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Science
Astronomy
Theory

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Astronomy: Theory

This page covers astronomical terms, as well as things like the history of the Universe.

The Life Cycle of Stars

Cloud of dust and gases from which a star may form Click to show or hide the answer
Imprecise term (not approved by astronomers) for a typical star like the Sun – more correctly known as a 'G–type main–sequence star' Click to show or hide the answer
Imprecise term for a star with about 50–75% of the Sun's mass: even those in our neighbourhood are invisible to the naked eye because of relatively low surface temperature and hence low luminosity Click for more information Click to show or hide the answer
Formed when the hydrogen at the centre of a star is exhausted, and nuclear reactions start to move outwards into its atmosphere Click for more information Click to show or hide the answer
Glowing shell of hot gas, cast off towards the end of the life of the red giant phase of a low–mass star (such as the Sun) Click for more information Click to show or hide the answer
Compact, extremely dense core of a low–mass star at the end of its life (after a planetary nebula event) Click for more information Click to show or hide the answer
Formed when a high–mass star has come to the end of its life and explodes (after the red supergiant phase) Click for more information Click to show or hide the answer
Formed when a supernova collapses into its core, becoming so compressed that the atoms are smashed apart Click for more information Click to show or hide the answer
Formed when a very large supernova collapses into its core Click for more information Click to show or hide the answer

Bigger than a planet, but smaller than a star – too small for nuclear reactions to ignite in the core Click for more information Click to show or hide the answer

Other

Age of the universe (billions of years) Click to show or hide the answer
Age of the oldest stars in our galaxy, the Milky Way (billions of years) Click to show or hide the answer
Age of the solar system (billions of years) Click to show or hide the answer

Distance of the Earth from the Sun (approx. 93,000 miles or 150,000 km) Click to show or hide the answer
Miles in a light–year Click to show or hide the answer
Astronomical units in a light–year (approx.) Click to show or hide the answer
Light–years in a parsec (approx.) Click to show or hide the answer

The angle between the north point and the perpendicular projection of the star down onto the horizon Click to show or hide the answer
Theory that the universe began from 'a cataclysmic event from a singularity'; proposed c. 1930 by Belgian priest and astronomer Georges Lemaitre, as the "hypothesis of the primeval atom" Click to show or hide the answer
Name for the end of the universe, corresponding to the Big Bang Click to show or hide the answer
A pair of stars bound together by gravity – term coined by William Herschel Click to show or hide the answer
Area in space where gravity is so strong that not even light can escape; believed to be created when a star collapses Click to show or hide the answer
Defines the distance of the planets from the Sun Click to show or hide the answer
Alignment of two or more celestial bodies, as viewed from Earth Click to show or hide the answer
Hypothetical form of energy, a way of explaining the apparent increase in the rate of expansion of the universe Click to show or hide the answer
Hypothetical material thought to constitute about 85% of the universe – does not react to electromagnetic radiation (and is therefore not detectable by current science) Click to show or hide the answer
Co–ordinate of a point on the celestial sphere, corresponding to latitude Click to show or hide the answer
Light year, parsec and astronomical unit (AU) are units of Click to show or hide the answer
Occurs when one body passes through the shadow of another Click to show or hide the answer
The geometric plane (in space) that includes the orbit of the Earth Click to show or hide the answer
The times of the year (strictly moments in time) when the sun is directly above the equator – when the tilt of the earth's axis is "side on" to the sun – resulting in days of (almost) exactly 12 hours Click to show or hide the answer
Layman's term (derived from a traditional children's story) for the (circumstellar) habitable zone – the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure Click to show or hide the answer
Declination, right ascension: celestial equivalents of Click to show or hide the answer
Term used for the apparent brightness of a star as viewed from Earth Click to show or hide the answer
Imaginary circle on the celestial sphere, perpendicular to the horizon and passing through the north and south points on the horizon Click to show or hide the answer
Difference between a meteor and a meteorite Click to show or hide the answer
The point in the heavens that's directly below the observer (opposite the zenith) Click to show or hide the answer
A concentration of hydrogen and dust in space, from which a star may form (from the Latin for 'cloud', 'mist' or 'fog') Click to show or hide the answer
Nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star. cf. Supernova Click to show or hide the answer
Eclipse of a star by a planet Click to show or hide the answer
The distance from the Earth to a star that has a parallax of one arcsecond – an arcsecond being one 3,600th of a degree – 3.26 light years Click to show or hide the answer
Co–ordinate of a point on the celestial sphere, corresponding to longitude Click to show or hide the answer
Time taken for the Earth to rotate on its axis (23 hours, 56 minutes, 4.1 seconds) Click to show or hide the answer
Found at the centre of a black hole Click to show or hide the answer
The times of the year (strictly moments in time) when the sun is at its most northerly or southerly extreme (furthest from the equator) – when the earth's axis is tilted directly towards (and directly away from) the sun – resulting in the longest and shortest days in the two respective hemispheres. Name derived from Latin, meaning "sun standing still" Click to show or hide the answer
Alternative to the Big Bang theory, proposed in 1948 and supported by Fred Hoyle; now largely discounted Click to show or hide the answer
Brief burst of extremely luminous radiation that occurs when a star collapses under its own gravity, having expended all its fuel, and explodes. Lasts a few weeks or months; can radiate as much energy as the Sun in its entire lifespan. Cf. Nova Click to show or hide the answer
Alignment of three bodies in the same gravitational system (e.g. the sun, earth and moon) Click to show or hide the answer
Latin word for a shadow – used in astronomy for the shadow of one body on another (e.g. the earth's shadow on the moon in a lunar eclipse) Click to show or hide the answer
The point in the heavens that is directly above the observer (cf. Nadir) Click to show or hide the answer

© Haydn Thompson 2017