All about Wolf-Rayet stars
Some facts about Wolf-Rayet Stars
Wolf-Rayet stars are named the ‘most massive and brightest stars known’. This is because their temperatures start at 30,000 degrees celsius. They also have strong stellar winds which blow away their outer atmospheres, revealing the stars’ inner layers. Blowing at over ten million miles per hour, the stars shed about 2 thousand billion billion tons of material every year. That’s equivalent to the mass of three Earths! Since these stars have trouble holding themselves together, they don’t last very long, burning up their fuel quickly and blasting mass into space, eventually tearing themselves apart. They are usually members of binary stars with O or B stars (types of stars classed by temperature) as companions.vd
The French astronomers Charles Wolf (1827-1918) and Georges Rayet (1839-1906) co-discovered this type of unusual, hot star, which are now named after them. They discovered Wolf-Rayet stars by using the Paris Observatory’s 40cm Foucault telescope in 1867 to observe three stars whose spectra had strong, broad emission lines, but few absorption lines, which is unusual for stars.
For decades, the reason for these emission bands remained a mystery. Eventually, it was discovered that these lines resulted from the presence of helium, which was discovered in 1868, one year after the original observation. E.C.Pickering also compared Wolf-Rayet spectra with nebula spectra, and noticed similarities between them. This led to his discovery that some or all Wolf-Rayet stars are in the centre of nebulae.
In 1929, doppler broadening (the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules) was being used to explain the width of the emission bands. It was concluded that the gas surrounding Wolf-Rayet stars must be moving with velocities of 300-2400 km/s, and therefore they are continually ejecting gas into space. This produces an expanding envelope, or bubble, of nebulous gas. The force that ejects this gas was then discovered to be radiation pressure.
Later, Rayet became Director of the Bordeaux Observatory, and to this day, we have discovered over 500 Wolf-Rayet stars in our galaxy.
About the most massive star, R136a1, which is a Wolf-Rayet star
R136a1 is the most massive star known, which is a Wolf-Rayet star. It is 163,000 light years away from the Sun, and is located in the Large Magellanic Cloud. It is also part of the R136 super star cluster, and has the mass of 315 suns. Despite being the most massive star known and shining 9 million times brighter than the sun, it requires a telescope to see. An interesting fact about R136a1 is that it defies what scientists know about how stars form. A popular hypothesis among scientists is that R136a1 did not form directly from the collapse of a molecular hydrogen cloud, but rather from two massive stars colliding.
About WR 124, a Wolf-Rayet star
WR 124 is 15,000 light years away from the Sun. Its spectral type is WN, meaning it falls into the hottest ‘O’ spectral type of stars, but is referred to as ‘W’ from Wolf-Rayet. The ‘N’ means it shows strong emission lines of nitrogen. WR 124 is the glowing star in the centre of a huge, fiery nebula. WR 124 has a surface temperature of around 50,000 degrees celsius, and is one of the hottest known Wolf-Rayet stars. It is a massive, unstable star which is blowing itself apart. Its material is travelling at up to 150,000kph. The nebula that surrounds the star, M1-67, consists of vast arcs of glowing gas which is violently expanding outwards into space. M1-67 is quite young, only 10,000 years old, and it contains clumps of material within it with masses 30 times the mass of Earth and diameters of 150 billion km.
WR 7, another Wolf-Rayet star
WR 7 is also 15,000 light years away from the Sun. It produced the emission nebula NGC 2359, which is also known as Thor’s Helmet because it looks like a helmet with wings. The nebula has a diameter of around 30 light years, and has WR 7 at its centre. Its surface temperature is between 30,000 degrees celsius and 50,000 degrees celsius, which is 6 to 10 times the temperature of the sun. It is an incredibly unstable star, ejecting stellar material into the interstellar medium at speeds which approach 7.2 million kph! Even though it is a massive star, it loses the mass of the Sun every thousand years. Material ejected from the star is done so in a spherical manner, which produces a bubble of material. This bubble has been shaped further by its interactions with the surrounding interstellar medium. WR 7 lies at the edge of a dense, warm molecular cloud, which is unusual.
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