Glossary

This page provides definitions of scientific terms from our course. If you want more detail, we encourage you to also look up these topics on sites such as Wikipedia, NASA.gov, NIH.gov, and other fine websites.

Abiogenesis - The process by which life, specifically the earliest living cells, arose from non-living materials such as amino acids. Also known as "abiotic synthesis." Contrast with spontaneous generation, an older theory that has been disproven.
Absorption Spectrum - The colors of light absorbed by an object when it is illuminated. The exact colors depend on the composition of the object, and are identical to those in the Emission Spectrum.
Acidic - Having an excess of hydrogen ions. Acids are known for their sour taste and caustic nature, but most are very mild. Acids are often found in digestive processes, such as in the human stomach. Opposite of Alkaline. See also pH.
Alkaline - Having the ability to remove hydrogen ions from other chemicals. Alkaline substances (or "bases") are known for their bitter taste and caustic nature, but most are very mild. Bases are often found in cleaning compounds. Opposite of Acidic. See also pH.
Amino Acid - An organic molecule from which larger, more complicated proteins are created. A particular subset of amino acids are found in all living creatures on Earth, and they are considered one of the fundamental building blocks for life.
Angular Size - How large an object appears to be from a certain viewpoint. While an object's actual size (as measured by a ruler) does not change, a particular object might have an angular size of 70 degrees when viewed up close, or just a few degrees when viewed from far away.
Archaea - Single-celled microorganisms which lack membrane-bound organelles. They reproduce asexually. Many thrive in extreme environments, thanks in part to the structure of their cell membrane. One of the three domains of the Darwinian Tree of Life with eukaryotes and prokaryotes.
Asteroid - A minor planet orbiting the Sun, especially one in the inner Solar System (closer to the Sun than Jupiter). They are solid objects, stony or metallic in composition.
Astronomical Unit - The distance from Earth to the Sun: 150 million kilometers, or 93 million miles.
Atmosphere - The layers of gases surrounding a planet. Planets with stronger gravity have thicker atmospheres. Atmospheric pressure and temperature determine the gaseous or liquid state of the atmosphere.
ATP Molecule - Adenosine triphosphate is a highly efficient energy-storage molecule within a cell. It is used to transfer energy during cellular metabolism.
Bacteria - Single-celled microorganisms which lack membrane-bound organelles. Most have the shape of spheres, rods or spirals. Bacteria are prokaryotes.
Binary Star System - A star system that consists of two stars orbiting around their common center of mass. These systems are very important in astrophysics because calculations of their orbits allow the masses of the two stars to be directly determined, in which in turn allows the estimation of other stellar parameters, such as radius and density. More Info in Course
Bio-molecule - Any molecule that is present in a living organism; especially used to describe larger molecules that are produced by living organisms.
Brown Dwarf - Substellar objects which do not have enough mass to maintain hydrogen fusion. They have masses between those of the largest planets and the smallest stars.
Captured Asteroid - An asteroid which has been captured by a planet's gravity but neither collides with the surface nor moves back into space. Instead, it orbits the planet. Many of our our own solar system's outer planets have moons that are probably captured asteroids.
Captured Comet - A comet which has been drawn by a planet's gravitational pull into orbit. See also Captured Asteroid.
Catalyst - A substance which speeds up or controls the rate of a chemical reaction, but survives the reaction unchanged.
Cell Division - This process occurs as part of the cell cycle whereby a parent cell is divided into two or more daughter cells with identical genetic material.
Cell Membrane - A protective layer shielding the inside of the cell from the outside world, but allows nutrients to enter and waste byproducts to leave.
Cell Replication - The process of cell division that creates two cells from one parent. The process assures that each new cell produced inherits all of the genetic material essential to survival and reproduction.
Chemical Bond - An interaction between two or more atoms where electrons are shared and exchanged, resulting in the formation of a molecule.
Coma - The cloud of dust and gas that surrounds a comet.
