Glossary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A
Asteroids: A class of Small Solar System Bodies (SSSB) in orbit around the Sun. They have also been called planetoids, especially the larger ones.
Astronomical objects: Gravitationally bound structures that are associated with a position in space, but may consist of multiple independent astronomical bodies or objects. These objects range from single planets to star clusters, nebulae or entire galaxies
Astronomy: The study of objects and matter outside the Earth’s atmosphere and of their physical and chemical properties. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics
Astrophysics: The branch of astronomy dealing with the behavior, physical properties, and dynamic processes of celestial objects and phenomena. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.
Atmosphere: A layer of gases that surround, a material body of sufficient mass (star, planet, moon) and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere’s temperature is low. Some planets consist mainly of various gases, but only their outer layer is their atmosphere.
Atmospheric boundary layer (ABL) or planetary boundary layer (PBL): the lowest part of the atmosphere which behaviour is directly influenced by its contact with a planetary surface.
Axial tilt or obliquity: The angle that a planet’s poles are offset from being perpendicular to the plane of the planet’s orbit around a star.
B
Black holes: Objects in space so dense that not even light can escape their gravity, although powerful jets of light and energy can be emitted from a black hole’s vicinity as gas and stars are sucked into it.
- Stellar black holes result from the collapse of individual stars.
- Intermediate-mass black holes are a rather uncommon class of black holes, somewhere in between in the size of stellar black holes and supermassive black holes. They are believed to form not from single supernovae, but possibly from a number of lesser stellar black holes in a star cluster.
- Supermassive black holes contain mass ranging from 1 million to 10 billion stars the size of our sun. They are located the centers of most galaxies
Bulge: A round structure at the center of spiral galaxies composed mostly of old stars and some gas and dust.
C
Celestial body: A natural single, cohesive structure that is bound together by gravity (and sometimes by electromagnetism). Examples include the asteroids, moons, planets and the stars.
Chondrites: Meteorites that were formed from the solar nebula that surrounded the Sun over 4.6 billion years ago. They are valuable to scientists because of their direct relationship with the early Solar System and the primordial material they contain.
E
Electromagnetic radiation (E-M radiation or EMR): A form of energy that exhibits wave-like behavior as it travels through space. It is classified according to the frequency of its wave. The electromagnetic spectrum, in order of increasing frequency and decreasing wavelength, consists of radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Evaporating Gaseous Globules (EGGs): Dense centers of hydrogen gas and dust that act as incubators for new starsdense centers of hydrogen gas and dust that act as incubators for new stars.
F
Filaments: The largest known cosmic structures in the universe, thread-like structures that form the boundaries between large voids in the universe, or threadlike structures extending outward from galaxies and stars (a solar prominence is also a filament).
G
Gamma radiation (gamma-rays and denoted as γ): electromagnetic radiation with a wavelength less than 10 picometres and a frequency above 10 exahertz (or >1019 Hz).
Goldilocks Zone or Habitable Zone: The region around a star where a planet (and moons) with sufficient atmospheric pressure can maintain liquid water on its surface. Since liquid water is essential for all known forms of life, planets (and moons) in this zone are considered the most promising sites to host extraterrestrial life.
H
Habitable Zone: The region around a star where a planet (and moons) with sufficient atmospheric pressure can maintain liquid water on its surface. Since liquid water is essential for all known forms of life, planets (and moons) in this zone are considered the most promising sites to host extraterrestrial life. (Sometimes also called Goldilocks Zone.)
Hertz (symbol Hz): The unit of frequency defined as the number of cycles per second. Commonly used multiples are kHz (kilohertz, 103 Hz), MHz (megahertz, 106 Hz), GHz (gigahertz, 109 Hz), THz (terahertz, 1012 Hz), PHz (petahertz, 1016 Hz) and Ehz (exahertz, 1019 Hz).
