Hypergiant star studied

Dec 10, 2012
Artist's rendition of the hypergiant HR 8752 traversing the 'Yellow Evolutionary Void'. The graph plots the temperature at the surface of the star observed over a century. It rose from 5000 to 8000 degrees between 1985 and 2005, while the radius of the hypergiant decreased from 750 to 400 times the radius of the Sun. Credit: © A. Lobel, Royal Observatory of Belgium

A European research team has published the results of a 30-year study of an extraordinary hypergiant star. They have found that the surface temperature of the super-luminous star HR 8752 increased by about 3000 degrees in less than three decades, while it went through an extremely rare stage called the 'Yellow Evolutionary Void'. The discovery marks an important step closer to unravelling the evolution of the most massive stars.

A team of astronomers from six European countries, including the Royal Observatory of Belgium (ROB), has investigated the hypergiant star HR 8752 for 30 years while it traversed the 'Yellow Evolutionary Void'. The 'Void' is a short stage in the lives of the most when they become very unstable. The team finds that the surface temperature of HR 8752 rose surprisingly fast from 5000 to 8000 degrees in less than 30 years. The research results were very recently published in the journal . The discovery is an important step forward to resolve the enigma of the hypergiants, the most luminous and massive stars of the Galaxy.

Hypergiants can shine millions of times brighter than the Sun, and they often have a diameter several hundred times greater. HR 8752 is a quarter million times more luminous than the Sun. The powerhouse is therefore visible with normal binoculars at large distance from the Earth in the Northern constellation of . There are currently only 12 hypergiants known in our Galaxy.

The 'Yellow Evolutionary Void' is a unique stage in the short life of a hypergiant when its temperature and can quickly change. The team has discovered that the are very unstable inside the Void because outwardly directed forces act equal or sometimes even stronger than the . Due to the unstable atmosphere, hypergiants lose tremendous weight in this 'forbidden zone', which can sometimes amount to the in a year. When a hypergiant enters the 'Evolutionary Void' the star tries to it leave as quickly as possible. That is why almost all hypergiants are found outside the Void.

The team finds that HR 8752 is a very rare hypergiant which has partly traversed the Void. The changes of its atmosphere were closely monitored with regular observations over 30 years.

Alex Lobel, co-author of the study and ROB scientist explains that "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."

Between 1900 and 1980 the atmospheric temperature of HR 8752 stayed almost constant around 5000 degrees, but it rose very rapidly to 8000 degrees between 1985 and 2005. The team calculates that the stellar radius decreased from 750 to 400 times that of the Sun. In 1985 the team embarked on a long-term spectroscopic observing program when it found that the remarkable hypergiant was exactly at the border of the 'Yellow Void' and started to cross over. "HR 8752 had to struggle through the Void which has changed the physical properties of its atmosphere", Lobel adds.

The team further demonstrates that the Void actually consists of two parts in which the atmosphere of the hypergiant is unstable. They result from ionization of large amounts of hydrogen and helium gas in the atmosphere, divided by a small zone around 8000 degrees where it becomes more stable.

The fate of HR 8752 is currently unclear but there are strong hints that these massive hypergiants may perish in a powerful supernova explosion. Or they quickly traverse the Void and transform into a hotter type of erratic stars known as the "Luminous Blue Variables". In either case that will not pass unnoticed according to Kees de Jager and Hans Nieuwenhuijzen, the of the Netherlands Institute for Space Research in Utrecht who directed the research of HR 8752 over the past three decades.

The discovery is an important new step for explaining the existence of these extreme stars. A number of other hypergiants with similar spectacular properties is expected to exist in the Milky Way. The search for these remarkable stars with dramatic changes over human timescales has just begun, but has been forever put on track.

Explore further: Computers beat brainpower when it comes to counting stars

More information: Arxiv pre-print: dx.doi.org/10.1051/0004-6361/201117166

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User comments : 4

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jonnyboy
1 / 5 (3) Dec 10, 2012
great information however the article writer needs to go to school.
dschlink
5 / 5 (3) Dec 10, 2012
More of a translation problem. The link has some really strange word usage.

Great work though and an excellent opportunity. The most interesting scenario, of course, would be if the star evolves in a fashion that doesn't fit current theory.
JamesL
5 / 5 (3) Dec 10, 2012
Watching a star for 30 year is just awesome. Written in plain and clear language. Where are my binoculars?
GSwift7
1 / 5 (2) Dec 11, 2012
The paradox of unusual objects like this is that we are able to observe it due to the extremely unusual brightness of it. This is similar to quasars, which also are bright enough to allow us to see what would otherwise be too dim to see much about. The result is that we end up knowing more about the exceptions to the rules than we do about the more common, but not as easily observable objects. Most stellar births are shrouded in so much dust and gas that we hardly ever get a good look at what happens there, for example.

Hubble opened up a whole new look at our Universe and we learned things we didn't even imagine we could learn. I look forward to the discoveries we might make if the JWST is successful. I'll be amazed if everything on JWST works as planned. It is truely one of the biggest engineering gambles of modern science.

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