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		Nebula
Imprecise term (not approved by astronomers) for a typical star like the Sun – more correctly known as a 'G–type main–sequence star'		Yellow dwarf
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		Red dwarf
Formed when the hydrogen at the centre of a star is exhausted, and nuclear reactions start to move outwards into its atmosphere		Red giant
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)		Planetary nebula
Compact, extremely dense core of a low–mass star at the end of its life (after a planetary nebula event)		White dwarf
Formed when a high–mass star has come to the end of its life and explodes (after the red supergiant phase)		Supernova
Formed when a supernova collapses into its core, becoming so compressed that the atoms are smashed apart		Neutron star
Formed when a very large supernova collapses into its core		Black hole

As stated in the above table, the Sun is a yellow giant (more correctly known as a 'G–type main–sequence star'). According to Wikipedia, "when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand, eventually transforming the Sun into a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable in five billion years. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, and no longer produce energy by fusion, but still glow and give off heat from its previous fusion."

Bigger than a planet, but smaller than a star – too small for nuclear reactions to ignite in the core

Brown dwarf

Other

Age of the universe (billions of years)		13.8
Age of the oldest stars in our galaxy, the Milky Way (billions of years)		13.6
Age of the solar system (billions of years)		4.55

Distance of the Earth from the Sun (approx. 93 million miles or 150 million kilometres)		Astronomical Unit
Miles in a light–year		5.879 × 1012
Astronomical units in a light–year (approx.)		63,241.1
Light–years in a parsec (approx.)		3.26

The angle between the north point and the perpendicular projection of the star down onto the horizon		Azimuth
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"		Big Bang
Name for the end of the universe, corresponding to the Big Bang		Big Crunch
A pair of stars bound together by gravity – term coined by William Herschel		Binary star
Area in space where gravity is so strong that not even light can escape; believed to be created when a star collapses		Black hole
Defines the distance of the planets from the Sun		Bodes' Law
Alignment of two or more celestial bodies, as viewed from Earth		Conjunction
Hypothetical form of energy, a way of explaining the apparent increase in the rate of expansion of the universe		Dark energy
Hypothetical material thought to constitute about 85% of the universe – does not react to electromagnetic radiation (and is therefore not detectable by current science)		Dark matter
Co–ordinate of a point on the celestial sphere, corresponding to latitude		Declination
Light year, parsec and astronomical unit (AU) are units of		Distance
Occurs when one body passes through the shadow of another		Eclipse
The geometric plane (in space) that includes the orbit of the Earth		Ecliptic
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		Equinoxes
Name coined by the Austrian–born British physicist Wolfgang Rindler (1924–2019) for the notional boundary around a black hole, beyond which no matter or energy can escape		Event horizon
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		Goldilocks zone
Declination, right ascension: celestial equivalents of		Latitude and longitude
Term used for the apparent brightness of a star as viewed from Earth		Magnitude
Imaginary circle on the celestial sphere, perpendicular to the horizon and passing through the north and south points on the horizon		Meridian
Difference between a meteor and a meteorite		Meteorites hit the Earth; meteors burn up
The point in the heavens that's directly below the observer (opposite the zenith)		Nadir
Name given to a concentration of dust and ionised gases (especially hydrogen and/or helium) in space, from which a star may form: from the Latin for 'cloud', 'mist' or 'fog'; the Crab, the Horsehead, the Eagle (featuring the so–called Pillars of Creation) and the Great one in Orion are the best–known examples		Nebula
Nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star; cf. Supernova		Nova
Eclipse of a star by a planet		Occultation
When two heavenly bodies are on opposite sides of the celestial sphere, as observed from a given body (usually Earth), they are said to be in, or at		Opposition
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 (206,000 au, or 19.2 trillion miles)		Parsec
A small, extremely dense neutron star, which rotates rapidly and emits a bright beam of electromagnetic radiation from its magnetic poles – so it can only be observed (from any one point in space) at regular intervals		Pulsar
Co–ordinate of a point on the celestial sphere, corresponding to longitude		Right ascension
Time taken for the Earth to rotate on its axis (23 hours, 56 minutes, 4.1 seconds)		Sidereal day
Found at the centre of a black hole		Singularity
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"		Solstices
Alternative to the Big Bang theory, proposed in 1948 and supported by Fred Hoyle; now largely discounted		Steady State
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		Supernova
Alignment of three bodies in the same gravitational system (e.g. the sun, earth and moon)		Syzygy
The line that divides the light and dark sides of a planet or moon (especially Earth's Moon) – also known as the twilight zone		Terminator
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)		Umbra
The point in the heavens that is directly above the observer (cf. Nadir)		Zenith

© Haydn Thompson 2017–23