Glossary of Common Astronomy Terms

Astronomy - The scientific study of objects and events beyond Earth, including stars, planets, galaxies, and the universe.

Term	Definition	Learn more
Astronomy	The scientific study of objects and events beyond Earth, including stars, planets, galaxies, and the universe.	In AstroGuide: Learn What Astronomy Really Is 7 matching references in this guide section. Visual Astronomy vs Astrophotography 5 matching references in this guide section. External references Wikipedia: Astronomy Overview of astronomy as the scientific study of the universe. NASA Universe glossary NASA glossary for common universe and astronomy terms.
Astrophotography	The practice of photographing objects in space. It uses cameras, telescopes, and image processing to reveal details that are too faint for our eyes to see.	In AstroGuide: Learn Preface 4 matching references in this guide section. Visual Astronomy vs Astrophotography 4 matching references in this guide section. External references Wikipedia: Astrophotography Background on photographic methods used to image astronomical objects.
Aperture	The diameter of a telescope's main lens or mirror. A larger aperture collects more light and can potentially reveal more detail.	In AstroGuide: Learn Aperture 6 matching references in this guide section. Myth: Bigger Aperture Always Wins 4 matching references in this guide section. External references Wikipedia: Aperture Optics background for the opening that admits light into an instrument.
Bortle Scale	A nine-level scale used to describe the brightness of the night sky caused by light pollution. Lower numbers represent darker skies.	In AstroGuide: Learn Light Pollution 6 matching references in this guide section. Myth: Lower Bortle Always Produces Better Images 5 matching references in this guide section. External references Wikipedia: Bortle scale Summary of the nine-level dark-sky brightness scale.
Celestial Object	Any natural object found in space, such as stars, planets, moons, asteroids, nebulae, and galaxies.	In AstroGuide: Learn Visual Astronomy vs Astrophotography 1 matching reference in this guide section. Understanding the Night Sky 1 matching reference in this guide section. External references Wikipedia: Astronomical object General reference for natural objects and structures in space.
Deep-Sky Object (DSO)	Objects outside our solar system, such as nebulae, star clusters, and galaxies.	In AstroGuide: Learn Myth: Fast Telescopes Are Always Superior 4 matching references in this guide section. Moonlight 2 matching references in this guide section. External references Wikipedia: Deep-sky object Overview of non-Solar-System targets such as nebulae, clusters, and galaxies.
Exposure	The amount of time a camera collects light. Longer exposures allow more faint details to be recorded.	In AstroGuide: Learn Why Great Images Take Time 4 matching references in this guide section. Myth: Stacking Fixes Everything 3 matching references in this guide section. External references Wikipedia: Exposure Photography background on exposure time and captured light.
Field of View (FOV)	The amount of sky visible through a telescope or camera system. A wider field of view captures larger areas of the sky.	In AstroGuide: Learn Focal Length 6 matching references in this guide section. Optimal Focal Length: 400–1,500 mm Related guide topic for this term. External references Wikipedia: Field of view Reference for angular extent visible through an optical system.
Focal Length	The distance between a telescope's optical system and the point where light is focused. It determines image scale and field of view.	In AstroGuide: Learn Focal Length 9 matching references in this guide section. Myth: Long Focal Length Produces Better Images 6 matching references in this guide section. External references Wikipedia: Focal length Optics reference for focal distance and image scale.
Focal Ratio	The relationship between a telescope's aperture and focal length, expressed as an f-number (such as f/4 or f/10). It affects light-gathering speed and imaging characteristics.	In AstroGuide: Learn Myth: Fast Telescopes Are Always Superior 19 matching references in this guide section. Focal Ratio 13 matching references in this guide section. External references Wikipedia: F-number Reference for focal ratio and how it relates aperture to focal length.
Galaxy	A massive collection of stars, gas, dust, and dark matter held together by gravity. Our home galaxy is the Milky Way.	In AstroGuide: Learn Galaxies 22 matching references in this guide section. Star Clusters 5 matching references in this guide section. External references NASA: Galaxies NASA overview of galaxy types, structure, and evolution. Wikipedia: Galaxy Broad reference on galaxies and their components.
Light Pollution	Artificial light that brightens the night sky and reduces the visibility of faint astronomical objects.	In AstroGuide: Learn Light Pollution 6 matching references in this guide section. Myth: Lower Bortle Always Produces Better Images 5 matching references in this guide section. External references Wikipedia: Light pollution Reference for causes and effects of artificial nighttime light.