Comet - Small bodies composed of rock, ice, frozen gases and dust. Comets often have very large and highly elliptical orbits. When they pass into our solar system, the sun heats the frozen gases and releases some materials, creating the visible coma and tail.
Companion Star - The smaller of two stars in a binary star system.
Core - The central part of an object. Terrestrial planets, ice giants, gas giants, and stars all have cores that are different from their outer layers. For instance, terrestrial planets often have molten metallic cores, while their surfaces are colder and most often solid.
Cosmic Abundance - a measurement of how common a particular element is, and/or how much of the element is present in a given environment. Abundance is typically measured by comparison to all other elements. More Info in Course
Cosmos - A complex and orderly system, such as our Universe; an alternate name for the universe.
Crust - The solid, low-density surface layer of a planet that has undergone differentiation. More Info in Course
Cytoplasm - Part of a cell which includes ribosomes, organelles (if present), and other inclusions, suspended in a combination of water, salt, and other organic molecules.
Density - A physical property of matter defined as mass per unit volume.
Direct Imaging - A method for detecting planets which involves taking an image of a star, and then filtering or blocking out the star from the light itself, enabling us to see its planets.
DNA - Deoxyribonucleic acid, a long-chain molecule that contains the genetic instructions used for all living organisms and viruses. DNA consists of two polymer strands formed into a double helix. Between the strands are nucleotides composed of either guanine (G), adenine (A), thymine (T), or cytosine (C).
Doppler Effect - When an object moves toward or away from an observer, that observer will detect waves from that object as being higher frequency (toward) or lower frequency (away). The faster the motion, the stronger the effect will be. This applies to sound, light, and any other waves. The effect also applies if the observer is moving.
Doppler Spectroscopy - A method for detecting extrasolar planets. Doppler Spectroscopy looks for changes in the spectrum of a star, which indicate that it is moving back and forth. The more the star is moving, the more massive the planet must be that is orbiting it. Also known as the Wobble Method.
Dust - Tiny solid flecks of material. In astronomy, we often discuss interplanetary dust (found within a star system) or interstellar dust (found between the stars in a galaxy).
Dwarf Planet - Planet-like objects which are in orbit of a star and assume hydrostatic equilibrium, but have not cleared their orbits of other objects.
Element - A pure chemical substance. Atoms of different elements can be identified by the number of protons in their atomic nucleus. Molecules are made up of elements; elements are made up of protons, neutrons, and electrons.
Emission Spectroscopy - The measurement of light as it is emitted from a hot object. In the visible range, this means light coming from stars, but planets emit enough infrared light for this technique to be useful in that spectrum.
Emission Spectrum - The colors of light emitted by an object when it is heated or subjected to a very strong electrical current. The exact colors depend on the composition of the object, and are identical to those absorbed in the Absorption Spectrum.
Endosymbiotic Theory - An evolutionary theory which seeks to explain the origin of eukaryotic cells from prokaryotes. According to this theory, several key organelles of eukaryotes originated as symbiosis between separate single-celled organisms. Also known as symbiogenesis.
Escape Velocity - The speed at which an object must move in order to escape from a planet forever, without any rockets or other propulsion at work.
Eukaryotes - A eukaryotic organism is made of complex cells that contain a nucleus (stores the DNA), cytoskeleton, and organelles (membrane bound structures that do work within the cell). One of the three domains of the Darwinian Tree of Life with archaea and prokaryotes.
Evolution - The gradual change in populations of living organisms. Evolution is responsible for transforming life on Earth from its primitive origins to the great diversity of life seen today.
Exoplanet - Short for "extrasolar planet," a planet that does not orbit the Sun and instead orbits a different star, stellar remnant, or brown dwarf. More than 1,800 exoplanets have been discovered. There are also rouge planets, which do not orbit any star and which tend to be considered separately, especially if they are gas giants. More Info in Course
Extrasolar Planet - see Exoplanet
Extremophile - A living organisms that is adapted to conditions that are"extreme" by human standards, such as very high or low temperature or a high level of salinity or radiation.
