Chapter 20. Death of a High-Mass Star

20.1 Introduction

This shows an artist's conception of a supernova that occurred 10 billion years ago - the oldest supernova ever discovered!

The goals of this module: After completing this exercise, you should be able to:

1. Describe how the evolution of a massive star creates a path on the Hertzsprung-Russell diagram that is distinctly different from that of a low-mass star.
2. Explain the theoretical sequence of events that occur in the core of a high-mass star just before it becomes a supernova, and the key physical processes that drive them.
3. Describe the appearance of a high-mass star through to the remnants that remain after the supernova explosion.
4. Relate observations to the theoretical internal changes of a high-mass star.

In this module you will explore:

1. The role mass plays in the post-main-sequence evolution of a high-mass star.
2. What is meant by neutron degeneracy pressure, and how it affects both the supernova explosion and the end state of the star.
3. How observations of supernovae confirm theoretical predictions.

Why you are doing it: High-mass stars experience interesting and exotic evolution, far beyond what their lower mass counterparts achieve. Studying the lives and deaths of these very massive stars not only introduces us to new phenomena, but also teaches us of our beginnings: the deaths of high-mass stars provide the material for the birth of new stars, planets, and all living things!

20.2 Background

This H-R diagram shows that the evolutionary paths of low and high-mass stars are very different.

In a separate activity, we explored the post main-sequence evolution of a low-mass (0.4 M_{\(\odot \)}- 4.0 M_{\(\odot \)}) star, in other words, a star like our Sun. To summarize, after leaving the main sequence a low-mass star evolves through the red giant, horizontal branch and asymptotic giant branch phases, before finally shedding its outer layers as a planetary nebula and dying as a white dwarf. The Hertzsprung-Russell diagram (H-R diagram) nicely summarizes the changing outward appearance of the star during this process. It shows how the star's luminosity and surface temperature change as the star evolves. While the evolutionary path just described is typical of all low-mass stars, the individual mass of a star is the key factor that drives the internal changes that in turn determine the details of its evolutionary path. I suggest that you do the low-mass activity first, and then come back to this one.

Mass also plays an important role in determining the evolution of high-mass stars (stars with masses greater than 4.0 M_{\(\odot \)}). While high-mass stars have some early evolutionary stages and characteristics in common with their lower mass counterparts, the later stages are very different. In this activity, we will study these differences in detail, and we will explore the role mass plays in determining the evolutionary path of these massive stars. We will also find that high-mass stars experience explosively dramatic evolution that low-mass stars never dream of!

20.3 Fusion After the Main Sequence

(Top) High mass stars expand and cool as they evolve. (Bottom) The end of core helium fusion.

Like its lower-mass counterparts, when hydrogen fusion ends in the core of a high-mass star, its core begins to contract and heat while the hydrogen starts fusing in a shell above the core, making the outer layers of the star expand. A high-mass star easily reaches the temperatures and pressures needed for helium fusion to begin, and this new, stable phase of evolution begins quietly. After exhausting its supply of helium in the core, a high-mass star also begins to fuse helium in a shell above the contracting and heating carbon core. Its outer layers expand even further due to the high-energy output from these two shell sources, and the star becomes a truly giant star. In fact, a star of 8 M_{\(\odot \)} or more can grow to be 10-100 times larger than the Sun's size now!

Unlike a lower-mass star, however, helium fusion is not the end of the road for a massive star; it is able to reach the temperatures and pressures needed for carbon fusion, so it continues to evolve. As each successive element burns, first in the core and then in a shell, the star continues to grow larger. Strangely enough, its luminosity remains nearly the same since the outer layer cools as it expands.

This happens because luminosity depends on the product of radius (squared) and temperature (to the fourth power). The star's radius is increasing, but this is offset by its dropping surface temperature.