I
Infrared (IR) light: Electromagnetic radiation with a wavelength measured from the nominal edge of visible red light at 0.74 micrometres (µm), and extending conventionally to 300 µm. These wavelengths correspond to a frequency range of approximately 1 to 400 THz (terahertz = 1012 Hz).
Intergalactic medium (IGM): A rarefied plasma that consist mostly of ionized hydrogen (a plasma consisting of equal numbers of electrons and protons), that is thought to possess a cosmic filamentary structure and that is slightly denser than the average density in the universe. The IGM is thought to exist at a density of 10 to 100 times the average density of the universe (10 to 100 hydrogen atoms per cubic meter).
Intermediate-mass black hole: A rather uncommon class of black holes, somewhere in between in the size of stellar black holes and the massive black holes theorized to reside in the center of many galaxies. Because of this, they are believed to form not from single supernovae, but possibly from a number of lesser stellar black holes in a star cluster.
Interstellar Cloud: Dust and gas existing between stars.
Interstellar planet (also known as Rogue planet, Nomad planet or Orphan planet): A planet, dwarf planet or larger moon that has been ejected from its system and is no longer gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly.
L
Light-year: The distance that light can travel in a year, which is 9.46 trillion km (5.86 trillion miles).
M
Mesopause: The upper boundary of the mesosphere, which can be the coldest naturally occurring place on Earth with temperatures below 130 Kelvin, usually at heights near 100 km, except at middle and high latitudes in summer where it descends to heights of about 85 km.
Mesosphere: The layer of the Earth’s atmosphere that is directly above the stratosphere and directly below the thermosphere. The upper boundary is the mesopause.
Meteor: The visible path of a meteoroid that has entered the Earth’s atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km. Many meteors appearing seconds or minutes apart are called a meteor shower.
Meteorite: A natural object originating in outer space that survives impact with the Earth’s surface. A meteorite’s size can range from small to extremely large. Most meteorites derive from meteoroids, but they are also sometimes produced by impacts of asteroids.
Meteoroid: A sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth’s (or another body’s) atmosphere is called a meteor, a shooting star or a falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite.
Microwaves: Radio waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (megahertz = 106 Hz), and 300 GHz (gigahertz = 109 Hz).
Mid-mass Black Holes: A newly discovered type of black hole that has a mass of 500 – 1,000’s of Suns.
Millisecond pulsars: The fastest-spinning and strongly magnetized, old neutron stars in binary systems. They have been spun up to high rotational frequencies by accumulating mass and angular momentum from a companion star. They are highly stable and keep time more accurately than atomic clocks.
Minor planet: An astronomical object in direct orbit around The Sun that is neither a dominant planet nor originally classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, and other trans-Neptunian objects.
N
Neutron star: The remnant after a massive star explodes at the end of its “normal” life. With no nuclear fuel left to produce energy to offset the stellar remnant’s weight, its material is compressed to extreme densities. The pressure squeezes together most of its protons and electrons to form neutrons.
Nomad planet (also known as Rogue planet, Interstellar planet, or Orphan planet): A planet, dwarf planet or larger moon that has been ejected from its system and is no longer gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly.
O
Obliquity or Axial tilt: The angle that a planet’s poles are offset from being perpendicular to the plane of the planet’s orbit around a star.
Observable universe: galaxies and other matter that we can in principle observe from Earth in the present day, because light (or other signals) from those objects has had time to reach us since the beginning of the cosmological expansion.
Orbit: The gravitationally curved path of an object around a point in space. (Orbits of planets are typically elleptical.)
Orphan planet (also known as Rogue planet, Nomad planet or Interstellar planet): A planet, dwarf planet or larger moon that has been ejected from its system and is no longer gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly.
P
Planet: A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
Planetary boundary layer (PBL) or the atmospheric boundary layer (ABL): the lowest part of the atmosphere which behaviour is directly influenced by its contact with a planetary surface.
Planetoids or Asteroids: A class of Small Solar System Bodies (SSSB) in orbit around the Sun. They have also been called planetoids, especially the larger ones.
