30-year study shows hypergiant star going through rare stage of evolution

Scott Sutherland
December 12, 2012

A group of European astronomers have released the results of their 30-year study of the immense star known as HR 8752 — also known as V509 Cassiopeiae — which show the star going through an extremely rare stage of stellar evolution that has never been observed before — 'The Yellow Evolutionary Void'.

HR 8752 is located around 4,500 light years away from Earth, in the constellation Cassiopeia, and is one of only seven hypergiant stars in The Milky Way. Hypergiant stars are incredibly large, up to hundreds of times the size of our Sun, and incredibly intense, shining up to a million times brighter than the Sun.

HR 8752 started its life as a fairly typical star, likely between 25 and 40 times the mass of our Sun, and sometime in the past it ballooned into a red giant star, around 750 times the size of the Sun. Put into context, if HR 8752 had suddenly swapped places with our Sun, Mercury, Venus, Earth, Mars and the asteroid belt would have been instantly consumed.

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Taking regular observations of HR 8752 from the early '80s until the present day, the astronomers found that, between 1985 and 2005, the star's temperature rose from around 5000 K to 8000 K, and it also exploded off huge amounts of its atmosphere into space in that time, creating a shell of material around itself and shrinking down to about 400 times the size of the Sun.

"HR 8752 was around 1980 identical to the eruptive hypergiant Rho Cas of spectral type F, but then the temperature of HR 8752's atmosphere rapidly increased by 3000 degrees and now shows the spectral properties of a hotter A-type star. We are baffled about the tremendous changes of HR 8752 in that period of time," says study co-author Dr. Alex Lobel, from the Royal Observatory of Belgium.

Furthermore, the study has found that the Yellow Evolutionary Void looks to be split into two regions. HR 8752 has now passed through the first part as its temperature rose to 8000 K, where there is apparently a very narrow stable range. It is unclear exactly what will happen with Hr 8752 now.

The star may undergo a cataclysmic explosion, erupting into a supernova, or it may slip into the second unstable area of the Void, and continue its evolution towards becoming a class of star known as 'Luminous Blue Variables'.

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According to the two astronomers that directed this research, Kees de Jager and Hans Nieuwenhuijzen, from the Netherlands Institute for Space Research, no matter what happens to HR 8752, it certainly won't go unnoticed.