Wolf-Rayet stars are a rare class of stars discovered by the French astronomers Charles Wolf and Georges Rayet. As can be seen in the full frame pictures (see revisions), all stars around display continuous spectra, some with prominent absorption lines. An example can be found in my gallery, the spectrum of Vega that I used for calibration (Spectroscopy of Vega and M57; Revision B). Wolf-Rayet stars are characterized by intense broad emission lines of helium (He), carbon (C) and nitrogen (N). When using the StarAnalyser100, these emission lines appear as roundish blobs of light on the continuous spectrum of the star. They are smaller copies of the stars zero order spectrum/image, because the emission line is only a fraction of the starlight. The spectrum therefore indicates that heavier elements are present on the surface of the star, meaning that the stars hydrogen is depleted, and the heavy elements are forged in the core of the evolved/old star. Hydrogen emission lines are therefore absent or weak. Some extremely massive young stars still fuse hydrogen in its core and can be identified by the hydrogen emission lines. To burn helium or heavier elements a star must have enough mass. Wolf-Rayet stars have ten to 215 solar masses and are the hottest and most luminous known stars. Their surface temperatures can reach 210000K, that is roughly 35 times the suns surface temperature. Depending on the mass of the star, its stage of development the chemical composition, surface temperature and radiation differs, causing a wide variety of spectral profiles. Wolf Rayet stars lose mass in form of fast moving hot winds and are therefore surrounded by nebulas. Their mass is therefore just a fraction of the mass they were born with.

WR136 (Top) became a red supergiant ~200000 years ago and will detonate as supernova in the next few hundred thousand years. Its spectrum displays hydrogen lines next to those of helium, nitrogen, carbon and maybe silicon. The spectrum is that of a WN6h Wolf-Rayet star, 20 times more massive than our sun and 70000K hot. The low-resolution spectrum reveals the N IV line at 710nm, a strong He II line at 541nm and a prominent N III line at 464nm as well as other, weaker ones.

Some central stars of planetary nebulas are in fact helium burning stars. Campbells Hydrogen Star (Mid) is an old star, approximately half as massive as our sun that began its live with less than eight solar masses. It is also counted as a Wolf-Rayet star because of the similarities in their spectra, the prominent broad emission lines. Some of these central stars are rich with hydrogen, others are not. These central stars should not be confused with the main class of massive Wolf-Rayet stars. Even though it is a hot stellar core that ejects mass in form of hot winds, their massive counterparts are a different beast. Campbells star displays a strong C III and a strong N III line at 569 and 465nm as well as weak C IV and He I lines. The most prominent lines in the continuous spectrum are the hydrogen Balmer lines.

Planetary nebula like the Blue Snowball (Bottom) are remnants of the outer shell of red giants ejected into space. The dying stars UV-emissions ionize the gas and excited atoms and ions. When relaxing, the energy difference of the excited state to the ground state is emitted as light with photons of a definite energy that means wavelength and therefore colour. The central stars are not very luminous in the visible part of the spectrum, but they are bright in the UV meaning they are hot (up to 30000K). Therefore, they are absent in the spectrum and only the emission of the gases is detected. The emission lines of hydrogen are detected as smaller blobs, a bigger blueish blob results from the O III line at 500nm. The O III line is the result of a forbidden line that can only exist due to the low density of the gas in space. Though the star is not massive and hot enough to fuse carbon, neon or heavier elements, the planetary nebula shines in the colours of hydrogen and oxygen, the ash of the helium fusion. Burning helium creates carbon and when carbon fuses with helium, oxygen is forged in the star. To fuse carbon to neon, oxygen and magnesium, a star needs to begin its life with at least 9 to 11 solar masses.
Planetary nebula are relatively short lived, but the dense white dwarf will radiate its stored heat for over 10 billion years. Out of fuel, the stellar corpse gradually cools. The universe is too young to find a single black dwarf, a cooled down white dwarf.

All three have in common that they forge the elements of life. The most massive and brightest of them live a short life, produce more elements while the smaller, less luminous ones are still "alive" and will be for billions of years until they cool. They are all more or less stellar cores surrounded by gas but their developement could not be more different.

Blue Snowball - 40 x 10s - gain 400
Campbells star - 46 x 6s - gain 400
WR136 - 41 x 20s - gain 400

5 to 10 darks each



ps: Though astronomy is not my scientific field of expertise, critique is welcome for me to advance my knowledge and correct the description.