Astrophysics Cheat Sheet

The core ideas of Astrophysics distilled into a single, scannable reference — perfect for review or quick lookup.

PiqCue — piqcue.com/astrophysics/cheatsheet

Quick Reference

Stellar Nucleosynthesis

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.

General Relativity

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.

Black Holes

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.

Cosmic Microwave Background (CMB)

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.

Dark Matter

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.

Dark Energy

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.

Hertzsprung-Russell Diagram

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.

Gravitational Waves

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.

Hubble's Law

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 and Pulsars

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.

Key Terms at a Glance

Accretion Disk:A rotating disk of gas, dust, and other material that forms around a massive object such as a black hole or protostar, as matter spirals inward due to gravity and angular momentum conservation.

Baryonic Matter:Ordinary matter composed of protons, neutrons, and electrons, which makes up atoms, molecules, stars, and planets. It constitutes only about 5% of the total energy content of the universe.

Black Hole:A region of spacetime where gravity is so intense that nothing, including light, can escape from within its event horizon. Black holes can be stellar-mass, intermediate-mass, or supermassive.

Blueshift:The decrease in wavelength of light from an object moving toward the observer, shifting the light toward the blue end of the electromagnetic spectrum.

Cosmic Microwave Background:The thermal radiation filling the observable universe almost uniformly, representing the oldest light in the universe emitted approximately 380,000 years after the Big Bang.

Dark Energy:The unknown form of energy that permeates all of space and drives the accelerating expansion of the universe, comprising roughly 68% of its total energy density.

Dark Matter:Non-luminous matter that interacts gravitationally but not electromagnetically, inferred from its effects on galaxy rotation curves, gravitational lensing, and large-scale structure formation.

Doppler Effect:The change in observed frequency or wavelength of a wave due to relative motion between the source and the observer. In astrophysics, it manifests as redshift or blueshift of spectral lines.

Event Horizon:The boundary around a black hole inside which the escape velocity exceeds the speed of light, making it a point of no return for matter and radiation.

Exoplanet:A planet that orbits a star outside our solar system. Thousands of exoplanets have been discovered using methods such as the transit method and radial velocity measurements.

Gamma-Ray Burst:The most energetic electromagnetic events in the universe, consisting of intense bursts of gamma rays lasting from milliseconds to minutes, associated with supernovae, neutron star mergers, or collapsars.

Gravitational Wave:A ripple in the curvature of spacetime that propagates outward from its source at the speed of light, generated by the acceleration of massive objects such as merging compact binaries.

Habitable Zone:The range of orbital distances around a star within which a planet's surface could support liquid water, a key condition for life as we know it. Also called the Goldilocks zone.

Hubble Constant:The current rate of expansion of the universe, denoted $H_0$, with a value of approximately 67 to 73 km/s/Mpc depending on the measurement method. It relates a galaxy's recession velocity to its distance.

Luminosity:The total amount of energy emitted by a star or other astronomical object per unit time, typically measured in watts or in units of solar luminosity.

Get study tips in your inbox

We'll send you evidence-based study strategies and new cheat sheets as they're published.

We'll notify you about updates. No spam, unsubscribe anytime.