# Intro - The Behemoth Stars Though the cutoff between the 'intermediate-mass' and 'high-mass' star isn't well-defined, we commonly use the 8 $M_\odot$ limit. Be it the cutoff for supernova (dependent on a myriad of factors including [[Metallicity (Astro)|metallicity]]) or even just its late-stage evolution, it's possible to greatly simplify the process of teaching When a massive, MASSIVE star (>20 $M_\odot$) initially expands, its core burns so bright and hot that despite its expanded radius, it is still able to burn blue-white, giving it the spectrum of an early B-Type star. These stares are **blue supergiants** and constitute some of the brightest stars in our night sky, such as Hadar (Beta Centauri) %%link%% or Alnilam (Epsilon Orionis) %%link%%. Unlike the cores of sun-like stars, which stop and start as degenerate matter is ignited, evolved massive stars are powered consistently through alternating shells. Much like the [[Red Giants and the Pre-Death of Stars (Astro)#AGB Stars|AGB stage]] of such stars, the shells within a supergiant stops and starts. Where one shell burns through its material, the shell above it (made of the products from the fusion of the original shell) begins to burn equally as bright, keeping the star in near-constant flux. What's typical of these stars is that their dense cores permit convection only within the core, so the material being fused actually *mixes* within it. With each shell that is depleted, the star is forced to compress further and further to stop itself from collapsing inwards, burning its remaining material brighter and brighter. During this stage, a sufficiently (>120 $M_\odot$) massive star instead undergoes a 'false nova', throwing off its outer layers in a cataclysmic ignition of its degenerate helium, carbon and so forth. ![[Pasted image 20240823233054.png]] *A massive star in its carbon-fusing stage!* For massive stars, this steady compression leads to the removal of the star's outer layers by outward radiation pressure. What's left is known as a **Wolf-Rayet star**, a husk of a star's inner, evolved layers. And that's what we'll be covering today. # Wolf-Rayet Stars Otherwise known as **Type W** stars, Wolf-Rayet stars are the remnant inner layers of massive (>20-25 $M_\odot$) stars. By the time a star transitions into a Wolf-Rayet star, its surface temperature will have increased from anywhere between 20,000 to **210,000** Kelvins. At the upper end of those temperatures, the star will appear a brilliant violet, shooting off high-energy gamma rays and x-rays into deep space. To explain the large temperature range, we've got to understand that different Wolf-Rayet stars typically lie at different stages in evolution. These "microstates" represent the main element that the core is fusing, giving the star a distinct spectra. For example, a star burning helium in its core would have a drastically different spectrum from a star burning oxygen in its core. Remember that a massive star's core is convective, allowing for *dredge-ups* - the movement of material from the star's interior to its surface - to happen. %%DIAGRAM!! OF !!! DREDGE!! UPS!!! %% The coolest of the WR stars are the **WNh** stars. They are in the first stages of increased mass loss, where the solar wind enveloping them is still relatively diffuse. %%draw a diagram for each one!%% %%WCstars%% Finally, we have the **WO** stars. To have reached this point in stellar evolution shows to us astronomers that these stars are burning ***oxygen***, meaning that its days are more than numbered. With only the inner layers of their cores visible, it's no surprise that these stars are also blisteringly hot, with surface temperatures ranging from 140,000 to 210,000 Kelvins. These are also the rarest type of WR star - oxygen burning lasts for roughly 600 years, meaning that the discovery of one is a serendipitous event. Wolf-Rayet stars are important. Massive stars burn fast and bright, meaning that the WR stars we can see now are direct reflections of their respective [[Star-Forming Regions|interstellar mediums']] compositions. we can do this by looking at the WR star's stellar wind - often violent and highly ionised, the light emitted is perfect for our spectrographs to analyse. Though these stars will end their lives in spectacular supernovae, they're not any risk to our blue marble - as of writing, the closest Type W star to us lies in the Gamma Velorum system, 2,500 parsecs away. More than far enough away to dodge any negative effects of such a stellar (geddit?) explosion! # Luminous Blue Variables When these massive stars begin to show variations in their brightness, they're known instead as **Luminous Blue Variables**, or LBVs. # IWantToGiveSources Read more! Learn more! Consider yourself encouraged. 1. https://www.sciencedirect.com/topics/physics-and-astronomy/wolf-rayet-star 2. https://www.sciencedirect.com/science/article/abs/pii/S1387647313000031