Hypergiant Stars

Hypergiant Stars

The exact definition of hypergiant stars is somewhat nebulous. They were first identified separately from other supergiant stars because of their significantly large luminosity and mass as compared to similar stars.

Creation of Hypergiant Stars

All stars spend the majority of their lives on the main sequence. However, once the hydrogen fuel in their cores are depleted they will leave the main sequence and evolve into different types of stars all together.

However, the process that ensues is completely dependent on their mass. Once high mass stars have exhausted the hydrogen fuel in their cores they expand into much larger supergiant stars and begin fusing helium into carbon and oxygen in order to avoid gravitational collapse.

At this stage they will oscillate between the red supergiant and blue supergiant phases as the fusion rates in their cores vary for various reasons. (They will also appear as yellow supergiants as they transition between the two.) The different colors are due to the fact that the star is swelling in size to hundreds of times the radius of our Sun in the red supergiant phase, to less than 25 solar radii in the blue supergiant phase.

In these supergiant phases the stars are losing mass quite rapidly, and therefore are quite bright. It was noticed though, that some of the supergiants were brighter than expected, and follow up observations revealed that they were also some of the most massive stars ever measured.

With some of these stars exceeding 100 times the mass of our Sun - with the largest some 265 times its mass - they were incredibly bright. It was these characteristics that gave rise to a new classification of star - hypergiants. These stars are essentially supergiants (either red, yellow or blue) that have very high mass and high mass-loss rates (effectively making them very luminous.)

Death of Hypergiants

Because of their high mass and luminosity, hypergiants only live a few million years; quite short compared to the billions of years that stars like our Sun will exist.Eventually the core will fuse heavier and heavier elements until a predominantly iron core is reached. At this point, the endothermic nature of the iron fusion reaction means that fusion can no longer sustain the hydrostatic equilibrium that prevents the star from collapsing upon itself. The resulting core collapse causes a supernova explosion of incredible magnitude. In fact, some theorize that instead of a typical type II supernova either a Gamma-ray Burst (GRB), sometimes called a hypernova, occurs. The most likely remnant left behind from such an event would be a black hole, or perhaps a neutron star or magnetar.

Yellow Evolutionary Void

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 massive stars 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 Astronomy and Astrophysics. 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 Earth in the Northern constellation of Cassiopeia. 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 luminosity can quickly change. The team has discovered that the atmospheres are very unstable inside the Void because outwardly directed forces act equal or sometimes even stronger than the force of gravity. Due to the unstable atmosphere, hypergiants lose tremendous weight in this 'forbidden zone', which can sometimes amount to the mass of the Sun 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 astronomers 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.