20.4 The Onion Skin

(Right) The evolved, high-mass star Betelgeuse is as big as Jupiter's orbit

Stars with masses between 4.0 M_{\(\odot \)} and 8.0 M_{\(\odot \)} will fuse carbon into heavier elements, such as oxygen, neon, sodium and magnesium, but the cycle stops there. They are simply not massive enough to burn heavier elements in their cores.

For stars with masses greater than 8.0 M_{\(\odot \)}, however, the fusion cycle continues with neon, oxygen, and then silicon as the "fuel" for fusion reactions. As each fuel is depleted, the core contracts, the surrounding layers begin to fuse that same fuel in a shell (which is surrounded by fusion shells of past core fuels) and the outer layers of the star expand and cool. Thus, these very massive stars cycle through periods of stable core fusion and expanding giant phases (caused by the heat from the contracting core, which increases the rate of fusion in the surrounding shells).

As shown in the figure, the interior of the star is made up of layers of fusion shells, often compared to the layers of an onion! The outer layers of the star, responding to the energy output of multiple fusion shells, expand and cool greatly, creating a super giant star much brighter and larger than any giant star produced by a low-mass star. Such a massive super giant star can swell to nearly fill the orbit of Jupiter - more than 100 times the size of the Sun and tens of thousands of times brighter!

20.5 Core Collapse

After silicon fusion, a massive star has run out of fuel. At this point, the core of the star is almost entirely composed of iron, which, no matter how dense and hot the core becomes, it cannot fuse to provide an energy source. Without a source of support against the downward pull of gravity, the core begins to contract and heat, as it did at the end of each previous fusion cycle. This time, however, there is nothing to stop the collapse.

To make matter worse for the core, as it contracts and heats, it reaches temperatures hot enough for a process called photodisintegration to occur. Photodisintegration occurs when the internal temperature is nearly 10 billion Kelvin; at this point, the photons in the core are energetic enough to split nuclei apart - even tightly bound nuclei like iron! In just a fraction of a second all of the nuclei created by millions of years of fusion are completely destroyed, leaving only elemental particles - protons and neutrons - in their place. Unfortunately for the star, photodisintegration requires a lot of energy, which it gets by absorbing some of the heat energy released by the collapse of the core. Thus, this process actually creates even more of an imbalance in the core, speeding its collapse.

After only a tenth of a second more, the collapse of the core raises its density to the point that the protons and electrons inside it are pushed together. This does not stop the collapse, however, and eventually even the protons and electrons are squeezed together, forming neutrons. This process creates neutrinos, which are able to escape even from the high densities in the core, and travel quickly outward through the star's outer layers, carrying away additional energy. It seems that once the iron core is formed, every process which occurs works to speed its collapse!

Stopping the collapse of the core: Degeneracy Pressure vs. Gravity

About a quarter of a second after the rapid collapse of the core begins, its collapse finally begins to stop as the core reaches high enough densities that its individual neutrons are squeezed together, as depicted in this figure. When this happens, the neutrons themselves resist further collapse by exerting a pressure, known as neutron degeneracy pressure, much as electrons do in low-mass stars (discussed further in a separate activity). This occurs when the density in the core reaches 10¹⁷ kg/m³ - one hundred trillion times denser than the current average density of the Sun!

Play the following animation to see the evolution of the core.

20.6 An Explosive Death

Supernova 1987A in the Large Magellanic Cloud

The ball deforms slightly as it hits the wall, then bounces back.

The powerful pressure provided by degenerate neutrons is strong enough to stop the runaway collapse of the core, but it cannot do so instantaneously. The free-falling material in the core has enormous inertia, making it difficult to stop. Neutron degeneracy pressure thus only slows the collapse at first. By the time it comes to a full stop, the core has passed its equilibrium, and is up to a million times denser than it was when neutron degeneracy pressure first began to slow its collapse.

