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A structure formed by diffused material in orbital motion around a massive central body, such as the many rings of debris that form around newly developing stars.

A hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe.

A quantity of rotation of an object, which is the product of its moment of inertia and its angular velocity. In celestial mechanics, it is conserved in closed systems.

An extremely bright meteor that often explodes in the atmosphere and is capable of causing a sonic boom.

A cloud of ionized gas emitting light of various colors. The most common source of ionization is high-energy photons emitted from a nearby hot star.

The visible streak of light from a meteoroid or a micrometeoroid, comet or asteroid as it enters the Earth's atmosphere, known as a shooting star or falling star.

A boundary in spacetime beyond which events cannot affect an outside observer. In the case of a black hole, it represents the point of no return.

A hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe. Its presence is implied by gravitational effects on visible matter and the structure of the universe.

The totality of space and time, all forms of matter and energy, the physical laws and constants that govern them, and all the cosmic structures that exist within it.

The succession of methods by which astronomers determine the distances to celestial objects. It forms a series of 'rungs' from the Earth to galaxies far, far away.

The minimum speed needed for an object to 'break free' from the gravitational attraction of a massive body, without further propulsion.

A unit of distance used in astronomy, equal to about 3.26 light-years, or just under 31 trillion kilometers (19 trillion miles). One parsec represents the distance at which one astronomical unit subtends an angle of one arcsecond.

A celestial object that is not massive enough to sustain nuclear fusion of hydrogen in its core, unlike a main-sequence star, but still has some similarities to stars.

The maximum mass (approximately 1.4 times that of the sun) that a white dwarf star can have before it potentially explodes as a Type Ia supernova or collapses into a neutron star.

The scientific study of the large scale properties of the universe as a whole. It involves the study of the physical origins, structure, evolution, and ultimate fate of the universe.

A type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They are one of the three main types of galaxy morphology.

The distance that light travels in one year, which is approximately 5.88 trillion miles or 9.46 trillion kilometers.

An event occurring when the Earth passes between the Sun and the Moon, casting a shadow on the Moon, causing it to appear reddish in color.

The range of all types of electromagnetic radiation. Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation.

A small rocky body orbiting the sun, found mainly between Mars and Jupiter in the asteroid belt.

A powerful and luminous stellar explosion that occurs at the end of a star's life cycle, often resulting in the creation of heavier elements and leaving behind a neutron star or black hole.

An instrument designed to observe distant objects by collecting electromagnetic radiation, such as visible light.

The point in the sky or celestial sphere that is directly above an observer on the ground. Opposite to the nadir.

The vast, bubble-like region of space which surrounds and is created by the Sun. In this region, the solar wind is detectable.

An icy small Solar System body that, when passing close to the Sun, warms and begins to release gases—a process called outgassing.

A group of stars forming a recognizable pattern that is traditionally named after its apparent form or identified with a mythological figure.

A planet that is located outside of our solar system and orbits a star other than the Sun.

A huge collection of gas, dust, and billions of stars and their solar systems, all held together by gravity.

A pattern of stars recognized in the Earth's night sky. It is not an official constellation but can be part of one or more constellations.

The center of mass of two or more bodies that orbit each other and where they balance each other.

An extremely luminous active galactic nucleus, powered by a supermassive black hole at the center of a galaxy that is accreting matter.

The gravitationally curved trajectory of an object, such as the path of a planet around a star or a natural satellite around a planet.

An event that occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks the Sun.

Three laws describing the motion of planets around the sun. 1) The orbit of a planet is an ellipse with the Sun at one of the two foci. 2) A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. 3) The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

A type of compact star that is the remnants of a supernova explosion and is composed almost entirely of neutrons.

The average distance between Earth and the Sun, approximately 149.6 million kilometers.

Radiation that is a remnant from an early stage of the universe, also known as relic radiation, is detected in all directions of the sky and has a thermal black body spectrum.

A celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, is not a satellite, and has not cleared its neighboring region of other objects.

The observation in physical cosmology that galaxies are moving away from each other at speeds proportional to their distance. The formula is $v = H_0 \times d$ where $v$ is the galaxy's recessional velocity, $H_0$ is the Hubble Constant, and $d$ is the distance from the galaxy.

The matter and radiation that exists in the space between the star systems in a galaxy.

A star that never sets below the horizon as viewed from a particular latitude on Earth. Always observable all night and every night.

A term introduced by Einstein in his field equations of General Relativity, representing a constant energy density filling space homogeneously.

An orbit directly above the Earth's equator and following the direction of the Earth's rotation. An object in such an orbit has an orbital period equal to the Earth's rotational period and thus appears motionless, at a fixed position in the sky, to ground observers.

The region around a star where planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces. Also known as the 'Goldilocks Zone'.

The leading explanation on how the universe began, starting with a small singularity and expanding over the next 13.8 billion years to the cosmos that we know today.

A highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed when the beam of emission is pointing towards the Earth.

The displacement of spectral lines towards longer wavelengths (the red part of the spectrum) in the light from distant galaxies and celestial objects; this is indicative of the Doppler effect due to the objects moving away from us.

The increase or decrease in the frequency of sound, light, or other waves as the source and observer move toward or away from each other. The effect causes the sudden change in pitch noticeable in a passing siren, as well as the redshift seen by astronomers.

A region in space where the pulling force of gravity is so strong that light is not able to escape. The boundary surrounding this region is called the event horizon.

An event that occurs when one celestial body moves into the shadow of another celestial body.

A vast cloud of dust and gas in space, sometimes formed by the explosion of a star. It can be a region where new stars are beginning to form.

A small, dense star that represents the final evolutionary state of stars not massive enough to become neutron stars, composed mostly of electron-degenerate matter.

A hypothetical structure linking disparate points in spacetime and is based on a special solution of the Einstein field equations.

A measure of how much light that hits a surface is reflected without being absorbed. It is a reflection coefficient and varies between 0 (no reflection) and 1 (perfect reflection).