20-9 White dwarfs in close binary systems can also become supernovae

In just a few seconds, a thermonuclear supernova completely destroys an entire white dwarf star

Astronomers discover dozens of supernovae in distant galaxies every year, but not all of these are the result of massive stars dying violently. A totally different type of supernova occurs when a white dwarf star in a binary system (possibly with a second white dwarf) blows itself completely apart. We now look at the different types of supernovae.

Types of Supernovae

There are two main mechanisms for a supernova explosion. We have already discussed one mechanism, which is a core-collapse supernova. A second mechanism involves at least one white dwarf in a binary system. Had astronomers understood these processes all along, they might have named the different supernovae according to their underlying mechanisms. However, the first indication that there were different types of supernova came from their differing spectral lines. As a result, supernovae are labeled not by their underlying mechanism, but by their spectra.

Supernovae with hydrogen emission lines are called Type II supernovae; these are core-collapse supernovae of the sort we described in Section 20-6. They are caused by the deaths of highly evolved massive stars that still have ample hydrogen in their atmospheres when they explode. When the star explodes, the hydrogen atoms are excited and glow prominently, producing hydrogen emission lines. SN 1987A (the topic of Section 20-7 and Section 20-8) was a Type II supernova.

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Hydrogen lines are missing in the spectrum of a Type I supernova (Figure 20-18), which tells us that little or no hydrogen is left in the debris from the explosion. Type I supernovae are further divided into three important subclasses. Type Ia supernovae have spectra that include a strong absorption line of ionized silicon. Type Ib and Type Ic supernovae both lack the ionized silicon line. The difference between them is that the spectra of Type Ib supernovae have a strong helium absorption line, while those of Type Ic supernovae do not.

Figure 20-18: Supernova Types These illustrations show the characteristic spectra and the probable origins of supernovae of (a) Type Ia, (b) Type Ib, (c) Type Ic, and (d) Type II.
(Spectra courtesy of Alexei V. Filippenko, University of California, Berkeley)

Astronomers suspect that Type Ib and Ic supernovae are caused by core-collapse in dying massive stars, just like Type II supernovae. The difference is that the progenitor stars of Type Ib and Ic supernovae have been stripped of their outer layers before they explode. A star can lose its outer layers to a strong stellar wind (see Figure 20-12) or, if it is part of a close binary system, by transferring mass to its companion star (see Figure 19-21b). If enough mass remains for the star’s core to collapse, the star dies as a Type Ib supernova. Because the outer layers of hydrogen are absent, the supernova’s spectrum exhibits no hydrogen lines but many helium lines (Figure 20-18b). Type Ic supernovae have apparently undergone even more mass loss prior to their explosion; their spectra show that they have lost much of their helium as well as their hydrogen (Figure 20-18c).

As Figure 20-18 shows, dividing supernovae by their underlying explosive mechanism leads to those involving white dwarfs (Type Ia), and core-collapse (Type II, Type Ib, and Type Ic). We now look at how to detonate a white dwarf.

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Type Ia Supernovae: Detonating a White Dwarf

Type Ia supernovae are not the death throes of massive supergiant stars. Instead, Type Ia supernovae are thought to result from the thermonuclear explosion of a white dwarf star. Recall that the “thermo” in thermonuclear indicates that high-speed collisions at very high temperatures fuse the atoms together. Furthermore, a supernova resulting from this rapid release of energy is called a thermonuclear supernova. As we will see, a thermonuclear supernova begins with a white dwarf.

This may seem contradictory, because we saw in Section 20-4 that white dwarf stars have no nuclear reactions going on in their interiors. But these reactions can occur if a carbon-oxygen–rich white dwarf gains mass in a close, semidetached binary system with a red giant star (see Figure 19-20b and Figure 20-18a).

Figure 20-19 shows the likely series of events that lead to a Type Ia supernova involving only one white dwarf. Stage 1 in this figure shows a close binary system in which both stars have less than 4 M. The more massive star on the left evolves more rapidly than its less massive companion and eventually becomes a white dwarf. As the companion evolves and its outer layers expand, it overflows its Roche lobe and dumps gas from its outer layers onto the white dwarf (see Stage 2 in Figure 20-19). When the total mass of the white dwarf approaches the Chandrasekhar limit (1.4 M), the increased pressure applied to the white dwarf’s interior causes carbon fusion to begin there (Stage 3 in Figure 20-19). Hence, the interior temperature of the white dwarf increases.

Figure 20-19: A Type Ia Supernova This series of illustrations depicts our understanding of how a white dwarf in a close binary system can undergo a sudden nuclear detonation that destroys it completely. Such a cataclysmic event is called a Type Ia supernova or thermonuclear supernova.
(Illustration by Don Dixon, adapted from Wolfgang Hillebrandt, Hans-Thomas Janka, and Ewald Müller, “How to Blow Up a Star,” Scientific American, October 2006)

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If the white dwarf were made of ordinary matter, the temperature increase would cause a further increase in pressure, the white dwarf would expand and cool, and the carbon-fusing reactions would abate. But because the white dwarf is composed of degenerate matter, this “safety valve” between temperature and pressure does not operate. Instead, the increased temperature just makes the reactions proceed at an ever-increasing rate, in a catastrophic runaway process reminiscent of the helium flash in low-mass stars (Stage 4 in Figure 20-19). The reaction spreads rapidly outward from the white dwarf’s center, with its leading edge (called the flame front) being propelled by convection and turbulence, in a manner analogous to what happens to the shock wave in a core-collapse supernova. Within seconds the white dwarf blows apart, dispersing 100% of its mass into space (Stage 5 in Figure 20-19).

