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Dark Energy is a mysterious form of energy that is hypothesized to be responsible for the observed acceleration of the expansion of the universe. In astrophysics, it constitutes about 68% of the total energy in the observable universe.

The Hertzsprung-Russell diagram is a two-dimensional graph in which the absolute magnitudes or luminosities of stars are plotted against their spectral types or temperatures. In astrophysics, it is used to classify stars and to understand stellar evolution.

A pulsar is a highly magnetized, rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. In astrophysics, pulsars are used as precise clocks to study space-time and also to detect gravitational waves.

A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of high-energy electromagnetic radiation, such as X-rays and gamma rays. In astrophysics, magnetars are studied due to their extreme magnetic fields and energetic events.

The Cosmic Microwave Background is the thermal radiation left over from the time of recombination in Big Bang cosmology. In astrophysics, it is used as evidence for the Big Bang and to study the early universe since it provides a snapshot of the universe about 380,000 years after the Big Bang.

A neutron star is the collapsed core of a massive star, which had a total mass of between 10 and 29 solar masses before the supernova explosion. In astrophysics, neutron stars are important for understanding the end stages of stellar evolution and the extreme conditions of matter.

The Big Bang Theory is the prevailing cosmological model for the universe from the earliest known periods through its subsequent large-scale evolution. In astrophysics, it describes how the universe expanded from an extremely hot and dense initial state.

Inflation is a theory that suggests a period of extremely rapid exponential expansion of the universe during its early moments. In astrophysics, it addresses various puzzles of the Big Bang model, such as the horizon problem and the flatness problem.

The interstellar medium is the matter that exists in the space between the star systems in a galaxy. In astrophysics, it consists of gas, dust, and cosmic rays, and is the material from which stars and planetary systems are formed.

A white dwarf is the remnant core of a low to medium mass star after it has expended all its nuclear fuel and expelled its outer layers. In astrophysics, white dwarfs are studied as an end stage of stellar life cycles and provide insight into the future of our Sun.

A supernova is a powerful and luminous stellar explosion that occurs at the end of the life cycle of certain types of stars. In astrophysics, supernovae are important for distributing elements throughout the universe and for contributing to the formation of new stars.

A black hole is a region of spacetime where gravity is so strong that nothing, no particles or even electromagnetic radiation such as light, can escape from it. In astrophysics, black holes are studied to understand the limits of gravity and the end results of stellar collapse.

A main sequence star is a star that is in the most stable part of its lifecycle, during which it is fusing hydrogen into helium at its core. In astrophysics, the main sequence is where stars spend the majority of their lifetime and is defined by the Hertzsprung-Russell diagram.

Redshift is the increase in the wavelength of light from an object, which makes the light appear redder. In astrophysics, it is used to determine how fast an object, such as a galaxy, is moving away from Earth, which is often due to the expansion of the universe.

Spectroscopy is the study of the interactions between matter and electromagnetic radiation. In astrophysics, it is a tool used to determine the composition, temperature, density, and motion of objects in the universe by analyzing their light spectra.

The event horizon is a boundary surrounding a black hole beyond which no light or other forms of radiation can escape. In astrophysics, it represents the point of no return for matter falling into a black hole.

A supermassive black hole is a black hole with a mass that is millions to billions of times that of the Sun. In astrophysics, it is believed to exist at the center of nearly every large galaxy, including our own Milky Way.

An accretion disk is a structure formed by diffuse material orbiting around a massive central body, such as a star, planet, or black hole. In astrophysics, it is observed in active galactic nuclei, protoplanetary disks, and binary star systems where material is transferred from one star to another.

Dark Matter is a form of matter that does not emit, absorb, or reflect light, making it invisible. In astrophysics, it is used to explain anomalies in the rotation rates of galaxies and the movement of clusters, since its presence affects the gravitational field.

A solar flare is a sudden flash of increased brightness on the Sun, usually observed near its surface and in close proximity to a sunspot group. In astrophysics, these flares are important for understanding the Sun's magnetic field and its effect on the heliosphere and Earth's space weather.

A quasar is a highly luminous object powered by a supermassive black hole at its center. In astrophysics, it represents extremely active galactic nuclei and can outshine their host galaxies. Quasars are used to probe the early universe and the growth of black holes.

The Doppler Effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. In astrophysics, it is utilized to determine the velocities of celestial objects by examining shifts in the wavelength of light or other forms of radiation.

A gamma-ray burst is an extremely energetic explosion that has been observed in distant galaxies. In astrophysics, they are the most luminous and energetic events since the Big Bang and are associated with the collapse of a massive star or the merger of neutron stars.

A helium flash is a very brief but intense episode of nuclear burning that occurs in the core of a star. In astrophysics, it marks the beginning of the helium-burning phase for a low or medium mass star after it has exhausted the hydrogen in its core.

Gravitational lensing is the bending of light caused by the gravitational field of a massive object between the light source and the observer. In astrophysics, it is used to study the distribution of dark matter and the properties of distant galaxies and clusters.

Cosmic Rays are high-energy particles that originate from outer space and strike Earth's atmosphere. In astrophysics, cosmic rays are crucial for studying high-energy processes and environments in the universe.

Hubble's Law is the observation that more distant galaxies are moving away faster than closer ones, with a velocity proportional to their distance from Earth. In astrophysics, it serves as the foundational evidence of the expansion of the universe. Mathematically, it is expressed as $v = H_0 \times d$, where $v$ is the galaxy's velocity, $H_0$ is the Hubble constant, and $d$ is the distance to the galaxy.

A galactic halo is a region of a galaxy that extends beyond its main, visible component. In astrophysics, it contains a galaxy's outermost stars, as well as dark matter and hot gas, and plays a role in the rotation curve of a galaxy.