NGC 7129 is a celestial object visible during summer nights in the northern hemisphere (circumpolar visible) in the central-western part of the Cepheus constellation, approximately 4 degrees away from the brightest star of the constellation, Alderamin (magnitude 2.4), in the northeast direction.
NGC 7129 is a reflection nebula located about 3,100 light-years away from Earth. Unlike emission nebulae that emit their own light due to ionization of gas, reflection nebulae do not emit light themselves. Instead, they shine by reflecting the light of nearby stars, typically young and hot stars, that are embedded within or close to the nebula. The light from these stars gets scattered and diffused by dust and gas particles in the nebula, creating a beautiful glow and giving the nebula its characteristic blue color.
NGC 7129 is associated with a young open star cluster, containing young, hot, and massive stars that formed relatively recently from the same molecular cloud of gas and dust.
Reflection nebulae like NGC 7129 typically show characteristic emission spectra with prominent spectral lines of ionized hydrogen (H-alpha) and other elements. The H-alpha line is often observed in red light due to the ionized hydrogen gas.
The dust grains within the reflection nebula can cause interstellar extinction, meaning they absorb and scatter light from background stars, making them appear fainter and redder.
NGC 7129 represents the remnants of a just hatched cocoon of O and B spectral class stars, making it a nursery of sorts. The recently born, young, and powerful stars, estimated to be a few hundred thousand years old, particularly the O-class stars, are among the hottest and most luminous in the universe. Their surface temperatures are significantly higher, exceeding 30,000 degrees Celsius, approximately six times hotter than our star (which shines with yellow light at about 5,600 degrees Celsius). This high temperature makes the O-class stars very bright and contributes to their bluish color.
Currently, their powerful stellar winds are sweeping away the remnants of their cocoons, which, when ionized, give the reflection nebula its visible color spectrum in the image.
Class O stars are stars with a fleeting life, quite different from our own. Due to their mass and high rate of nuclear fuel consumption, they have a short lifespan compared to other classes of stars, quickly (in astronomical terms) depleting their nuclear fuel and ending their existence as brilliant supernovae.
These stars are characterized by the presence of very strong spectral lines of ionized neutral hydrogen (known as H-alpha lines) and other lines of elements such as helium and nitrogen. The presence of these characteristic spectral lines helps astronomers identify and classify O-class stars.