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Glossary of astronomy

Glossary of astronomy is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Glossary of astronomy rather than just read about it. In short: This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth.

Glossary of astronomy — main illustration
Glossary of astronomy — illustration

Key takeaways

  • Glossary of astronomy belongs to astronomy; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Glossary of astronomy to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Glossary of astronomy from memory before moving on to harder problems.

Reference excerpt

This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of sophisticated terminology.

A

A-type main-sequence star In the Harvard spectral classification system, a class of main-sequence star having spectra dominated by Balmer absorption lines of hydrogen. Main-sequence stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times the mass of the Sun, and have surface temperatures of 7,600–10,000 kelvin.

absolute magnitude A measure of a star's absolute brightness. It is defined as the apparent magnitude the star would show if it were located at a distance of 10 parsecs, or 32.6 light-years.

accretion disk A roughly circular mass of diffuse material in orbit around a central object, such as a star or black hole. The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object.

active galactic nucleus (AGN) A compact region in the center of a galaxy displaying a much higher than normal luminosity over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. A galaxy hosting an AGN is called an active galaxy.

airborne observatory An airplane with an astronomical telescope, which relies upon altitude to reduce atmospheric absorption and improve seeing conditions. Drawbacks include the instability of the lifting platform and higher costs.

albedo A measure of the proportion of the total solar radiation received by an astronomical body, such as a planet, that is diffusely reflected away from the body. It is a dimensionless quantity typically measured on a scale from 0 (indicating total absorption of all incident radiation, as by a black body) to 1 (indicating total reflection). The albedo reported for an astronomical body may vary widely by the spectral and angular distribution of the incident radiation, by the "layer" of the body being measured (e.g. upper atmosphere versus surface), and by local variation within these layers (e.g. cloud cover and geological or environmental surface features).

albedo feature A large area on the surface of a reflecting object that shows a significant contrast in brightness or darkness (albedo) compared to adjacent areas.

Alfvén surface The boundary separating a star's corona from the stellar wind, defined as the point at which the coronal plasma's Alfvén speed equals the large-scale stellar wind speed.

Am star A chemically peculiar star belonging to the more general class of A-type stars. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals. They are generally slow rotators, and are often found in binary systems where tidal braking has slowed the rate of spin. See metallicity.

aphelion The point at which a body orbiting the Earth's Sun is furthest from the Sun. Contrast perihelion.

apoapsis The point at which an orbiting body is furthest from its primary. Contrast periapsis.

apogee The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is furthest from the Earth. Contrast perigee.

apparent magnitude Also visual brightness (V). A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower its magnitude.

appulse The closest approach of one celestial object to another, as viewed from a third body.

apsis In the orbit of a planetary body, one of the two extreme points of distance between the body and its primary – either the point of minimal distance, called the periapsis, or the point of maximal distance, called the apoapsis. The term may also be used to refer to the value of the distance rather than the point itself. All elliptical orbits have exactly two apsides.

argument of periapsis Also argument of perifocus or argument of pericenter. The angle from an orbiting body's ascending node to its periapsis, measured in the direction of motion. It is one of six canonical orbital elements used to characterize an orbit.

artificial satellite An object that has been intentionally placed into orbit by humans, often around the Earth but also around other bodies within the Solar System. Contrast natural satellite.

ascending node Also the north node. The orbital node at which an orbiting object moves north through the plane of reference (in geocentric and heliocentric orbits) or at which the orbiting object moves away from the observer (in orbits outside of the Solar System). The position of the ascending node with respect to a reference direction, called the longitude of the ascending node, is used along with other parameters to describe an orbit. Contrast descending node.

aspect The position of a planet or Earth's Moon with respect to the Sun, as viewed from Earth.

asterism Any pattern of stars recognizable in Earth's night sky. An asterism may form part of an official constellation or it may be composed of stars from more than one constellation.