Protoplanetary Disk (or Proplyd for short): A region of dense gas around a newly formed star.
Pseudobulge: The central group of stars found in most spiral galaxies. These bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the outer disk.
Pulsars: Spinning neutron stars that sling “lighthouse beams” of radio waves around as they rotate.
Q
Quasars (short for quasi-stellar object): The brilliant cores of galaxies where infalling material fuels a supermassive black hole. The black hole is so engorged that some of the energy escapes as powerful blasts of radiation from the surrounding disk of accreting material. This light can appear as a jet-like feature. If the beam shines in Earth’s direction the “accretion disk” and jet surrounding the supermassive black hole can appear as a quasar that can outshine its surrounding galaxy a hundred or a thousand times.
R
Radio waves: A type of electromagnetic radiation with wavelengths from 1 millimeter to 100 kilometers, with corresponding frequencies from 300 GHz (gigahertz = 109 Hz), to as low as 3 kHz (kilohertz = 103 Hz). Like all other electromagnetic waves, they travel at the speed of light.
Red dwarfs (Red dwarf stars also known as M dwarfs): Stars that have the spectral class M. This is the coolest of the seven classes in the simplest scheme for classifying stars accordingly to decreasing temperature and the appearance of their spectra.
Rogue planet (also known as an interstellar planet, nomad planet or orphan planet): A planet, dwarf planet or larger moon that has been ejected from its system and is no longer gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly.
S
Siderophiles: Chemical elements such as iridium, gold, platinum, rhenium and osmium that tend to bond with metallic iron.
Small Solar System body (SSSB): an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet, so all comets and all minor planets other than those classified as dwarf planets, i.e.: the classical asteroids, with the exception of Ceres; the centaurs and trojans; the trans-Neptunian objects, with the exception of Pluto, Haumea, Makemake and Eris.
Speed of light (denoted by c): 299,792,458 metres per second (approximately 186,282 miles per second).
Stellar-Black Holes: Black holes with a mass of about 5 – 100 Suns formed at the end of very massive star’s evolutionary cycle.
Stratosphere: The second major layer of Earth’s atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down.
Submillimeter galaxy: A type of galaxy which has intense star formation activity and is covered by large amounts of dust which block visible light. This situation hampers detailed observation of the galaxy with optical telescopes.
Super-Earths: Planets with a mass between one and ten times that of the Earth. There are no such planets in our Solar System, but they appear to be very common around other stars. Discoveries of such planets in the habitable zones around their stars are very exciting because — if the planet were rocky and had water, like Earth — they could potentially be an abode of life.
Supermassive-Black holes: Black holes with a mass of a million or more Suns located in the centers of galaxies.
T
Trans-Neptunian object (TNO): Any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune.
Tropopause: The atmospheric boundary between the troposphere and the stratosphere.
Troposphere: The lowest portion of Earth’s atmosphere. It contains approximately 80% of the atmosphere’s mass and 99% of its water vapor and aerosols. The lowest part of the troposphere is the planetary boundary layer. The border between the troposphere and stratosphere is called the tropopause.
U
Ultraviolet (UV) light: Electromagnetic radiation with a wavelength in the range between 10 nanometres (nm) to 400 nm, with a frequency range from 7.5 x 1014 Hz to 3 x 1016 Hz.
V
Visible light: Electromagnetic radiation that is visible to the human eye. Visible light has wavelength in a range from about 380 nanometres (nm) to about 740 nm, with a frequency range of about 405 THz to 790 THz (terahertz = 1012 Hz).
Voids: The huge spaces between galaxy clusters or between filaments (the largest-scale structures in the Universe), which contain very few, or no, galaxies.
X
X-radiation (X-rays): A form of electromagnetic radiation with a wavelength in the range of 0.01 to 10 nanometres (nm), corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz)
Xenology: The scientific study of extraterrestrials
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