Glossary of astronomy

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Earths orbit and ecliptic

Elliptic orbit

Glossary of Astronomy is a comprehensive list of terms and definitions used in the field of astronomy. Astronomy, the scientific study of celestial objects and phenomena, employs a unique vocabulary to describe its observations, theories, and findings. This glossary serves as a valuable resource for both professionals in the field and enthusiasts looking to deepen their understanding of the cosmos.

A

• Absolute magnitude – A measure of the intrinsic brightness of a celestial object, as it would appear at a standard distance of 10 parsecs or about 32.6 light-years from the observer.
• Astronomical unit (AU) – A unit of length used in astronomy equal to the average distance between the Earth and the Sun, approximately 149.6 million kilometers (93 million miles).

B

• Binary star – A system of two stars that orbit their common center of mass.

C

• Celestial coordinates – A system of spherical coordinates used to specify the positions of objects in the sky, typically using right ascension and declination.
• Comet – A small solar system body that, when passing close to the Sun, heats up and begins to release gases, a process called outgassing, creating a visible atmosphere or coma, and sometimes also a tail.

D

• Dark matter – A form of matter thought to account for approximately 85% of the matter in the universe. It is invisible through electromagnetic radiation and its presence can only be inferred from gravitational effects on visible matter.

E

• Eclipse – An event in which one celestial body moves into the shadow of another body, temporarily blocking the light from a source such as the Sun in the case of a solar eclipse, or casting a shadow on another such as in a lunar eclipse.

F

• Focal length – In an optical system, the distance over which initially collimated rays are brought to a focus. A longer focal length allows for a larger image of a distant object.

G

• Galaxy – A massive, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter.

H

• Hubble's Law – The observation in physical cosmology that galaxies are moving away from each other at speeds proportional to their distance. This law supports the Big Bang theory of the universe.

I

• Interstellar medium – The matter that exists in the space between the star systems in a galaxy. This medium includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays.

J

• Jovian planet – A term used to describe the gas giant planets of the solar system: Jupiter, Saturn, Uranus, and Neptune. These planets are characterized by their large masses and gaseous compositions.

K

• Kepler's laws of planetary motion – Three laws describing the motion of planets around the Sun. They include the law of orbits, the law of areas, and the law of periods.

L

• Light-year – A unit of length used to express astronomical distances. It is approximately 9.461 trillion kilometers (5.878 trillion miles), the distance that light travels in a vacuum in one Julian year.

M

• Magnitude (astronomy) – A measure of the brightness of a celestial object. The lower the number, the brighter the object.

N

• Nebula – An interstellar cloud of dust, hydrogen, helium, and other ionized gases. Nebulae are often the sites of star formation.

O

• Orbit – The gravitationally curved trajectory of an object, such as the path of a planet around a star or a moon around a planet.

P

• Parallax – The apparent shift in position of an object when viewed from different points, used in astronomy to measure distances to nearby stars.

Q

• Quasar – A very energetic and distant galaxy with an active galactic nucleus. Quasars are among the brightest and most distant objects in the known universe.

R

• Redshift – The increase in the wavelength of electromagnetic radiation received from a celestial object moving away from the observer, used as a measure of the speed at which an object is moving away in the universe.

S

• Solar eclipse – An eclipse that occurs when the Moon passes between the Earth and the Sun, blocking all or part of the Sun's light.

T

• Telescope – An instrument that aids in the observation of remote objects by collecting electromagnetic radiation, such as visible light.

U

• Universe – The totality of known or supposed objects and phenomena throughout space. The universe is believed to be at least 13.8 billion years old and possibly infinite in extent.

V

• Variable star – A star whose brightness as seen from Earth fluctuates.

W

• White dwarf – A small, dense star that is the final evolutionary state of stars not massive enough to become neutron stars or black holes.

X

• X-ray astronomy – The study of astronomical objects at X-ray wavelengths. It is used to observe high-energy phenomena such as black holes and neutron stars.

Y

• Year – A unit of time based on the orbit of the Earth around the Sun. In astronomy, it is used to measure periods of time related to the motion of planets and other celestial objects.

Z

• Zenith – The point in the sky directly above an observer on the Earth.

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