asteroid A minor planet of the inner Solar System. They are primarily found orbiting the Sun between Jupiter and Mars, but can approach the Earth or occupy trojan orbits with Jupiter. Asteroids are somewhat arbitrarily distinguished from many different types of similar objects: small Solar System bodies primarily composed of dust and ice instead of mineral and rock are known as comets; bodies less than one meter in diameter are known as meteoroids; very large asteroids are sometimes called planetoids or planetesimals; and bodies similar to asteroids in size and composition but which lie beyond Jupiter are known as distant minor planets.

asteroid belt The circumstellar disc in the Solar System located roughly between the orbits of Mars and Jupiter that is occupied by numerous irregularly shaped small Solar System bodies ranging in size from dust particles to asteroids and minor planets. The asteroid belt is often called the main asteroid belt or main belt to distinguish it from other asteroid populations in other parts of the Solar System.

… excerpt ends here. Continue reading the full article.

Illustrations

Glossary of astronomy: Two bodies of similar mass orbit a common barycenter external to both, as usual in binary star systems
Two bodies of similar mass orbit a common barycenter external to both, as usual in binary star systems
Glossary of astronomy: A diagram of the relationships between the Earth's axis of rotation, its celestial equator, and the plane of its orbit around the Sun, known as the ecliptic. Note that the Earth's rotational axis is not perpendicular to the ecliptic but rather is tilted; this means that the path of the Sun, as viewed from Earth, appears to move both above and below the celestial equator during the course of the year.
A diagram of the relationships between the Earth's axis of rotation, its celestial equator, and the plane of its orbit around the Sun, known as the ecliptic. Note that the Earth's rotational axis is not perpendicular to the ecliptic but rather is tilted; this means that the path of the Sun, as viewed from Earth, appears to move both above and below the celestial equator during the course of the year.
Glossary of astronomy: Circumstellar debris disks as detected by the Hubble Space Telescope, and artist's impressions of the disks' orientations around their host stars
Circumstellar debris disks as detected by the Hubble Space Telescope, and artist's impressions of the disks' orientations around their host stars
Glossary of astronomy: The plane of the ecliptic (grey) is defined by the Earth's orbit around the Sun, and is distinct from the plane of the celestial sphere's equator (green), which is permanently tilted 23.4 degrees with respect to the ecliptic. To an observer on Earth, this means that the path the Sun appears to follow upon the celestial sphere is not a straight line (i.e. the ecliptic does not appear "flat"). Twice during each complete orbit, at the two equinoxes, the Sun's path appears to intersect the celestial equator, though the two planes are never in fact coplanar.
The plane of the ecliptic (grey) is defined by the Earth's orbit around the Sun, and is distinct from the plane of the celestial sphere's equator (green), which is permanently tilted 23.4 degrees with respect to the ecliptic. To an observer on Earth, this means that the path the Sun appears to follow upon the celestial sphere is not a straight line (i.e. the ecliptic does not appear "flat"). Twice during each complete orbit, at the two equinoxes, the Sun's path appears to intersect the celestial equator, though the two planes are never in fact coplanar.
Glossary of astronomy: A relatively small body (such as a planet) orbiting a larger one (such as a star) in an elliptical orbit, with the larger body located at one of the focal points of the ellipse
A relatively small body (such as a planet) orbiting a larger one (such as a star) in an elliptical orbit, with the larger body located at one of the focal points of the ellipse

Worked examples

Example 1 — a first encounter with Glossary of astronomy

Start with the simplest possible case. Write down what Glossary of astronomy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Glossary of astronomy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Glossary of astronomy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Glossary of astronomy

In research
Glossary of astronomy appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Glossary of astronomy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Glossary of astronomy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astronomy, Astronomy-related lists, Glossaries of science, so understanding it makes those chapters shorter.
In everyday life
Look for Glossary of astronomy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Glossary of astronomy in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Glossary of astronomy means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Glossary of astronomy out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Glossary of astronomy in simple terms?

This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth.

Why does Glossary of astronomy matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Glossary of astronomy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Glossary of astronomy.

Tags

  • Astronomy
  • Astronomy-related lists
  • Glossaries of science

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