How Massive Stars Begin To Die

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  • 0:02 Endpoint of Stellar…
  • 0:53 Comparing Massive…
  • 3:26 Temperature, Density, & Time
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Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

The ways massive stars and smaller stars evolve near their end are very different. This lesson will explain where they diverge, why, and what factors influence the beginning of the end for massive stars.

The Endpoint of Stellar Nuclear Fusion

Two pieces of bread dough or two pieces of iron - which two are easier to mush together? The bread dough, obviously. That wasn't a trick question.

The release of energy through nuclear fusion within a star occurs when less tightly bound nuclei are mushed together into more tightly bound ones. It's like taking fluffy egg whites to make a cream, which is then used to make hard caramel. With each subsequent mushing of ingredients, things get more and more tightly bound.

But there comes a point where even a massive star can't mush ingredients together to make new ones; that point is the development of an iron core within a star and it's a dead end for that star. It will be clear to you why as this lesson will explain how massive stars begin to die.

Comparing Massive Stars to Smaller Ones

Low to medium mass stars die when they run out of fuel by forming a planetary nebula and then becoming white dwarfs. It's a meek death compared to that of massive stars, as the lessons on supernovae make clear. A planetary nebula is a shell of illuminated gas surrounding certain dying stars. White dwarfs are dying stars of initially low to medium mass.

Nevertheless, like their smaller brothers and sisters, massive stars use up their initial core of hydrogen fuel first. This eventually produces a helium core with a hydrogen shell. The hydrogen shell ignites and the star expands into a giant. The helium core then contracts and ignites to fuse helium. Helium fusion eventually produces a carbon-oxygen core, and so the cycle of core ignition and shell ignition thereafter continues on and on in stage after stage with every new nuclear fuel. Such a process stops at the carbon core for smaller stars and they become white dwarfs thereafter.

But massive stars, such as those with a solar mass of about eight times that of our sun, end their lives very differently. They can't die as a white dwarf; they are too massive to do so. Their mass allows for the start of carbon fusion, a thermonuclear reaction that consumes carbon to produce oxygen, neon, sodium, and magnesium. The temperature necessary for this is one billion Kelvin, about ten times higher than that which is necessary for helium fusion to begin.

And here's where things get colorful. What happens next reminds me of those multi-layered jawbreaker candies. The central region of a supergiant, only about the size of Earth, has a series of concentric shells or layers undergoing nuclear fusion and, therefore, producing energy. As you can tell, after fusion of carbon, fusion of neon will begin, then of oxygen to make silicon, which then fuses to make iron.

Note how it's silicon not silicone that fuses to make iron. Silicone with an 'e' is made by 'e'arthlings (it's manmade), while silicon is naturally occurring. The problem with iron is, the star can't combine iron nuclei in any nuclear reaction in order to release energy and, thus, the evolution of our massive star ceases as a result. It is this iron core that eventually leads to a massive explosion that kills the star off once and for all.

Temperature, Density, and Time

The way these massive stars die and form these layers ties in with important concepts related to temperature, density, and even time. As each consecutive fuel is ignited, its nuclear fusion reactions must become faster and faster. For example, the stage of hydrogen fusion (hydrogen being our first nuclear fuel) is in the order of millions of years for a star with 25 solar masses. But when we get to silicon fusion, the duration of this stage is about one day.

Why is that? There are several things to consider to help understand why that's the case. First, each successive nuclear fusion stage is faster than the one preceding it because the temperature and density of the core increases. Furthermore, massive stars, like massive cars, must use up their fuel more quickly than smaller ones in order to support their own weight.

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