To understand why this happens, imagine a tennis ball hitting a brick wall. The wall is strong enough to stop the ball, but the ball has momentum, so when it hits the wall one side of it becomes momentarily squashed - compressed by the force of the wall pushing back on it. As the ball bounces back off of the wall, it expands, regaining its original shape. Likewise, the core of a high-mass star bounces, or rebounds, after stopping. The difference is that for the stellar core, this bounce occurs at nuclear densities - meaning that the density is so high that nuclear particles such as neutrons have been squished together. A bounce at densities that high initiates an incredibly powerful pressure wave that compresses and heats the material it travels through. This pressure wave is speeded on its way up to the surface by the convection and turbulence in the layers surrounding the core, and it quickly begins to move faster than the speed of sound in the star's outer layers. Such a wave is known as a shock wave. As this shock wave speeds through the star's outer layers, it propels them outward, causing the star to both lose most of its mass and become temporarily thousands of times brighter. This explosive outburst of light and material is known as a supernova. The image below shows a computer simulation of a supernova as it explodes through the outer layers of a massive star.

A simulated supernova 5 ½ hours after the core "bounce" (red = hydrogen, green = helium, turquoise and blue = carbon, oxygen, silicon, and iron)

20.7 Out of Death Comes Life

The violent death of a massive star has a silver lining, however. As the shock wave passes through the star, it compresses and heats the material enough to generate thermonuclear reactions which create many new chemical elements - some even heavier than iron! Reactions of this type require too much energy to occur during normal fusion in the core of even the most massive stars; it is only in the extreme conditions created by the shock wave that they flourish. This means that most of the metals and other heavy elements found on Earth - such as silver or gold jewelry - were theoretically created during a supernova explosion!

These newly created elements are blasted outward (along with the outer layers of the star) by the supernova into the surrounding interstellar medium, where they become the raw material out of which new stars (and planets) can form. In fact, supernova explosions themselves often directly trigger new waves of star formation, giving birth to the next generation of stars even as the massive star dies.

The supernova process is summarized in the animation below:

20.8 What's Left?!

The Gum Nebula: a Spectacular Supernova Remnant. The supernova explosion occurred about 11,000 years ago, and the remnant now has a diameter of about 700 pc (2300 ly).

The outer layers of the star that are blasted into space by the shock wave are called a supernova remnant. The expanding gases of the remnant gradually slow and cool as they travel away from what is left of the star, and eventually (after tens- to perhaps hundreds- of thousands of years) simply become part of the interstellar medium. Before that happens, however, these gases may collide with the existing material of the interstellar medium, heating it and causing it to radiate light. Such collisions can also initiate the collapse of a cloud and thus the formation of a new generation of stars. Supernova remnants create some of the most beautiful sights in astronomy, as can be seen in the figures.

Looking inward, we find that the violence of a supernova explosion leaves most of the star's inner core intact. The fate of this core depends on its mass after the explosion, which depends on both the initial mass of the star itself and on the details of the supernova explosion. If the mass of the core is less than 2–3 M_{\(\odot \)}, the star ends as a neutron star. A neutron star is an object that is so dense that not even the degenerate pressure of electrons can support it against collapsing. This means that it is even denser than a white dwarf - the remnant of low-mass star formation. Instead, it contracts until the pressure of individual neutrons being pushed together (neutron degeneracy pressure) stops the contraction. Neutron degeneracy pressure is the same pressure that halted the collapse of the core before the supernova occurred; it is one of the most powerful forces in the universe. Neutron stars are very compact objects, usually about 20 km in diameter (the size of a large city).

If the core is more massive than 2-3 M_{\(\odot \)} then nothing can halt its collapse, and the object becomes a black hole. When this happens, the core simply keeps collapsing, becoming more and more dense, until it disappears within its own event horizon, the radius from which nothing can escape from the black hole. Black holes and neutron stars are truly bizarre objects and are presented in more detail in separate activities.

20.9 Observations of Supernova - Images

We have been lucky enough to observe several Type II supernovae in great detail. Type II supernovae are created by the violent death of a high-mass star. (Type I supernovae are discussed in a separate activity.)

