European astronomers have reported results of a thirty-year study of a hypergiant star dubbed HR 8752 that passed through an evolutionary stage known as the Yellow Evolutionary Void – an unstable stage that can profoundly alter the evolution of a star.

Hypergiants are the most luminous stars known in the Universe. They are very rare objects, only 12 of them being known in our Milky Way Galaxy. They can attain a luminosity millions times that of the Sun and reach a size of several hundred solar radii, with surface temperatures ranging from 3,500 K to 35,000 K.

HR 8752, also known as V509 Cassiopeiae, is a yellow hypergiant star located some 4,500 light years away in the northern constellation of Cassiopeia. It is about 250 thousand times as luminous as our Sun and can be observed with binoculars.

According to new results published in the journal Astronomy and Astrophysics, HR 8752’s surface temperature has risen rapidly from 5,000 to 8,000 K during the period from 1973 to 2005. This result marks the discovery of a fundamental missing link in the evolution of hypergiant stars.

Astronomers have discovered that the atmospheres of hypergiants are unstable in the Yellow Evolutionary Void (YEV) – an evolutionary stage in which the temperature and luminosity of a star can dramatically change its history – because the direct outward forces on their atmospheres equal or even exceed the inward gravitational pull.

When giant stars enter this forbidden zone in the evolution of their stellar parameters, they lose tremendous amounts of mass – as much as a solar mass in less than a year – owing to the instability of their atmospheres. For this reason, once the star is in this zone, it has to leave it as quickly as possible. That is why there are many stars to either side of it but very few within it.

The hypergiant HR 8752 was ‘captured’ in its passage through this zone, allowing astronomers to make a detailed study of the physics of the region, thus ascertaining what has happened to the star, how it lost its mass, how quickly its atmosphere has changed.

“Way back in 1999 in a study we published in the Astrophysical Journal Letters we realized that this hypergiant had increased its temperature by 3,000 K in less than 30 years. A similar phenomenon was discovered in another hypergiant called Rho Cassiopeiae, when a spectacular eruption took place in the year 2000,” said study co-author Dr Garik Israelian of the Instituto de Astrofísica de Canarias.

“These stars attract a great deal of attention because we expect them to explode as supernovae in less than 1000 years. Given the distances a which they are located, it’s possible that some of them will cease to exist forever.”

“Stars like HR 8752 are full of interesting physical enigmas still to be revealed. They are excellent objects for networks of robotic telescopes all over the world,” he said.

While the analysis of previous photometric observations shows that, at least between 1900 and 1980, HR 8752 maintained an almost constant surface temperature of 5000 degrees, towards 1985 the team observed certain indications that this star was fairly close to, or even beyond the lower temperature limit of the YEV. With the open question of what had happened, the scientists decided to embark on a long and systematic program of spectroscopic observations that would last for three decades until the present.

These spectroscopic observations have now revealed that, in the twenty-year period between 1985 and 2005, the surface temperature of the star had risen rapidly from 5,000 to 8,000 degrees as it passed through a series of events that brought about large losses of its mass. During this time, the radius of the star had decreased from 750 times that of the Sun as measured in 1985 to a mere 400 times the solar radius in 2005.

“Our team made a tremendous combined effort to observed HR 8752 and now we are delighted to see these marvelous results after so many years. We knew that this was the hypergiant to observe and we have been rewarded,” concluded lead author Dr Hans Nieuwenhuijzen of the SRON Laboratory for Space Research in Utrecht, the Netherlands.

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Bibliographic information: Nieuwenhuijzen H et al. 2012. The hypergiant HR 8752 evolving through the yellow evolutionary void. A&A, vol. 546, A105; doi: 10.1051/0004-6361/201117166