Astronomy / Stellar Evolution — UPSC Mains Model Answer

A Type Ia supernova originates in a binary stellar system where a white dwarf accretes matter from a companion star. When the accumulated mass pushes the white dwarf beyond the Chandrasekhar Limit (approximately 1.4 solar masses), runaway nuclear fusion triggers a catastrophic explosion. Unlike core-collapse supernovae, no neutron star or black hole remnant is left behind.

Because the triggering mass threshold is nearly constant, the resulting thermonuclear explosion releases a consistent peak luminosity across different events. This uniformity allows astronomers to calculate the actual brightness of the explosion and compare it with observed brightness to determine precise distances — the defining characteristic of a 'standard candle' in observational astronomy.

Systematic surveys of Type Ia supernovae in the 1990s revealed that distant supernovae appeared fainter than expected, implying they were farther away than standard models predicted. This evidence pointed to an accelerating expansion of the universe, leading to the postulation of dark energy as a dominant cosmic component — a discovery recognised with the Nobel Prize in Physics.

Type Ia supernovae are not perfectly uniform; variations in progenitor systems and host galaxy environments introduce scatter in luminosity measurements. Calibration techniques such as the Phillips relation (linking light-curve decline rate to peak luminosity) partially correct for this, but residual uncertainties remain a source of debate in precision cosmology.