Before exploding, the white dwarf contained primarily carbon and oxygen and almost no hydrogen or helium, which explains the absence of hydrogen and helium lines in the spectrum of the resulting supernova. Silicon is a by-product of the carbon-fusing reaction and gives rise to the silicon absorption line characteristic of Type Ia supernovae.

CAUTION!

Different types of supernovae have fundamentally different energy sources. We saw in Section 20-6 that core-collapse supernovae (which we have now sorted into Types II, Ib, and Ic) are powered by gravitational energy released as the star’s iron-rich core and outer layers fall inward. The core undergoes nuclear reactions during the supernova, but these reactions consume energy provided by gravity. Therefore, the nuclear reactions that occur are gravity-powered; gravity “fuels” the explosion. Type Ia supernovae, by contrast, are powered by nuclear energy released in the explosive thermonuclear fusion of a white dwarf star. In these supernovae, gravity only acts as a trigger—just as a spark can light a candle—but, the source of energy is thermonuclear carbon-fusion reactions; nuclear energy contained in the carbon atoms is the “fuel.” To highlight the source of the energy, we also use the term thermonuclear supernova to refer to a Type Ia supernova. The different types of supernova also differ in their energy output. While Type Ia supernovae typically emit more energy in the form of visible light than supernovae of other types, they do not emit copious numbers of neutrinos because there is no core collapse. If we include the energy emitted in the form of neutrinos, the most energetic supernovae by far are those of Type II. A Type Ia supernova consumes its nuclear fuel in much the same way as a fusing star, but instead of lasting a stellar lifetime, the thermonuclear reactions are over within seconds.

The Decay of a Supernova: Light Curves

In addition to the differences in their spectra, different types of supernovae can be distinguished by their light curves (Figure 20-20). All supernovae begin with a sudden rise in brightness that occurs in less than a day. After reaching peak luminosity, Type Ia, Ib, and Ic supernovae settle into a steady, gradual decline in luminosity. By contrast, most Type II light curves have a long plateau, where they remain at a similar brightness for about 100 days, before rapidly fading. For all supernova types, during the period of declining brightness—after the peak—energy comes from the decay of radioactive isotopes. These decays heat the surrounding material enough to emit visible light.

Figure 20-20: Supernova Light Curves A Type Ia supernova reaches maximum brightness in about a day, followed by a gradual decline in brightness. A Type II supernova reaches a maximum brightness only about one-fourth that of a Type Ia supernova and usually has alternating intervals of steep and gradual declines.

Merging White Dwarfs

Another possible mechanism for a Type Ia supernova involves the merger of two white dwarfs, where enough mass is concentrated in the merger to ignite a nuclear explosion. While the initial scenario is quite different than a binary system consisting of a white dwarf and a red giant, under certain circumstances, the predicted outcome of merging white dwarfs can be quite similar. One motivation for this mechanism is that some Type Ia supernovae fail to show evidence, before or after the event, for any red giant companion.

However, stronger evidence would involve a unique signature predicted by the merger of two white dwarfs. In 2011 the Type Ia supernova SN 2011fe exploded, and it is close enough that observations rule out a red giant companion; this supernova generates a lot of excitement because it is our best case for a nearby white dwarf merger. One signature that might indicate a white dwarf merger is that such a merger is expected to produce a greater proportion of radioactive cobalt atoms that power its light curve (unique from those in Figure 20-20). For SN 2011fe, it is predicted that after several years of monitoring the light curve, this higher abundance of cobalt atoms might be revealed and tell us if the event was most likely due to merging white dwarfs.

Type Ia Supernova Are Used to Measure Distances to Other Galaxies

A number of astronomers are now measuring the distances to remote galaxies by looking for Type Ia supernovae in those galaxies. This is possible because there is a simple relationship between the rate at which a Type Ia supernova fades away and its peak luminosity: The slower it fades, the greater its luminosity. Hence, by observing how rapidly a distant Type Ia supernova fades, astronomers can determine its peak luminosity—not just its apparent brightness, but its actual intrinsic luminosity at its peak.

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Then, a measurement of the supernova’s peak apparent brightness tells us (through the inverse-square law) the distance to the supernova, and, therefore, the distance to the supernova’s host galaxy. In this way, the Type Ia supernova is being used as a “standard candle” of known luminosity, and by comparison to its apparent brightness, its distance is determined.

The tremendous luminosity of Type Ia supernovae allows this method to be used for galaxies more than 109 ly distant. In Chapter 25 we will learn what such studies tell us about the size and evolution of the universe as a whole. In fact, we will see that distances determined from Type Ia supernovae underlie such discoveries as “dark energy.”

CONCEPT CHECK 20-15

If a supernova is observed with no lines in emission or absorption of hydrogen, helium, or silicon, did it result from core-collapse, or involve a white dwarf (consult Figure 20-18)? What “type” is it?