5.3 Stellar death: planetary nebulae and supernovae

Stellar Death: Planetary Nebulae and Supernovae

Every star eventually runs out of fuel, and what happens next depends almost entirely on its mass. Low-to-intermediate-mass stars shed their outer layers to form planetary nebulae, while massive stars end in violent supernova explosions. Both processes scatter heavy elements into the interstellar medium, seeding the raw material for future generations of stars and planets.

Formation of Planetary Nebulae

Stars between about 0.8 and 8 solar masses don't explode. Instead, they die gradually during the asymptotic giant branch (AGB) phase, when intense stellar winds (with mass-loss rates up to $\sim 10^{-4} \, M_\odot \, \text{yr}^{-1}$) push the hydrogen- and helium-rich envelope outward into space.

What's left behind is the hot, exposed core: a white dwarf with a surface temperature that can exceed 100,000 K. Ultraviolet radiation from this remnant ionizes the expanding shell of gas, causing it to glow. That glowing shell is the planetary nebula. (The name is a historical misnomer; these have nothing to do with planets.)

A few key properties to know:

• Composition: Mostly hydrogen and helium, enriched with carbon, nitrogen, and oxygen dredged up from the stellar interior during the AGB phase.
• Morphology: Shapes range from spherical to bipolar to highly elliptical. The shaping mechanisms likely involve binary companions, magnetic fields, and asymmetric mass loss.
• Size and lifespan: Typically 0.1 to 1 light-year across, visible for roughly 10,000 years before dispersing into the interstellar medium.
• Emission spectrum: Dominated by strong emission lines from ionized gases, including forbidden lines of [O III], [N II], and [S II]. These forbidden lines arise because the gas density is extremely low, allowing atoms to de-excite via transitions that would be collisionally suppressed in denser environments.

Types of Supernovae

Supernovae fall into two fundamentally different categories based on their explosion mechanism, even though the observational classification is based on spectra.

Type Ia (Thermonuclear)

A carbon-oxygen white dwarf in a binary system accretes matter from a companion star. When its mass approaches the Chandrasekhar limit ($\sim 1.4 \, M_\odot$), carbon fusion ignites throughout the star in a runaway thermonuclear explosion. The white dwarf is completely destroyed, leaving no compact remnant. Because the trigger mass is nearly the same every time, Type Ia supernovae have relatively uniform peak luminosities, making them valuable as standard candles for measuring cosmological distances.

Core-Collapse Supernovae (Types II, Ib, Ic)

Stars more massive than about 8 $M_\odot$ build up an iron core through successive stages of nuclear fusion. Iron cannot release energy through fusion, so once the core reaches roughly 1.4 $M_\odot$, electron degeneracy pressure can no longer support it. The core collapses in milliseconds. Here's the sequence:

1. The iron core collapses at speeds up to $\sim 0.25c$, and protons and electrons combine to form neutrons and neutrinos (neutronization).
2. The inner core reaches nuclear densities ($\sim 10^{14} \, \text{g cm}^{-3}$) and stiffens as neutron degeneracy pressure halts the collapse, creating a "bounce."
3. An enormous burst of neutrinos (carrying away $\sim 99\%$ of the gravitational binding energy, roughly $3 \times 10^{46}$ J) deposits a fraction of its energy into the infalling outer layers.
4. A shock wave propagates outward, expelling the star's envelope at thousands of km/s.

The spectral subtypes reflect what's left of the star's envelope at the time of explosion:

• Type II: Hydrogen lines present. The star retained its hydrogen envelope.
• Type Ib: No hydrogen lines, but helium lines present. The hydrogen envelope was stripped (by winds or a binary companion).
• Type Ic: Neither hydrogen nor helium lines. Both outer layers were stripped before collapse.

The Role of Mass in Stellar Fate

Mass is the single most important variable determining how a star dies. The boundaries below are approximate and depend on metallicity and mass-loss history:

At very high masses ($\gtrsim 130 \, M_\odot$ for low-metallicity stars), pair-instability supernovae become possible. In these events, high-energy photons in the core produce electron-positron pairs, reducing radiation pressure and triggering a catastrophic collapse that can completely unbind the star.

Throughout all of these scenarios, mass loss through stellar winds plays a critical role. A star that begins at 25 $M_\odot$ may lose a significant fraction of its mass before death, potentially shifting it from the black hole track to the neutron star track. Wind-driven mass loss is especially strong for high-metallicity stars, since metals provide more opacity for radiation to drive the wind.

Impact of Stellar Explosions

Stellar deaths are not just endpoints; they actively shape galactic evolution through several interconnected feedback mechanisms.

Chemical enrichment. Supernovae and planetary nebulae return processed material to the interstellar medium. Type Ia supernovae are the dominant source of iron-peak elements, while core-collapse supernovae produce most of the alpha elements (oxygen, magnesium, silicon). Planetary nebulae contribute carbon and nitrogen. This ongoing enrichment drives galactic chemical evolution, progressively increasing the metallicity of each new stellar generation.

Triggering star formation. Supernova blast waves compress surrounding interstellar gas, which can push marginally stable clouds past the Jeans mass threshold and initiate gravitational collapse. The Orion Nebula is a well-known example of a star-forming region shaped by feedback from previous generations of massive stars.

Dust production. Both supernova ejecta and planetary nebula shells produce solid dust grains (silicates, graphite, and other compounds). This dust is a significant contributor to the interstellar dust population, which plays a role in molecule formation, radiative cooling, and eventually planet building.

Cosmic ray acceleration. Supernova remnant shock fronts accelerate charged particles to relativistic energies through diffusive shock acceleration (Fermi acceleration). These cosmic rays influence the ionization state of molecular clouds and contribute to the pressure balance and magnetic field structure of the galaxy.

Observable remnants. Supernova remnants like the Crab Nebula (the remnant of the Type II supernova observed in 1054 CE) expand over thousands of years, sweeping up interstellar material and producing emission across the electromagnetic spectrum, from radio to X-rays. These remnants serve as laboratories for studying shock physics, particle acceleration, and the properties of neutron stars (the Crab Pulsar, at the center, spins at about 30 times per second).

Galactic-scale feedback. Collectively, supernovae inject enough energy and momentum into the interstellar medium to regulate star formation rates across entire galaxies. In extreme cases, concentrated supernova activity drives galactic outflows (superwinds) that expel gas from the disk, influencing the baryon cycle and the galaxy's long-term evolution.