20-6 High-mass stars violently blow apart in core-collapse supernova explosions

Our present understanding is that stars of about 8 M_⊙ or less shed most of their mass in the form of planetary nebulae. The burned-out core that remains settles down to become a white dwarf star. But the truly massive stars—stellar heavyweights that begin their lives with more than 8 solar masses of material—do not pass through a planetary nebula phase. Instead, they die in spectacular core-collapse supernova explosions.

The Violent End of a High-Mass Star

To understand what happens in a core-collapse supernova explosion, we rely on theoretical models based on the known behavior of gases and atomic nuclei. The following model accounts for the observed properties of supernovae fairly well, which lends support to the overall picture. And, as we will see in Section 20-8, a special kind of “telescope” has allowed us to glimpse the interior of at least one relatively nearby supernova.

The core of an aging, massive star gets progressively hotter as it contracts and ignites successive stages of nuclear fusion (see Stage 1 in Figure 20-14). But even as more of the star’s core is converted into iron, this iron is unable to undergo further fusion. Without this heat-producing nuclear energy, the usual pressure that supports a star’s core from collapsing is absent. Nonetheless, the core is held up briefly by degenerate electron pressure, which is the same pressure that keeps a white dwarf from collapsing. However, degenerate electron pressure can only hold up a 1.4 M_⊙ core against its own tremendous inward gravitational pull (this is the Chandrasekhar limit, see Section 20-4). When enough iron builds up to cross this threshold of mass, the result is a cataclysmic core collapse (Stage 2 in Figure 20-14). The collapse is so sudden and gravity is so strong that the outer core comes rushing in at over 20% the speed of light!

Figure 20-14: A Core-Collapse Supernova This series of illustrations depicts our understanding of the last day in the life of a star of more than about 8 M_⊙.

(Illustration by Don Dixon, adapted from Wolfgang Hillebrandt, Hans-Thomas Janka, and Ewald Müller, “How to Blow Up a Star,” Scientific American, October 2006)

Even though steady fusion reactions in the core end with iron, the collapsing core soars in temperature due to the gravitational implosion. Temperatures skyrocket up to about 10 billion K within a fraction of a second. As a result, gamma-ray photons emitted by the intensely hot core have so much energy that when they collide with iron nuclei, they begin to break the iron nuclei down into much smaller helium nuclei. This process is called photodisintegration. As Table 20-1 shows, it takes a high-mass star millions of years and several stages of nuclear reactions to build up an iron core, but, within a fraction of a second, photodisintegration blasts apart much of that core.

Within another fraction of a second, the core becomes so dense that the negatively charged electrons within it are forced to combine with the positively charged protons to produce neutrons. In fact, most of the core is converted into neutrons. This process also releases a flood of neutrinos (see Section 16-4), denoted by the Greek letter ν (nu):

There are other processes that produce neutrinos as well, and because these neutrinos carry away a substantial amount of energy from the core, the core cools down and condenses even further.

At about 0.25 seconds after its rapid contraction begins, the core is less than 20 km in diameter and its density is in excess of 4 × 10¹⁷ kg/m³. This is nuclear density, the density with which neutrons and protons are packed together inside nuclei. (If Earth were compressed to this density, it would be only 300 meters, or 1000 feet, in diameter!)

Matter at nuclear density or higher is extraordinarily difficult to compress. Just as compressed electrons experience a degenerate electron pressure, these neutrons in the core attain a degenerate neutron pressure. Thus, when the density of the neutron-rich core begins to exceed nuclear density, the core suddenly becomes very stiff and rigid. The core’s contraction comes to a sudden halt, and the inner part of the core actually bounces back and expands somewhat. This core bounce sends a powerful wave of pressure, like an unimaginably intense sound wave, outward into the outer core (see Stage 3 in Figure 20-14).

Initially, it was thought that the core bounce was sufficient to generate a shock wave that propelled a star’s matter outward during a supernova. While there is evidence for a shock wave, more energy is needed than can be provided by the core bounce alone. One of the mechanisms that can provide additional energy to the shock wave appears to involve neutrinos.

At first glance, you might not expect neutrinos to have an explosive effect because they interact so weakly with matter. Recall that after a trillion or so neutrinos pass through your head each second—which is our Sun’s normal output—most of them pass right through Earth without getting absorbed. In a collapsing core, however, the density of matter is so high during the collapse that enough neutrinos are absorbed to significantly heat matter around the core.

