Wolf-Rayet Star: My Favourite Stellar Object

Artist impression of a Wolf-Rayet star (NASA)

Wolf-Rayet (WR) stars are my favourite stellar objects bar none. Due to the excitement factor I find them even more interesting than black holes, pulsars and quasars. Why? Well, they are a significant period of a massive star’s lifetime making its violent, self-destructive death, possibly culminating in a supernova or gamma ray burst (GRB). WR stars blast out dense stellar winds creating a bubble of matter that completely obscures the star’s surface from any attempts at observation. They are also very noisy neighbours, disrupting binary partners and messing up huge volumes of space. If you thought a star might die quietly, the WR phase ensures this isn’t the case and astronomers are paying attention, making some of the most detailed observations of WR stars yet…

WR stars may sound pretty exotic, but we’ve actually been observing them since the 1860s. The first observations were made by the French astronomers Charles Wolf and Georges Rayet and this exciting phase of a star’s life was forever designated the “Wolf-Rayet” phase. They are massive stars, usually in excess of 20 solar masses and they are very hot with surface temperatures of 25,000K to 50,000K (compared with the Sun’s ~6,000K photospheric temperature). WR stars shed 10-5 solar masses per year (compared with our Sun’s 10-15 solar mass loss per year), creating fast and dense stellar winds. It is for this reason WR stars are surrounded by huge nebulous clouds of gas and dust. These stellar winds have been measured (via spectroscopic analysis of non-thermal broadening of emission lines) to blast from WR stars up to a velocity of 2,000 km/s. This is all very impressive, but why is the WR phase so violent?

Wolf-Rayet 104 as viewed from Earth - the spiral nebula is produced by the stellar winds (Univ. Sydney)

Toward the end of a massive star’s life, all the hydrogen has been burnt through nuclear fusion and heavier elements such as helium, lithium, nitrogen, carbon or oxygen take over. Depending on the enrichment of which element in the WR star, these unstable stellar objects are designated as nitrogen-rich (WN), carbon-rich (WC) or oxygen-rich (WO) stars. As each progressive fuel is depleted, the core of the WR star will become iron-rich. Iron is the heaviest element that can be created by normal stellar nucleosynthesis (heavier elements are formed via red giant stars or supernovae), so normal fusion reactions will stop in the WR star core, allowing gravity overcome thermonuclear expansion, collapsing the star. It is generally accepted that this is the seed for the rapid implosion that leads to a supernova, and in some cases a GRB (the most energetic explosion measured in the Universe). The phase before this occurs will be naturally violent, giving the impression that WR stars are rather suicidal.

I’ve written a few articles about the nature of Wolf-Rayet stars on Astroengine and Universe Today, but here are two of my favourites:

A Wolf-Rayet star WR124 with surrounding nebula M1-67 (NASA)

The Wolf-Rayet Star, Gamma Ray Burst Connection
In January 2008, the first ever supernova observed as it exploded was observed. And what star created this supernova? Yep, it was a Wolf-Rayet star. Although astronomers had a good idea that the WR phase ended in a supernova, it had never been directly observed till this year. So WR stars generate supernovae, how about GRBs? Can the largest explosions observed be attributed to WR stars? According to Dutch researchers at the University of Utrecht, a recent GRB observed by the “Pie of the Sky” observatory and NASA Swift mission could be down to a WR progenitor. With some complex tidal interactions with a binary partner, the WR star may be “spun up” by a stellar object like a neutron star, eventually causing the WR star to collapse in such a way that a huge GRB is possible. This “collapsar model” is very attractive to astrophysicists as it may help aid the prediction of GRBs in the future.

Wolf-Rayet Stars Disrupt the Stellar Neighbourhood
Long-period gamma ray bursts have been observed and astronomers have been at a loss to explain the reasons behind the phenomenon. Assuming a Wolf-Rayet star collapses and generates a GRB in a densely packed stellar environment, particularly inside a cluster of O-type stars, the shock formed through interactions with stellar winds can generate a secondary GRB after the primary event through a process known as inverse Compton scattering.

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