In astrophotographs NGC 2359 has an ethereal quality. Surprisingly the outward appearance strongly contradicts the violent events which gave rise to the peculiar nebula. NGC 2359 is a prototypical wind blown bubble powered by the extremely massive and unstable Wolf-Rayet star HD 56925. These types of stars (only about 300 are known presently) represent a late evolutionary phase of massive O-type blue giants which have become unstable in the late stages of their short stellar life. WR stars heavily influence the surrounding interstellar medium. There are several basic types of nebulae associated with WR stars which run the gamut from concentric rings (NGC 6888), wind blown bubbles (NGC 7635), to filamentary type nebulae (NGC 2359). NGC 2359 consists of two distinct components. The outer component is a "U" shaped diffuse HII region of illuminated gases ejected at an earlier time by the O type progenitor of the WR star. The inner component is the central filamentary bubble blown more recently by the winds of the WR star HD 56925. The stars get their name from the French astronomers Georges A. Rayet (1839-1906) and Charles J. Wolf (1827-1918) who first reported their existence in 1867.
Wolf-Rayet stars (WR) are characterized by copious mass loss and dense stellar winds. The powerful winds smash into the surrounding stellar medium at speeds up to millions of kilometers per hour. The impact of the winds on the stationary interstellar medium can create bow-shaped structures of shocked gases (bow shocks) which glow in the brilliant colors we see at visual wavelengths. WR stars are typically 100,000 to even 1,000,000 times more luminous than our sun and contain over 25 solar masses. Their instability peaks as they enter the Wolf-Rayet stage. During this stage the star's hydrogen envelope is depleted leaving its helium core exposed. The torrential mass loss of about one earth sized mass per year accelerates the star's fate as they are doomed to explode as supernovae within a short time. Several subclasses of Wolf-Rayet stars are known to exist. The classes include type WN (nitrogen dominant), type WC (carbon dominant), and the rare type O (oxygen dominant). All WR stars have specific emission lines of helium and a relative absence of hydrogen in their spectra, a signature which helps to identify them especially at extragalactic distances.