why do hypergiants shed mass before death?

Question

I'm reading on this website here and will be using lots of quotes from it.

Hypergiant Star Seen Shedding Mass Ahead Of Explosive Death As Supernova

Astronomers using a telescope in Chile have observed a hypergiant star shedding massive amounts of mass, suggesting it is about to end its relatively short life in a massive supernova explosion. The red hypergiant star VY Canis Majoris, one of the largest stars ever found in the Milky Way, is losing enormous amounts of its mass as it deteriorates, they say. It is 30 to 40 times as massive as our sun and 300,000 times as bright. If it sat in the center of our solar system, it would encompass the orbit of Jupiter.

This interested me so I kept reading,

Radiation pressure is the force exerted by starlight, and is very weak, which is why only large dust grains have enough surface area to be affected and cause the star to lose mass, the researchers say. VY Canis Majoris, around 3,800 light-years away from us, is expelling an amount of dust and gas every year equal to 30 times the mass of the Earth, they say.

As the star died wouldn't the radiation pressure decrease? Less fusion = less radiation = less radiation pressure right? Why does increase of radiation pressure cause gas and dust to be pushed out of the star.

Answer 1

Towards the ends of their lives, massive stars exhaust hydrogen from their cores and burn heavier nuclear fuels. The sequence of events is that hydrogen core burning is followed by hydrogen burning in a shell around the core, then He core burning, then H+He shell burning, then C/O core burning, and so on.

During phases when shell burning dominates the luminosity of the star, it becomes a red (super)giant, both its radius and luminosity increase. The central misconception is that the luminosity decreases - that isn't true, what happens in broad terms is that as the mean mass per particle increases (because of fusion) in the center, the temperature must increase to provide the necessary pressure. But the nuclear reactions are very temperature dependent, so the star burns through the heavier fuels even quicker, leading to higher luminosity.

In the extended outer layers of the star, it is possible for dust to condense. The dust is coupled to the gas but scatters radiation from the interior. The momentum absorbed by the dust may be sufficient to lift the outer layers away from the star, because the "surface" gravity is relatively low.

In practice, dust alone may be insufficient. Dust cannot form until it is far enough away from the star to fall below the dust condensation temperature. Radiative driving of gas may also play a role closer in. This is where light is absorbed in atomic and molecular transitions, accelerating the gas. As it gains velocity, the transition is redshifted with respect to the radiation so that new photons are able to be absorbed and impart momentum.

The net effect of these processes, which are not fully understood, is that luminous red supergiants appear to be losing mass at the rate of $10^{-4}$ solar masses every year. This rate is highly dependent on the metallicity of the star, since this controls how much dust can form and how opaque to radiation the outer atmosphere is. Higher metallicity means more mass loss.

Answer 2

First, a better reference would have been better. That's a very commercial site, with a lot of advertisements and enticements. They gave the actual paper reference, it's more serious. It is at http://www.eso.org/public/archives/releases/sciencepapers/eso1546/eso1546a.pdf

Second, these super or hyper giants have a short lifetime, maybe millions of years instead of billions. They are so massive that they burn their hydrogen fusion fuel much more rapidly. There is a limit of luminosity limit, the Eddington limit, based on the balance of the outward luminosity pressure and the gravitational attraction. See the Wikipedia article which explains most of it, and talks about some remaining issues as well.

https://en.m.wikipedia.org/wiki/Hypergiant

The radiation or luminosity pressure is there even while the nuclear fuel is burning, and it causes them to loose mass. It is so for large stars with somewhat less fuel, as well as more more compact ones, but still giants. This pressure also causes a stellar wind, and for the larger stars there is less gravity holding them (just a lower density and larger) so they loose a lot of the atmosphere.

It also depends on whether the atmosphere has more metallic elements (more spectral lines to have absorption). That means they can absorb or scatter light more. So older stars with less gravity, big, and more metallic elements loose more of their atmosphere and mass.

The dynamic is not simply more or less fuel, it is also the dynamics of the radiation pressure possible as function of the makeup of the outer layers of the star.

Answer 3

when a super giant undergoes contraction due to the breaking of equilibrium between the gravitational force and the radiation pressure(near the end of its life time) , the outer layers of the star undergo nuclear reaction(due to the contraction) and the resulting radiation pressure blows up the outer layers. of-course, the star nearing its death has less radiation pressure due to the reaction happening at the center while shedding mass is chiefly due to the nuclear reaction at the outer layers.