Frequency - The more often a wave oscillates, the higher its frequency is. Measured in Hertz. A wave that has an oscillation of, for example, 47 times per second, would have a frequency of 47 Hertz (Hz). Frequency is inversely related to wavelength, so higher frequencies mean shorter wavelengths. Frequency in light is directly related to energy: higher frequency means higher energy.
Galaxy - A large collection of stars that is gravitationally bound together, and is distant from other such collections. Galaxies are typically home to billions of stars, though they vary widely. Our home galaxy is called the Milky Way.
Gamma Ray - A very-high-energy, very-high-frequency form of light. Gamma rays are very damaging to living creatures.
Gas Giant - A giant planet composed mainly of hydrogen and helium. In our solar system Jupiter and Saturn are considered gas giants. The term gas giant was initially synonymous with the term "giant planet," but in the 1990s, Uranus and Neptune were reclassified as ice giants due to their composition of heavier volatile substances.
General Relativity - Our best current theory of gravity and acceleration, which describes everything from gravity on Earth and planets in orbit to the effects of black holes. General Relativity is Einstein's generalization of his Special Theory of Relativity, best-known for its description of time dilation at near-light-speed.
Genetic Material - The information encoded in the DNA or RNA, found in either the nucleus or cytoplasm, which determines the inherited characteristics of an organism.
Gradient - A change in a quantity from one point to another. Some examples: A hillside has a height gradient. The end of a freshwater river as it runs into the salty ocean has a salt gradient. There is a gravity gradient that exists between the surface of our planet and deep space. This is a more general term than the mathematical "gradient" operation.
Gravity - A force exerted by all objects that have mass on all other objects that have mass. The larger the masses involved, the greater the force. Gravity obeys the Inverse Square Law and is described by General Relativity.
Gravitational Lensing - The magnification or distortion (into arcs, rings, or multiple images) of an image caused by light bending through a gravitational field. Gravitational lensing was predicted by Einstein's general theory of relativity, and was one of the early pieces of evidence for the accuracy of that theory.
Habitable Zone - The region around a star in which planets could potentially have surface temperatures at which liquid water could exist.
Heteroatoms - Any atom in a hydrocarbon that is not carbon or hydrogen; also used to indicated that non-carbon atoms have replaced carbon in the backbone of the molecular structure.
Hydrocarbon - An organic compound made up of hydrogen and carbon, possibly with other elements as well.
Hydrogen Bond - A bond formed between a hydrogen atom with an electronegative atom, such as fluorine, oxygen or nitrogen. Hydrogen bonds are weaker than most other chemical bonds.
Hydrogen Fusion - The process by which two hydrogen nuclei fuse together to make a single, more massive nucleus, and release energy in the process. This process is also known as atomic fusion or nuclear fusion; hydrogen fusion is only the most common type.
Hydrophilic - Molecules that are attracted to and tend to be dissolved by water are called hydrophilic.
Hydrophobic - Molecules that are not attracted to a mass of water and tend not to interact with water are called hydrophobic.
Hydrostatic Equilibrium - A balanced state in which forces pulling in one direction are countered by forces pushing in the other direction. This equilibrium is what makes stars and planets approximately round.
Ice Giant - A giant planet whose composition includes substances heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, sulfur, methane, and ammonia. In our solar system there are two ice giants, Uranus and Neptune. Those two have become known as ice giants because their constituent compounds were ices when they were incorporated into the planets during their formation, either directly in the form of ices or trapped in water ice. See gas giants.
Image Processing - The analysis and alteration of images by computer. Direct Imaging techniques use image processing to remove the light of a star while leaving the light from its planets. More Info in Course
Infrared Light - Light with a wavelength that falls in the portion of the electromagnetic spectrum between radio waves and visible light. Infrared light is fairly low-energy and low-frequency. It is best-known for use in "heat-vision" or "thermal imaging" cameras that see the infrared light emitted by warm objects.