Light-Year	The distance light travels in one year. It is used to describe distances between stars and galaxies.	In AstroGuide: Learn Galaxies 1 matching reference in this guide section. Understanding Light Related guide topic for this term. External references NASA Space Place: Light-year Beginner-friendly explanation of distance measured by light travel time. Wikipedia: Light-year Reference for the astronomical distance unit.
Magnification	How much larger an object appears through a telescope compared to viewing it with the naked eye.	In AstroGuide: Learn Seeing 2 matching references in this guide section. Myth: Long Focal Length Produces Better Images 1 matching reference in this guide section. External references Wikipedia: Magnification Optics reference for apparent size increase.
Nebula	A large cloud of gas and dust in space. Some nebulae are regions where new stars are forming.	In AstroGuide: Learn Nebulae 21 matching references in this guide section. Other Types of Nebulae 14 matching references in this guide section. External references Wikipedia: Nebula Overview of gas and dust clouds in space. NASA: Hubble's Nebulae NASA overview of nebula types and star-forming regions.
Narrowband Filter	A filter that allows only specific wavelengths of light to pass through. It is commonly used to capture emission nebulae.	In AstroGuide: Learn Filters 10 matching references in this guide section. Moonlight 3 matching references in this guide section. External references Wikipedia: Narrowband Reference for narrow passbands that transmit a limited range of wavelengths. Wikipedia: Optical filter General optics reference for filters that selectively transmit light.
Pixel Scale	The amount of sky covered by each pixel of a camera image. It affects the level of detail that can be recorded.	In AstroGuide: Learn Focal Length 4 matching references in this guide section. Focal Ratio 2 matching references in this guide section. External references Wikipedia: Angular resolution Background on resolving fine angular detail in an image.
Seeing	The stability of Earth's atmosphere. Good seeing allows sharper views, especially for planets and high-resolution imaging.	In AstroGuide: Learn Seeing 7 matching references in this guide section. Choosing the Right Telescope: Optimal Aperture and Focal Length for General Sky Conditions 3 matching references in this guide section. External references Wikipedia: Astronomical seeing Reference for atmospheric turbulence and image sharpness.
Signal-to-Noise Ratio (SNR)	The amount of useful information compared to unwanted noise in an image. Higher SNR usually results in cleaner and more detailed images.	In AstroGuide: Learn Why Great Images Take Time 2 matching references in this guide section. Why Processing Matters 2 matching references in this guide section. External references Wikipedia: Signal-to-noise ratio Technical reference for useful signal compared with unwanted noise.
Stacking	Combining multiple images together to improve signal quality and reduce noise.	In AstroGuide: Learn Myth: Stacking Fixes Everything 9 matching references in this guide section. Why Great Images Take Time 4 matching references in this guide section. External references Wikipedia: Astrophotography processing Background on processing multiple astronomical images. Wikipedia: Lucky imaging Related technique that combines selected frames for sharper results.
Star Cluster	A group of stars held together by gravity. The two main types are open clusters and globular clusters.	In AstroGuide: Learn Star Clusters 18 matching references in this guide section. Light Pollution 4 matching references in this guide section. External references Wikipedia: Star cluster Overview of open clusters, globular clusters, and stellar groupings.
Telescope	An optical instrument that collects and focuses light to observe distant objects.	In AstroGuide: Learn Mounts 9 matching references in this guide section. There Is No Perfect Equipment 7 matching references in this guide section. External references Wikipedia: Telescope Reference for instruments that collect and focus electromagnetic radiation.
Transparency	The clarity of the atmosphere and how easily light from space can pass through it.	In AstroGuide: Learn Transparency 11 matching references in this guide section. Myth: Lower Bortle Always Produces Better Images 4 matching references in this guide section. External references Wikipedia: Atmospheric window Context for how Earth's atmosphere transmits light from space.
Tracking	The movement of a telescope mount to follow objects as Earth rotates, keeping them centred during observation or photography.	In AstroGuide: Learn Mounts 12 matching references in this guide section. Myth: More Expensive Telescope Equals Better Images 7 matching references in this guide section. External references Wikipedia: Equatorial mount Reference for mount geometry used to follow Earth's rotation. Wikipedia: Clock drive Background on mechanisms that move a telescope with the sky.