From the October 2019 issue

What makes stars explode?

Sound waves in collapsing stars may produce supernova explosions.
By | Published: October 14, 2019 | Last updated on May 18, 2023
A supernova detonates in a spiral galaxy in this illustration. These titanic explosions create and distribute most of the elements, stir up galactic gas and dust, and give astronomers beacons that shine across billions of light-years.
Adolf Schaller for Astronomy
No event in nature surpasses a supernova’s raw power. The flood of neutrinos accompanying the explosion of a single massive star releases as much instantaneous power as the rest of the visible universe combined.
Such blasts stir interstellar gas and dust, helping new stars form. More importantly, supernovae disperse most of the elements heavier than carbon — such as the iron in our blood — and create neutron stars and black holes.

After decades of debate, astrophysicists still aren’t sure how a star turns into nature’s grandest firecracker. Even the most complex supercomputer simulations haven’t solved the problem, but they have served up some surprises. For instance, sound waves in a collapsing star’s heart could help kick-start a stalled explosion, while a white dwarf’s detonation may arise when the star’s gravity turns a thermonuclear conflagration back on itself.

The big picture
By the 1930s, it was clear that some stellar flare-ups, called novae, were in a class by themselves. In 1933, astronomers Walter Baade at Mount Wilson Observatory and Caltech’s Fritz Zwicky began referring to the most luminous events as supernovae. They suggested the explosions occurred when a massive star collapsed and created a neutron star. Bear in mind this was more than three decades before pulsed radio signals from the Crab Nebula supernova remnant proved that neutron stars exist at all.
In 1941, Mount Wilson’s Rudolph Minkowski proposed supernovae come in two flavors based on the absence (type I) or presence (type II) of strong hydrogen spectral lines at peak brightness. Since then, the observational picture has become more complex as astronomers recognized new subclasses of both types. Nevertheless, astronomers generally agree that two scenarios likely account for most supernovae.

Type Ia supernovae occur in all galaxies among an older stellar population. All others — type II, plus types Ib and Ic associated with gamma-ray bursts — prefer galaxies sparkling with star-forming regions, which contain many hot, young, massive stars. Such stars explode when they use up their nuclear fuel and collapse.

Core collapse
Stars weighing more than about eight times the Sun’s mass burn through their hydrogen fuel quickly, but as a massive star runs low on one fuel, it taps into another. Its core contracts, growing hotter and denser until the previous nuclear reaction’s “ash” — helium, at first — undergoes fusion itself. As each fuel runs out, the star’s core responds in the same way, running through a succession of fuels: hydrogen, helium, carbon, neon, oxygen, and silicon.
But this is a game of diminishing returns. Each new fuel releases less energy, so the star burns through it even faster. Moreover, once carbon ignites and the core’s temperature approaches a billion degrees, neutrinos form and escape in greater numbers. Formed in many nuclear reactions, neutrinos don’t interact easily with other matter and quickly exit the star. To compensate for the energy loss, the core burns its nuclear fuel even faster.

 While such a star may take 10 million years or more to run through its “first course” of hydrogen fuel, it consumes its helium in 2 million years and its carbon in just 2,000 years. The last phase, when the core fuses silicon, lasts less than three weeks.
As silicon fusion ends, an Earth-sized iron-nickel core about 1.5 times the Sun’s mass resides in the star’s center. But iron-group elements have nature’s most tightly bound nuclei, so the core can’t resort to its old trick — fusing iron actually consumes energy. Neutrinos stream from the core. The core’s central density is so high that it forces electrons — the star’s main pressure source — inside nuclei. The electrons transform some protons into neutrons. Both processes — streaming neutrinos and squeezing protons and electrons together — remove pressure that supports the star. With pressure losses mounting and no new energy source to tap, the star’s battle with gravity is over.
The iron core collapses at about ¼ light-speed. In half a second or less, it transforms from an Earth-sized stellar core to a hot, dense proto-neutron star just 19 miles (30 kilometers) across. When the central density reaches about twice that of an atomic nucleus, the core stiffens and rebounds thanks to a repulsive component in the strong nuclear force. This core “bounce” acts like a spherical piston that drives into the star’s infalling gas.
“It was hoped this piston would generate a shock that would be the supernova in its infancy,” says Adam Burrows, who models supernovae at Princeton University. “That was sweet, and it made some sort of sense, but it doesn’t work.” As the shock moves out, it radiates lots of neutrinos, which saps its energy. “In addition, it’s trying to overcome all that stuff that’s still falling in, and it fails.”

The shock stalls a few milliseconds after it starts and simply sits there, heating the infalling gas. If nothing changed during the next second, the nascent neutron star would accrete a few tenths of a solar mass of matter and then become crushed into a black hole. No supernova.