Interstellar Medium - The gas and dust that fills the space between stars in a galaxy
Inverse Square Law - When a force obeys the Inverse Square Law, its strength drops off by the square of the distance involved. For instance, if two objects were moved from a certain separation distance to 5 times that distance, the force between them would decrease by a factor of 25.
Ion - An atom with extra or missing electrons, giving it a negative or positive electrical charge.
Isotope - A form of an element that has the same number of protons, but different numbers of neutrons.
Kuiper Belt - The comet-rich region of our solar system that spans the distance of about 30-100 astronomical units from the Sun. Kuiper belt comets have orbits that lie fairly close to the plane of planetary orbits and travel around the Sun in the same direction as the planets.
Light - A form of energy; specifically, light is electromagnetic radiation. Light includes a wide variety of phenomena, including radio waves, microwave radiation, infrared light, visible light, ultraviolet light, x-rays and gamma rays.
Light Curve - A graph that shows the intensity of light coming from an object versus time. Light curves are used in the Transit Method for detecting exoplanets. More Info in Course
Light Year - A distance; specifically, the distance that light travels in one year.
Lipid - Molecules that contain hydrocarbons, which make up the building blocks of the structure and function of living cells. Examples of lipids include fats, oils, waxes, certain vitamins, hormones, and most of the non-protein membrane of cells.
Mantle - The semi-molten layer of rock in a terrestrial planet, between the solid crust and the molten core.
Mass - A measure of the amount of matter in an object. More massive objects require more force to accelerate them.
Metabolism - All of chemical reactions that occur within a living system. The term "metabolize" is typically used to mean "chemically break down food so that its energy and nutrients can be used by an organism."
Microbe - A single-cell organism, so tiny that millions can fit into the eye of a needle. Microbes are the oldest form of life on Earth; their fossils date back more than 3.5 billion years. More info at Microbeworld
Microlensing - One of the methods of discovering exoplanets. Microlensing is a method which uses a special property whereby light from a distant object is bent by gravity as it travels; our telescopes therefore see the distant object at a different location from where it actually is. It may also appear brighter or dimmer, depending on whether we receive more or less of that light than normal, and often appears distorted. More Info in Course
Micrometeorite - An extraterrestrial particle that is collected on the Earth's surface. It used to be a part of micrometeoroid, which has survived entry through the Earth's atmosphere.
Microwave Radiation - Light with wavelengths in the range of micrometers to millimeters. Microwaves are generally considered to be a subset of the radio wave portion of the electromagnetic spectrum.
Minor Planet - An astronomical object that orbits around the Sun but is not considered a planet nor a comet. Some examples of a minor planet include dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, and other trans-Neptunian objects.
Molecule - A collection of any number of atoms that are chemically bonded to one another.
Momentum - The product of an object's mass and velocity.
Monomer - A single molecule that has the ability to chemically connect or bind to other molecules, forming a polymer.
Moon - A natural object that orbits a planet.
Noble Gas - The group of elements on the far right-hand side of the periodic table. These elements all have similar chemical properties - they are the most stable elements in the periodic table, and therefore the least reactive.
Nucleic Acid - Very large molecules that are essential to life. The backbone of a nucleic acids is made of alternating sugar and phosphate molecules bonded together in a long chain. Nucleic acids store information which details the specific structure of proteins that determine daily functions of our bodily systems. See also DNA and RNA.
Nucleus - The core of an atom, containing positively charged protons and neutral neutrons. The same word is also used for the central part of a living cell, which contains the cell's DNA.
Observable Universe - The portion of the entire universe that, at least in principle, can be seen from Earth at present time. Light and other signals from these objects have had time to reach the Earth since the beginning of the cosmological expansion.
Orbit - The path followed by a celestial body because of gravity. An orbit may be bound (elliptical), in which case the celestial body moves around a central point over and over, or unbound (parabolic or hyperbolic), in which case it is deflected in its path but never returns.