These neutrinos are essential to what follows. The absorbed neutrinos are thought to carry enough energy to produce hot inflating bubbles (Stage 4 in Figure 20-14). As the outer boundary of these bubbles pushes on matter around the core, the resulting shock wave ejects material from the star altogether. The result is an explosive supernova.

When the shock wave breaks out of the star, a portion of the explosive energy escapes in a torrent of light (see Stage 5 in Figure 20-14). At this point, the star has become a supernova (plural supernovae). Specifically, what we have described is the formation of a core-collapse supernova. We use this term because, as we will see in Section 20-9, it is possible for a different type of supernova explosion to occur that does not involve the collapse of the core in a massive star.

The details of the shock wave are not fully understood, neither the mechanism that drives it nor all the forms of energy that power it. Supernova explosions are still an active area of investigation, and as one researcher put it: “Astrophysicists still don’t know how to blow up a star.”

The Big Picture of Core-Collapse

One way to think about the evolution of a massive star’s core is that it undergoes an unrelenting gravitational contraction. Before the star arrives on the main sequence, hydrogen contracts until hydrogen fusion produces enough heat and pressure to temporarily halt further contraction. But as the resulting helium builds up, the core contracts until helium fusion turns on and produces a temporary halt to further gravitational contraction. This pattern continues all the way through to the production of iron in the core, with each stage of fusion—each “onion” layer in Figure 20-13—providing only a “pause” in gravity’s unrelenting contraction.

But these pauses in gravitational contraction required pressure that was produced by the release of nuclear energy. While that was possible when fusing elements lighter than iron, the iron core does not release nuclear energy, and offers nothing to support the surrounding layers against gravity.

With nothing to hold off gravity, the surrounding layers rush inward, gaining tremendous speed and kinetic energy. The core is also compressed further as a variety of processes unfold. However, more than anything else, the kinetic energy of the infalling matter is ultimately converted into neutrinos. In this sense, a supernova can be seen as a gravity-powered neutrino explosion.

For a few seconds during the collapse, the neutrino energy output is unimaginably intense: While a supernova can outshine an entire galaxy with its visible light, the energy in neutrinos is a hundred times greater. By depositing only a tiny fraction of this energy in surrounding layers of the star, the neutrinos produce hot inflating bubbles that propel most of the star’s matter into space.

The Wreckage of a Core-Collapse Supernova

Supercomputer simulations provide many insights into the violent, complex, and rapidly changing conditions deep inside a star as it is torn apart by a supernova explosion (Figure 20-15). For example, Figure 20-15a shows a computer simulation of a high-mass star several hours after the core bounce that initiates the supernova. The simulation produces turbulent swirls and eddies. Evidence in favor of such turbulence comes from images of the remnants of long-ago supernovae (Figure 20-15b). Such images show that material is ejected from the supernova not in uniform shells but in irregular clumps. These clumps are just what would be expected from a turbulent explosion. (We discuss supernova remnants further in Section 20-10.)

Figure 20-15: Turbulence in a Core-Collapse Supernova (a) This image from a supercomputer simulation shows a cross section of a massive star 2.8 hours into the supernova explosion. Density increases from black through red and orange to white. The darker inner region of the core is lower density because its heavier elements have been ejected into the outer layers. These dense metals appear almost as white flames as they undergo turbulence on the outermost shells of helium and hydrogen. The simulated supernova is SN 1987A. (b) Turbulence causes material to be ejected from the supernova in irregular “blobs,” as shown by these images of the supernova remnant Cassiopeia A. Each image was made using an X-ray wavelength emitted by a particular element. (Figure 18-26 shows a false-color image of Cassiopeia A made using visible, infrared, and X-ray wavelengths.)

(a: Konstantinos Kifonidis/Science Photo Library/Science Source; b: U. Hwang et al., NASA/GSFC)

Detailed computer calculations suggest that a 25-M_⊙ star ejects about 96% of its material to the interstellar medium for use in producing future generations of stars. Less massive stars eject a smaller percentage of their mass into space when they become core-collapse supernovae.

Before this material is ejected into space, it is compressed so much that a new wave of nuclear reactions sets in. These reactions produce many of the atoms on the periodic table of the elements (see Box 5-5) that are heavier than iron. Reactions producing the heavier elements require a tremendous input of energy (recall Section 20-5), and thus cannot take place during the star’s pre-supernova lifetime. Hence, much of the heavy elemental material that makes up our solar system, Earth, and even our bodies was produced in violent, explosive supernovae.