Presentation: massive stars

So far we've looked at the rather complex life and death of medium-mass stars. By comparison, massive star death is relatively simple...and more dramatic. (Video link: "Crab Supernova Explosion.")

In this presentation, we'll discuss how massive stars die.

First, the supergiant phase.

Last week you were invited to my house party(*). What if instead you were invited to a "Donner party?" Good times? Bad times?

(*There is no actual house party.)

The settlers in the Donner party were trying to get over the Sierras very late in the fall, and due to bad weather and bad luck were snowed in for a while. But they ate their provisions waiting for the weather to clear, but by that time had run out of provisions, and had to resort to eating their livestock, then less nutritive bark off of trees, and then boiling shoe leather to stave off starvation. What would you resort to in order to keep alive past that point? Would this be the point of no return for you?

Likewise for a massive star, after depleting all of its hydrogen at the end of its main-sequence life, it is trying to stave off "starvation" by fusing a sequence of heavier and heavier elements, at each stage more difficult to accomplish, and yielding less energy. A medium-mass stars can only go as far as fusing helium into carbon, but a massive star has greater pressures and higher temperatures in its core, so it will fuse carbon into oxygen, oxygen into silicon, and silicon into iron... Now iron can be fused into even heavier elements, but everything past this point would then require more energy to be put in that would be released afterwards. Fusion no longer becomes a source of energy for the massive star, and the massive star truly begins to die.

Second, the type II supernovae death.

When the core of a massive star can no longer be supplied with energy from fusion to support the crushing force of gravity, the outer layers will begin to crush the core. This gravitational contraction provides another new source of energy, and as we shall see, this causes the outer layers to explode off of the imploding core. (Video link: "A Cosmological Fantasia.")

To illustrate this implosion-explosion, we have an AstroBlasterTM toy. It is a stack of three superballs on a spindle, with a fourth small superball that is free to slide on and off the top of the spindle. The packaging says it "works like a real 'Super Nova.'" Gotta respect a toy that comes with its own (child-sized) safety goggles.

To work the AstroBlasterTM you merely place drop the spindle onto the floor, and as the spindle and three larger superballs rebound off the floor, the fourth small superball rapidly flies upwards. This happens so quickly in real time that if you blink, you'll miss the upwards motion of the small superball.

The reason for this can be seen in a simplified version, where a basketball and tennis ball stack are dropped. If done properly, the basketball will "squish" and then rebound up against the lighter tennis ball, shooting it upwards to rebound to a much higher height.

So the AstroBlasterTM does work like a real type II supernova as the lower balls are the inner layers that fall inwards (downwards) onto the collapsing core (the floor), and the smaller upper ball is the outermost layer of the massive star, falling inwards (downwards) to rebound explosively off of the "squishing" inner layers. (Video link: "Stacked Ball Drop.")

Remember that fusing iron into heavier elements not only requires immense pressures and extremely high temperatures, but takes in more energy to make happen that would be released afterwards. These are exactly the same conditions that occur during the final moments of a massive star. (Video link: "A Cosmological Fantasia.")

With all the crushing pressures, extreme temperatures, and excess energy produced by this implosion-explosion process, not only do the outer layers get flung outwards, but in that brief cataclysm iron is fused in just about every heavier element. Everything that is heavier than iron on the periodic table was produced in a type II supernova somewhere in the universe in the past. Got any bling on you? You're just dragging around the carcass of a massive star. (Video link: "A Cosmological Fantasia.")

As the outer layers of a massive star get strewn about the universe (with a sprinkling of heavy elements), is there anything left?

The imploding core may collapse into a neutron star...

...or may even collapse into a black hole, both of which we'll discuss in the next presentation.

To wrap it up so far, a massive main-sequence star will run out of hydrogen in its core, expand to a supergiant as it tries to fend of "star-vation," fusing everything up until iron, and then the outer layers will implode and explode off of the collapsing core, which eventually either becomes a neutron star or a black hole.

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