In a few cases, we have images of the star both before and after it becomes a supernova. These observations dramatically highlight the brightening that occurs during the explosion. This high luminosity is sustained first by the large amount of heating produced by the shock wave as it travels through the star's outer layers. As the expelled material moves outward from the central star, its expansion causes it to cool. The supernova's luminosity is then produced by the decay of the many radioactive isotopes produced during the explosion itself.

Supernova 1993J in the galaxy M81

More detailed observations show intricate structures and details of the explosion. This figure shows the ring-like structure produced by the interaction of the radiation and material expelled in the explosion with the gas in the surrounding medium. This surrounding medium was, itself, once part of the upper atmosphere of the massive star that was blown away during a previous super giant stage of evolution. The outer rings are produced as the burst of UV radiation, which occurs initially during the explosion, heats the interstellar medium. The middle ring marks the progress of the shock wave as it travels outward from the central core.

The Crab Nebula: a Supernova Remnant

Perhaps the most stunningly beautiful observations of supernovae are of the remnants – the outer layers of the star that are thrown out by the explosion. Supernova remnants show intriguingly complicated and intricate shapes, as shown above. These result both from the details of the explosion itself as well as the characteristics and distribution of the interstellar medium that the exploded material encounters. Measuring the speed of the supernova remnant also allows us to extrapolate back in time and determine when the explosion occurs. This technique allowed astronomers to identify the Crab Nebula, pictured below, with a supernova recorded by ancient Chinese astronomers in 1054 A.D.!

20.10 Observations of Supernova - Light Curves

A Supernova Light Curve

Analyzing how the light emitted by the supernova changes over time helps us to understand what happens during the explosion itself. The light curve of a Type II supernova shows how its luminosity peaks with the explosion and then tapers off in a step-like fashion, as shown below. The steps, or bumps, in the light curve indicate periods of steep and gradual decreases in the supernova's luminosity. The decay of the light curve is powered by the radioactive decay of heavy elements produced during the explosion. Type II supernovae all begin as massive stars with very close to the same internal (onion skin) composition and undergo the same processes to become supernovae, producing the same set of radioactive elements during the explosion. These similarities make their light curves very distinctive and a useful tool for identifying supernova explosions even in other galaxies.

20.11 Observations of Supernova - Spectra

Studying Type II supernova spectra also yields important information about the exploded star that helps to identify it. The figure below shows a typical Type II supernova spectrum. The most prominent feature is the very strong hydrogen line Hα. This shows that the material thrown out by the explosion was composed mainly of hydrogen, just as we would expect if this material came from the outer layers of a massive star.

Spectrum of a Type II Supernova: Notice the strong Hydrogen Alpha line, which forms due to the hydrogen in the star's outer layers.

20.12 Observations of Supernova - Neutrinos

The interior of the giant Super-Kamiokande detector in Japan

As explained earlier in this activity, the core of a dying, massive star emits short, strong bursts of neutrinos during the collapse. Observations of neutrinos from supernovae yield invaluable glimpses into the processes occurring in the core of the star before the supernova explosion actually occurs. Neutrinos are very difficult to detect, but two neutrino detectors on Earth successfully measured the neutrino output from supernova 1987A. The measurements showed that the burst of neutrinos occurred before the explosion, as predicted by theory. The number of neutrinos measured helped scientists to calculate the intensity of the explosion and to confirm theoretical details of what takes place in the very heart of the star before and during the explosion.

20.13 Summary

In this activity, you have seen the changes a massive star undergoes after it leaves the main sequence. Perhaps the most interesting part of that evolution is its death, which is both violent and spectacularly beautiful. Supernovae give us a sense of how connected and self-sustaining the Universe is, as the death of one star leads to the birth of others and to the production of elements that are a part of our every day life. This cycle is summarized in the following figure.

The Stellar Cycle of Life

20.14 Quick Check Quiz

Indepth Activity: Death of a High-Mass Star