projected along the western wall of the bubble is actually the ionized edge of a larger cloud that physically lies outside the bubble itself.
The Bubble Nebula, NGC 7635 is imbedded in the surrounding HII region Sharpless 162 (S162). The entire complex is located in the Perseus arm of the Milky Way. BD +60°2522 the sole exciting star of the nebula is a type O6.5 giant with a surface temperature of 34,000 degrees. It is considered a member of the Cas OB2 stellar association.
(See NGC 2359 and NGC 300 for more information on Wolf-Rayet Stars)