Star Endings: Pair Instability and Photodisintegration

Population III Stars

For the third part in the series of posts on star deaths, we look at deaths at the most extreme ends. They happen to enormously massive stars that were only common during the beginning of the universe, called the population III stars. Population III stars are a class of hypothetical first generation, never observed stars, that are believed to be over a hundred times more massive than the sun, and have none or almost no metal ( in astronomy any element of higher atomic number than helium).  Now, why they are called population III instead of I, don’t ask me, but turns out it is all reversed. Population II is the metal poor stars, and population I is metal rich stars like the sun.

While stars of 100+ solar mass exists in today’s universe (as seen here), they are extremely rare. In terms of probability, there is the fact that this kind of massive stars are extremely short lived compared to other stars, less than 10 million years compared to billions of years like the sun. In terms of the time scale of the universe, that is a blink of an eye, so more likely than not, you will miss it. Physically, huge stars form more easily in metal poor condition. The presence of metals allow for further cooling of gas to takes place, which inhibits the formation of huge stars. The temperature is important because the hotter the gas, the higher the gas pressure is, which counteracts the force of gravity. So, cooler gas contracts further, which gets fragmented in the process, creating smaller stars. In that way, metallicity is once again shown to be a powerful factor in determining the life and times of a star.

So, considering the hypothetical/theoretical nature of these objects and events, should we take all of this with a grain of salt? Well, yes and no. We do believe the theories are correct because they have worked in other situations. At the same time, that doesn’t mean we know all the details, and those details do matter when formulating accurate theories. Furthermore, there are the issues of rotation. The way I think of it is, the general picture is likely to be right, but if new observations come that contradict certain aspects of the models, then of course, I change my mind. That said, these are issues that are already being probed, with the search of population III stars being a high priority, and hey look, pair instability supernovas are already looking very likely.

Pair Instability End

Pair instability happens to stars beyond 140 solar mass. The energy in the core is such that the photons in there create electron-positron pairs. This happens because other forms of energy can be transformed into matter, and due to conservation principles, of charge (+1 for positive charge, -1 for negative charge) and particle number(+1 for matter, -1 for antimatter). Enough energy is taken away in the process that gravity dominates and the core starts contracting. Now, pair instability does occur in lower mass stars, around 100 to 140 solar masses, but it isn’t  enough to cause a runaway nuclear explosion. Instead, it will increase nuclear reaction until gravity and pressure balances out. This pulsation goes on until a star loses enough mass so pair instability doesn’t happen. Otherwise, the collapse of the core causes an uncontrollable reaction of oxygen and silicon to begin. While turning a huge amount of mass into Nickel 56, the core bounces back from collapse. What ends up happening is an extremely energetic explosion that leaves no remnant of the star behind. The star has pretty much been completely obliterated. Well, I guess you count count the gas and dust left by the star as remnant if you want…

Photodisintegration End

At the 250 solar mass point, a star dies by photodisintegration instability. At the energy generated by this star, a significant portion of the photons gets used up splitting atoms apart. The core collapses without the energy needed to hold up the star from its own gravity, and a black hole forms. This is pretty much core collapse, as in previous post, but with a different reason for the instability to happen, so some of the detail from the previous post about degeneracy and black holes applies here. I am not exactly sure at which part of the chain of nuclear reactions this happens at, though.

I wish I could say that there have been this kind of supernova observed. As far as I can see, I haven’t found any article regarding any observation of this type of supernova. So, for now you can put this entirely in the hypothetical area in your mind.

Conclusion

With this, I conclude the three series of posts on how stars die. Now you know the general process of how stars die, with some of the gory details displayed. If they felt dense, I feel for you. Even I learned a lot on the nuances of these processes, and had one hell of a time researching a lot of these facts. Even so, if these posts have informed you and made you interested in astronomy, well then I am happy. If it made you want to vomit, I am okay with that too, just tell me what went wrong!

Sources and Reading Materials

http://www.universetoday.com/24776/what-were-the-first-stars/

http://www.astro.yale.edu/larson/papers/Bromm02.pdf

http://scitation.aip.org/content/aip/magazine/physicstoday/article/64/4/10.1063/1.3580493

http://www.solstation.com/x-objects/first.htm

http://arxiv.org/pdf/astro-ph/0107037v2.pdf

http://arxiv.org/PS_cache/arxiv/pdf/0905/0905.0929v1.pdf

http://www.skyandtelescope.com/astronomy-news/a-super-duper-supernova/

http://news.nationalgeographic.com/news/2009/12/091202-biggest-star-explosion-supernova/

http://iopscience.iop.org/0004-637X/550/1/372/pdf/52134.web.pdf

http://arxiv.org/pdf/astro-ph/0212469v1.pdf

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