Astrophysics

The process by which new atomic nuclei are created from pre-existing protons and neutrons inside stars through nuclear fusion. Light elements such as hydrogen and helium fuse in stellar cores to produce heavier elements, while the heaviest elements are forged in supernova explosions and neutron star mergers.

Einstein's theory describing gravity not as a force but as the curvature of spacetime caused by mass and energy. It provides the mathematical framework for understanding black holes, gravitational lensing, and the expansion of the universe.

Regions of spacetime where gravity is so extreme that nothing, not even light, can escape once it crosses the event horizon. Black holes form from the gravitational collapse of massive stars or through mergers and accretion over cosmic time.

The faint thermal radiation left over from the early universe, emitted roughly 380,000 years after the Big Bang when the cosmos cooled enough for atoms to form and photons to travel freely. Its tiny temperature fluctuations encode information about the composition, geometry, and age of the universe.

A hypothetical form of matter that does not emit, absorb, or reflect electromagnetic radiation, making it invisible to telescopes. Its existence is inferred from gravitational effects on visible matter, such as the unexpectedly flat rotation curves of galaxies.

A mysterious component of the universe that drives its accelerating expansion. Discovered in 1998 through observations of distant Type Ia supernovae, dark energy constitutes roughly 68 percent of the total energy content of the universe.

A scatter plot of stars showing the relationship between their luminosity (or absolute magnitude) and their surface temperature (or spectral class). It is a fundamental tool for understanding stellar evolution, as stars occupy different regions of the diagram at different stages of their lives.

Ripples in the fabric of spacetime caused by the acceleration of massive objects, predicted by Einstein in 1916 and first directly detected by LIGO in 2015. They carry information about their sources that is inaccessible through electromagnetic observations alone.

The observation that galaxies are receding from us at velocities proportional to their distance, expressed as $v = H_0 d$. This relationship provides direct evidence for the expansion of the universe and is used to estimate its age and size.

Neutron stars are the ultra-dense remnants of massive stars that have undergone supernova explosions, packing roughly 1.4 solar masses into a sphere only about 20 kilometers across. Pulsars are rapidly rotating neutron stars that emit beams of radiation from their magnetic poles, observed as periodic pulses.