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Important terms in astronomy


The redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. This is the evidence for Big bang theory or expansion of the universe.


Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids.

Big Bang

The Big Bang theory is the prevailing cosmological model for the earliest known periods of the universe and its subsequent large-scale evolution. It states that the Universe was in a very high density state and then expanded.

After the initial expansion, the Universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies.

 The Big Bang theory does not provide any explanation for the initial conditions of the Universe; rather, it describes and explains the general evolution of the Universe going forward from that point on.

Black hole

A black hole is a mathematically defined region of space-time exhibiting such a strong gravitational pull that no particle or electromagnetic radiation can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform space-time to form a black hole.

Celestial coordinate system

A celestial coordinate system is a system for specifying positions of celestial objects: satellites, planets, stars, galaxies, and so on.

Coordinate systems can specify a position in 3-dimensional space, or merely the direction of the object on the celestial sphere, if its distance is not known or not important.

Celestial pole

The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the celestial sphere.

The north and south celestial poles appear permanently directly overhead to an observer at the Earth's North Pole and South Pole respectively.

As the Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other points appear to rotate around them, completing one circuit per day (strictly per sidereal day).

Circumpolar star

A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets (that is, never disappears below the horizon), due to its proximity to one of the celestial poles. Circumpolar stars are therefore visible from said location toward nearest pole for the entire night on every night of the year (and would be continuously visible throughout the day too, were they not overwhelmed by the Sun's glare).

All circumpolar stars are within the circumpolar circle. This was in fact the original meaning of "Arctic Circle", before the current geographical meaning, meaning "Circle of the Bears" (Ursa Major, the Great Bear; and Ursa Minor, the Little Bear), from Greek αρκτικός (arktikos), "near the Bear", from the word άρκτος (arktos) bear.

Cosmic rays

Cosmic rays are immensely high-energy radiation, mainly originating outside the Solar System.

Dark matter

Dark matter is a hypothetical kind of matter that cannot be seen with telescopes but accounts for most of the matter in the Universe. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the Universe. It has not been detected directly, making it one of the greatest mysteries in modern astrophysics.

Dark matter neither emits nor absorbs light or any other electromagnetic radiation at any significant level. According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Thus, dark matter is estimated to constitute 84.5% of the total matter in the Universe, while dark energy plus dark matter constitute 95.1% of the total mass–energy content of the Universe.

According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle. The search for this particle, by a variety of means, is one of the major efforts in particle physics today.

Although the existence of dark matter is generally accepted by the mainstream scientific community, some alternative theories of gravity have been proposed, such as MOND and TeVeS, which try to account for the anomalous observations without requiring additional matter.


The ecliptic is the apparent path of the Sun throughout the course of a year.


A galaxy is a gravitationally bound system consisting of stars, stellar remnants, interstellar gas and dust, and dark matter.Examples of galaxies range from dwarfs with just a few thousand stars to giants with one hundred trillionstars, each orbiting their galaxy's own center of mass.

Galaxies have historically been categorized according to their visual morphology, including elliptical, spiral(eg. Milky way), irregular, and starburst. Many galaxies are believed to have black holes at their active center. The Milky Way's central black hole, known asSagittarius A*, has a mass four million times that of our Sun.


A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases.

Nebulae are often star-forming regions. In these regions the formations of gas, dust, and other materials "clump" together to form larger masses, which attract further matter, and eventually will become massive enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.


Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

Pole star

A pole star is a visible star, preferably a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent position is close to one of the celestial poles, and which lies approximately directly overhead when viewed from the Earth's North Pole or South Pole.

In practice, the term pole star usually refers to Polaris, which is the current northern pole star, also known as the North Star.

The south celestial pole currently lacks a bright star like Polaris to mark its position. At present, the naked-eye star nearest to this imaginary point is the faint Sigma Octantis, which is sometimes known as the South Star.

While other stars' apparent positions in the sky change throughout the night, as they appear to rotate around the celestial poles, pole stars' apparent positions remain virtually fixed. This makes them especially useful in celestial navigation:


Precession refers to any of several gravity-induced, slow and continuous changes in an astronomical body's rotational axis or orbital path.

Examples: Precession of the equinoxes, perihelion precession etc..

Sidereal time

Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky.

Briefly, sidereal time is a "time scale that is based on the Earth's rate of rotation measured relative to the fixed stars.

Solar time

Solar time is a reckoning of the passage of time based on the Sun's position in the sky.

The fundamental unit of solar time is the day.

Two types of solar time are apparent solar time (sundial time) and mean solar time (clock time).


A supernova is a stellar explosion that briefly outshines an entire galaxy, radiating as much energy as the Sun or any ordinary star is expected to emit over its entire life span, before fading from view over several weeks or months.

A great proportion of primary cosmic rays comes from supernovae.

Supernovae can be triggered in one of two ways: by the sudden re-ignition of nuclear fusion in a degenerate star; or by the gravitational collapse of the core of a massive star.

They play a significant role in enriching the interstellar medium with higher mass elements.

Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.