Types of Supernovae

Why This Matters

Supernovae aren't just spectacular cosmic explosions—they're the universe's element factories and distance markers, making them central to nearly every major theme in Astrophysics II. You'll encounter these events when studying stellar evolution, nucleosynthesis, cosmological distance ladders, and compact object formation. The differences between supernova types reveal fundamental physics: how mass determines stellar fate, how binary interactions alter evolution, and how extreme conditions create the heavy elements that make up planets and people.

When you're tested on supernovae, you're really being tested on your understanding of degeneracy pressure, nuclear burning stages, and mass thresholds. Don't just memorize which type has hydrogen lines and which doesn't—know why those spectral differences exist and what they tell us about the progenitor star's history. Each supernova type is a window into a different physical process, and that's what exam questions will probe.

---

Thermonuclear Explosions: White Dwarf Detonations

These supernovae don't involve core collapse at all. Instead, runaway nuclear fusion in degenerate matter causes the entire star to be destroyed—no remnant left behind.

Type Ia Supernovae

• Thermonuclear explosion of a white dwarf—occurs in binary systems when mass accretion pushes the white dwarf past the Chandrasekhar limit ($\sim 1.4 M_\odot$)
• Consistent peak luminosity makes them crucial standard candles for measuring cosmological distances, including the discovery of dark energy
• No hydrogen lines in spectra and produces iron-group elements, distinguishing them completely from core-collapse events

---

Core-Collapse Supernovae: Massive Star Deaths

When stars above $\sim 8 M_\odot$ exhaust their nuclear fuel, the core collapses under gravity faster than pressure can respond, triggering an explosion that leaves behind a neutron star or black hole.

Type II Supernovae

• Gravitational collapse of massive stars ($> 8 M_\odot$) that retain their hydrogen envelopes through their entire evolution
• Strong hydrogen lines in spectra—the defining observational signature that distinguishes them from stripped-envelope types
• Forms neutron stars or black holes depending on progenitor mass, and creates supernova remnants that can trigger new star formation

Type Ib Supernovae

• Core collapse of hydrogen-stripped massive stars—outer layers lost to strong stellar winds or binary companion interactions before explosion
• Helium lines present, hydrogen absent in spectra, indicating the star retained its helium layer but nothing beyond
• Produces carbon and oxygen along with heavier elements; typically forms a neutron star remnant

Type Ic Supernovae

• Most stripped core-collapse type—progenitors have lost both hydrogen and helium layers before exploding
• Neither hydrogen nor helium lines in spectra; often associated with Wolf-Rayet stars that shed their envelopes through intense winds
• Linked to long gamma-ray bursts in some cases; produces iron and nickel during the explosion

---

Exotic Collapse Mechanisms: Edge Cases in Stellar Death

Not all supernovae fit neatly into the standard categories. Extreme masses and unusual core compositions create alternative pathways to explosion that test the boundaries of stellar physics.

Electron-Capture Supernovae

• Collapse triggered by electron capture onto magnesium and neon nuclei in stars with masses $\sim 8–10 M_\odot$—right at the boundary between white dwarf and neutron star formation
• Lower energy explosions than typical Type II events, producing fewer heavy elements but still contributing to galactic chemical evolution
• Forms neutron stars and provides key insights into the transition zone between intermediate and massive star fates

Pair-Instability Supernovae

• Occurs in extremely massive stars ($> 140 M_\odot$) when core temperatures create electron-positron pairs, removing pressure support
• Complete stellar disruption—no remnant survives because the entire star is blown apart by runaway thermonuclear burning
• Critical for early universe models—these explosions from Population III stars seeded the cosmos with the first heavy elements beyond hydrogen and helium

---

Quick Reference Table

---

Self-Check Questions

1. Which two supernova types leave no compact remnant behind, and what different physical mechanisms cause this outcome?

2. A spectrum shows strong helium lines but no hydrogen. What supernova type is this, and what does the absence of hydrogen tell you about the progenitor's evolution?

3. Compare and contrast Type Ia and Type II supernovae in terms of their progenitor systems, spectral signatures, and remnants produced.

4. Why are Type Ia supernovae useful as standard candles while core-collapse supernovae are not? What physical property makes the difference?

5. An FRQ asks you to explain how supernova type relates to progenitor mass. Outline the mass ranges for electron-capture, typical core-collapse, and pair-instability supernovae, and explain what happens at each threshold.