Organelle - A specialized subunit within a cell that has a specific function. Eukaryotic cells (like human cells or plant cells) have many organelles suspended in their cytoplasm; prokaryotic cells (like bacteria) have none.
Organic Molecule - Molecular structures that have carbon atoms; also known as organic compounds. Their importance is pronounced when it comes to forming life: each carbon atom can form strong, stable bonds with four other atoms at a time. The other elements commonly found in organic molecules are oxygen, hydrogen, nitrogen, sulfur and phosphorus. Carbon can also bond to other carbon atoms to form chains that are almost unlimited in length.
Parallax - The apparent shifting of an object against the background that is caused by viewing it from different positions.
pH - A measure of the acidity or basicity of an aqueous solution.
Photometrics - The science of the measurement of light, often phrased in terms of its perceived brightness to the human eye. Used in measuring transits of a planet against its star.
Pixel - An individual "picture element" on a screen, camera, or other device.
Planet - A moderately large object that orbits a star and shines primarily by reflecting light from its star. More precisely, according to a definition approved in 2006, a planet is an object that 1) orbits a star (but is itself neither a star nor a moon), 2) is massive enough for its own gravity to give it a nearly round shape, and 3) has cleared the neighborhood around it orbit. Objects that meet the first two criteria but not the third, including Ceres, Pluto, and Eris, are designated dwarf planets.
Planetoid - See minor planet
Plutoid - A body that orbits beyond Neptune that is large enough to be round in shape. Also known as an"ice dwarf" or "Trans-Neptunian dwarf planet."
Polar Molecule - A molecule in which chemical bonding creates an uneven distribution of electrons, resulting in an electrical field in that area. Many molecules are polar; water is an example of a polar molecule.
Polymer - Multiple molecules all strung together to form really long chains, such as DNA or proteins, which are fundamental to biological structure and function. The individual unit of a polymer is a monomer.
Primary Transit - When a planet passes in front of its star as seen from Earth. See also transit, secondary transit.
Prokaryotes - A single-celled organism that lacks a membrane-bound nucleus, mitochondria, or other membrane-bound organelles. One of the three domains of the Darwinian Tree of Life with eukaryotes and archaea.
Proteins - Large and complex molecules that play many critical roles in living organisms. Proteins do most of the work in cells and are required for the structure, function, and regulation of the body's tissues and organs. They are made up of hundreds or thousands of smaller units called amino acids.
Quasar - The nucleus of an active galaxy, which produces exceptional amounts of energy. Short for"quasi-stellar object",
Radial Velocity Method - Another name for the Wobble Method and Doppler Spectroscopy.
Radiation - A catch-all term for any fast-moving particles that are emitted from an object. Most radiation is in the form of light (electromagnetic radiation) or subatomic particles (nuclear radiation). Some radiation is damaging to living cells, some is not.
Radio Wave - Light with very low energy and low frequency (and hence long wavelength). Radio waves pass through many materials.
Radius - The distance from the center of a circle (or sphere) to its edge.
Red Dwarf - A small and relatively cool star, with a mass range between .075 to .5 solar masses, and a surface temperature of less than 4,000 K.
Ribosome - A large and complex molecular machine, found within all living cells, that serves as the primary site of biological protein synthesis (called translation). The ribosome links amino acids together in the order specified by Messenger RNA molecules.
Ribozyme - Ribonucleic acid enzyme (or RNA enzyme) catalyzes chemical reactions. They play a role in reactions such as RNA splicing, transfer RNA biosynthesis, and viral replication.
RNA - Ribonucleic acid, one of the three major biological macromolecules that are essential for all known forms of life (the others are DNA and proteins). Genetic information in a cell is passed on from DNA through RNA to proteins. RNA is like the DNA's"photocopy" of the cell - RNA carries the viral genetic information and plays an important role in regulating cellular processes from cell division, differentiation and growth to cell aging and death. More Info at the RNA Society site
RNA World Hypothesis - A theory which posits that RNA was the first major component of life that appeared on Earth, providing genetic material and acting as a catalyst for other reactions.
