23.3 Supernova Observations

Supernova 1987A

Supernova 1987A shook up our understanding of stellar explosions. This nearby blast, located in the Large Magellanic Cloud (about 168,000 light-years away), let scientists watch a star die in real-time, confirming some theories and challenging others. Its neutrino burst and chemical makeup provided key insights into how massive stars end their lives.

SN 1987A's blue supergiant progenitor surprised astronomers, who expected red supergiants to explode. This discovery led to updates in stellar evolution models. The supernova's light curve and spectral changes over time helped refine our knowledge of these cosmic cataclysms.

Features of SN 1987A

Brightness changes

SN 1987A's light curve didn't follow a simple rise-and-fall pattern. It rapidly increased in brightness over several months, reaching peak luminosity around May 1987 (about 3 months after first observation). Then it gradually declined over the following years, fading by several magnitudes by 1990. Tracking these brightness changes gave astronomers crucial data about the explosion's energy output and how the ejected material evolved over time.

Neutrino emissions

The neutrino detection from SN 1987A was a landmark moment for astrophysics. Three separate observatories (Kamiokande II, IMB, and Baksan) picked up a total of 24 neutrinos arriving 2–3 hours before the visible light from the supernova. That timing makes sense: neutrinos escape the collapsing core almost immediately, while light has to fight its way out through the star's outer layers.

The entire neutrino burst lasted less than 13 seconds, yet it confirmed that neutrinos carry away the vast majority of energy released during core collapse. This detection also spurred improvements in neutrino observatory design for catching future supernovae.

Confirmation of Supernova Theories

Core collapse model

Before SN 1987A, the core collapse mechanism was well-developed theory but lacked direct observational proof. The neutrino detections changed that. The number, energy, and timing of the neutrinos matched predictions from core collapse models, providing strong evidence that the theory was on the right track.

Nucleosynthesis of heavy elements

Spectroscopic observations of SN 1987A's ejecta revealed heavy elements like oxygen, silicon, and iron. This confirmed that supernovae are responsible for producing and dispersing elements heavier than iron into the interstellar medium. Without supernovae, the universe would lack most of the heavy elements that make up rocky planets (and us).

Progenitor star identification

The progenitor star, Sanduleak $-69°$ 202, was identified as a blue supergiant. This was a genuine surprise. The prevailing theory held that only red supergiants could produce Type II supernovae. Finding a blue supergiant as the progenitor forced astronomers to revise their stellar evolution models to account for this possibility.

Stages Before Type II Supernovae

A massive star goes through several distinct phases before it explodes as a Type II supernova:

1. Main sequence — The star fuses hydrogen into helium in its core, maintaining hydrostatic equilibrium between inward gravitational pull and outward radiation pressure.

2. Red supergiant phase — Once hydrogen is exhausted in the core, fusion continues in a shell surrounding the core. The core contracts and heats up while the outer layers expand and cool dramatically. Betelgeuse and Antares are examples of stars currently in this phase.

3. Core collapse — Through successive fusion stages, the star builds up an iron core. Iron can't release energy through fusion, so once the core reaches the Chandrasekhar limit ($\sim 1.4$ solar masses), electron degeneracy pressure can no longer support it against gravity. The core collapses in a matter of seconds, reaching temperatures of billions of degrees.

4. Neutrino burst and shock wave — During collapse, protons and electrons are squeezed together to form neutrons, releasing an enormous burst of neutrinos. These neutrinos carry away roughly 99% of the gravitational energy released. Meanwhile, infalling material bounces off the newly dense core, creating an outward-moving shock wave.

5. Supernova explosion — The shock wave propagates outward, heating and ejecting the star's outer layers. Rapid nucleosynthesis occurs behind the shock front, forging heavy elements. The expanding supernova remnant plows into the surrounding interstellar medium. Famous remnants include the Crab Nebula and Cassiopeia A.

Supernova Types and Outcomes

Not all supernovae happen the same way. The two main categories differ in their cause and what they leave behind.

• Type II supernovae (core-collapse) result from massive stars greater than about 8 solar masses. The process described above applies here. These explosions typically leave behind a neutron star, or in the case of the most massive progenitors, a black hole.
• Type Ia supernovae are fundamentally different. They occur when a white dwarf in a binary system accumulates enough mass from its companion star to trigger a thermonuclear explosion. Because they reach a consistent peak brightness, Type Ia supernovae serve as standard candles for measuring cosmic distances.

The type of supernova a star produces depends on its mass, composition, and whether it has a binary companion. Stellar evolution determines the path a star takes toward its eventual explosion.