The pause that refreshes
The central mystery of core-collapse supernovae is how this situation ever can turn itself around. “What people had suggested was, you wait a while, and neutrinos eventually heat up the material behind the shock enough that you relaunch an explosion,” says Burrows. He calls this happening “the pause that refreshes.”
The large number of neutrinos departing the core makes up for the low odds that a single neutrino will interact with the star’s matter as it leaves. The action pauses for just a few hundred milliseconds, but “that’s a long time in this game because things happen fast,” says Burrows.
In early computer simulations, which assumed the collapsing star was spherically symmetric, even this process didn’t work. Such 1-D calculations gave way to more demanding 2-D models, which assume symmetry around the star’s spin axis. They revealed fluid instabilities and turbulence that promised to aid the stalled shock.
“For a while, that was the prevailing view,” Burrows explains. “But with the best neutrino physics, it doesn’t look like this works in 2-D.” Will new effects in 3-D simulations help neutrinos deposit energy more efficiently?
“That’s still the hope,” he says.
In 2005, Burrows and his colleagues discovered a potentially important alternative energy source in collapsing stars: sound waves. In the team’s 2-D model, the stalled shock starts to wobble top-to-bottom along the star’s spin axis. “People hadn’t seen this before because they had waited for maybe 200 milliseconds after bounce,” Burrows explains. “And the shock just went up, it stalled, and it went back down.” Nothing more happened, so supernova modelers ended their expensive computer runs.
As matter streams onto the proto-neutron star, turbulence around the core sets it oscillating at around 300 hertz — musically, about F above middle C. Acoustic waves radiate back into the collapsing envelope. While the energy from neutrinos is far greater, only a fraction of it becomes deposited in the stalled shock, whereas matter absorbs sound almost completely. There’s enough acoustic power to blow the star apart half a second after core bounce in Burrows’ simulation.
How important this process is remains an open question. It’s the accreting material that keeps a lid on the explosion, preventing neutrinos from moving the shock out. “If the neutrino mechanism worked, we would have seen it in our model,” Burrows says.

The sound waves push streams of accreting matter to one side of the core while energizing the shock on the opposite side. So, by creating a path of least resistance, sound may help neutrinos revitalize a stalled shock. “It’s unproven,” he says, “but very interesting.” Moreover, the oscillating core could be a prominent source of gravitational radiation.

Shattered dwarfs
Large-scale computer simulations are also providing new insights into how white dwarfs, the end state of low-mass stars, destroy themselves as type Ia supernovae. Brighter and more uniform than core-collapse explosions, type Ia events are important probes of the distant universe. The discoveries of dark energy and cosmic acceleration add urgency to deciphering how they work.

A star similar to the Sun ends its days as a white dwarf, with the star’s carbon-oxygen-rich core crushed to Earth’s size. Most shine for billions of years, gradually cooling until they fade into dark stellar cinders. Electron pressure prevents further collapse, but it works only if the dwarf weighs less than 1.44 Suns — the so-called Chandrasekhar limit. Exceed that, and collapse resumes until the dwarf becomes a neutron star.

In 1960, University of Cambridge astronomer Fred Hoyle and Caltech’s William Fowler realized a white dwarf near this limit could be a giant thermonuclear bomb. Place a white dwarf in close proximity to a normal star, and the dwarf can gain mass until it nears the 1.44-Sun threshold and explodes. The dwarf gobbles up hydrogen gas from its partner at a probable rate of about 1/30 of an Earth mass per year. If it’s much slower than this number, the dwarf’s stellar wind prevents the gas from reaching the surface; if it’s any faster, the gas will flash-fuse rather than accumulate.
As a white dwarf tips the scale toward 1.44 Suns, its carbon ignites somewhere inside. Before 2004, no one could figure out how to make a carbon-oxygen star detonate, so theorists first invoked turbulent thermonuclear fusion. These simulations failed to match the energy and element mix of type Ia blasts. Models that followed a period of turbulent burning with a detonation better matched reality, but theorists simply decided where and when the explosion would occur and inserted it into the simulation. “I sometimes refer to this as the ‘Here, a miracle occurs’ mechanism,” says the University of Chicago’s Don Lamb.
For this reason, Wolfgang Hillebrandt and his group at the Max Planck Institute for Astrophysics in Munich, Germany, tried a different direction. They found that simulations using turbulent burning alone can better match observations, but, to do so, the dwarf’s thermonuclear fires must ignite in about 100 different points at once. That’s very unlikely. Says Lamb: “We worry one miracle has been replaced by another.”
In 2004, a team led by Alan Calder, then at the University of Chicago, including Lamb, stumbled onto a way to blow up a white dwarf. Thanks to the U.S. Department of Energy’s computational resources, the team had the hardware to simulate an entire white-dwarf star. After ignition, a narrow front of nuclear flame expanded through the star, leaving behind a 10-billion-degree ash bubble. When this bubble broke through the dwarf’s crust, less than 10 percent of the star’s mass had been fused — too little to disrupt the dwarf or produce a strong explosion. “It looked like it might be a dud,” Lamb recalls.
Then, team member Tomasz Plewa performed additional 2-D simulations to see what happens after the bubble breaches the star’s surface. The nuclear ash erupts, moving at around 6.7 million mph (10.8 million km/h), just shy of orbital speed. The hot cloud hugs the dwarf’s billion-degree surface and rapidly spreads. As it does so, it plows up cooler, unfused surface material. The superheated ash-cloud wraps around the white dwarf and meets itself at the point opposite its breakout. The collision compresses all of the unfused surface material, which explodes and rips the star apart.
The model, called “gravitationally confined detonation,” is the most complete description of a type Ia supernova to date — and the only one in which a full-scale detonation naturally occurs. “It’s a very promising model for most type Ia supernovae,” Lamb says. “It was a serendipitous discovery. And it is a perfect example of how large-scale numerical simulations can lead to discoveries of complex, non-linear phenomena that are very difficult to imagine ahead of time.”

More than 85 years after astronomers connected supernovae with stellar deaths, the universe’s most powerful explosions still tax astrophysicists. But even the most complete simulations don’t yet capture the complex environment of an exploding star. Modelers are beginning to probe how neutrino emission, magnetic fields, and rotation affect the picture. Observers watch and catalog new events, using them both as cosmic yardsticks and to find holes in current understanding. And new facilities designed to capture neutrinos and gravitational waves — signals that directly escape an exploding star’s core — one day soon may give us a glimpse of a supernova’s chaotic heart.