Secondary Transit - When a planet completely disappears behind its star as seen from Earth. See also transit, primary transit.
Small Solar System Bodies (SSSB) - An object in the Solar System that is neither a planet, nor a dwarf planet, nor a satellite. Examples of SSSB include comets, minor planes, asteroids, trojans, centaurs, and trans-Neptunian objects.
Solar Eclipse - The most dramatic example of a transit, when the moon passes between the Sun and Earth. The sunlight we receive dims dramatically because the moon blocks our view of the Sun.
Solar System - A star (sometimes more than one star) and all the objects that orbit it. Also known as star system.
Solar Wind - A stream of plasma released from the upper atmosphere of the Sun; it consists of electrons and protons. The stream of particles varies in density, temperature and speed.
Solvent - A substance that dissolves a chemically different substance. In a salt water solution, for example, water is the solvent.
Space Probe - An unmanned spacecraft that travels through space to collect science information. Probes send data back to Earth for scientists to study. More Info at NASA.gov
Spectrometry - A technique for measuring the distribution of light across the whole electromagnetic spectrum. Spectrometry usually focuses on a smaller range, such as the range from ultraviolet to infrared.
Speed of Light - 300,000,000 meters per second, which is 186,282 miles per second. This is a universal constant, a value that is the same everywhere across our universe.
Star - A collection of hydrogen (with traces of other elements) that is so large that is has collapsed under its own weight, causing hydrogen fusion in the core and making the whole object glow.
Stellar parallax - The apparent shift in the position of a nearby star (relative to distant objects) that occurs as we view the star from different positions in Earth's orbit of the Sun each year. A specialized case of parallax.
Sugar - A small hydrocarbon molecule that living cells use to produce or store energy.
Sun - When we say "sun" in this course, we typically mean our sun, the star closest to Earth. However, one could also look at the star closest to another planet and call that star the planet's "sun."
Super-Earth - A terrestrial planet that is larger than Earth, typically 2-10 times more massive.
Terrestrial Planet - Rocky planets similar in overall composition to Earth. Terrestrial planets typically have a crust and mantle with large amounts of silicon and oxygen, and a molten iron core.
Thermodynamic Equilibrium - A state where all the components of a system have the same temperature and there are no flows of material.
Transit - When one astronomical object moves across another from our point of view, that is called a transit. Solar eclipses are good examples of transits.
Transmission Spectroscopy - The measurement of light after it passes through a gas, such as an interstellar cloud or the atmosphere of a planet.
Transit Method - A method for detecting extrasolar planets that looks for brief decreases in the light given off by the planet's home star. These changes may be indicative of a planet moving across the face of the star in a transit.
Triple Point - The temperature and pressure at which the solid, liquid, and vapor phases of a pure substance can coexist in equilibrium.
Ultraviolet Light - A form of light with relatively high energy and frequency. Ultraviolet light can cause surface damage to living creatures and to individual cells.
Universe - All of the objects and space that are connected in some way to our own world. Another term for the cosmos.
Velocity - The combination of speed and direction of motion. Velocity can be stated as a speed in particular direction, such as 100 kilometers per hour due north or 40 meters per second upward.
Virus - A small infectious agent that replicates only inside the living cells of other organisms.
Visible Light - The portion of the electromagnetic spectrum that is visible to human beings. This light has moderate energies and frequencies, and is typically not damaging to living beings, though there are exceptions.
Wavelength - The length of one full oscillation of a wave, from peak to peak. The larger a wave's wavelength is, the lower its frequency and energy.
Wobble Method - A method for detecting extrasolar planets. The Wobble Method, also known as Doppler Spectroscopy, looks for changes in the spectrum of a star which indicate that it is moving back and forth. The more the star is moving, the larger the planet must be that is orbiting that star.
X-ray - A high-energy, high-frequency form of light. X-rays can travel through some materials but not others. Living beings who are exposed to x-rays for long periods of time have an increased risk of DNA damage.

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