Here you will find the comprehensive list of vocabulary used in Epo’s Chronicles and their definitions.

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


Accretion Disk – A disk of matter that forms when material is transferred to a gravitationally massive body, such a black hole. For black holes, the disks form outside the event horizons. For other objects, such as neutron stars or white dwarfs, the disks can extend down to the stellar surfaces. Friction and other forces heat the disks, which then emit a a wide range of different types of electromagnetic radiation including x-rays.

(protoplanetary) Accretion Disk – Accretion disks are not just found around black holes. When dust and gas orbit an object in a flattened disk-like shape, collisions within the disk cause most of the material to fall inward to create a central object. The remaining material moves outward, carrying with it all the angular momentum. In a protoplanetary accretion disk, a star will form at the center, while in the disk itself material can accumulate to form planets. Protoplanetary disks are a special type of accretion disk.

Active galaxy – A class of galaxy that emits enormous amounts of energies from its center.

Active galactic nucleus (AGN) – The central, compact region of of an active galaxy, which is the source of most of the host galaxy’s emitted energy. The plural of nucleus is nuclei.

Afterglow – In gamma-ray bursts, afterglows are the emissions seen after a GRB, which can be seen in X-rays, optical, and radio. Afterglows can last for days or even weeks.

Albedo – is the efficiency with which a particular surface reflects light. It is calculated by dividing the amount of light reflected by the surface by the amount of light that shines on it.

All Sky Survey – A map created by taking images of every region of the sky, generally at a particular set of wavelengths, in order to detect and catalog the objects found.

Altimeter – an instrument to measure altitude.

Altitude – the height of an object above a reference plane, usually the surface of a planet. On Earth, the sea level is used as the reference plane.

Annihilate – The process where a particle and its anti-particle collide, destroying both particles and leaving behind light energy (i.e. photons) in accordance with Einstein’s famous E = mc2 equation.

Apollo – NASA’s Apollo program included the first manned mission to the Moon and ran from 1963 to 1972.

Argument – is the variable that is being passed to a function.

Artificial intelligence (AI) – is a system of hardware and software that mimics human intelligence.

Asteroid – is a rocky object in space that can be a few feet to several hundred miles wide.

Asterism – is a pattern of stars as seen on Earth’s night sky as opposed to a constellation, which is a defined area of the sky as seen from Earth.

Astrobiology – is the science that tries to answer questions like: how do living organism originate? Is there life out in space, and what form does it have? What is the future for life on Earth and beyond?

Atmospheric science – The study of the atmosphere, its processes and the interactions between the atmosphere and other systems, especially the oceans. Atmospheric Science includes fields like meteorology (the study of weather systems) and climatology (the study of climates and climate change).

Axion – A hypothetical subatomic particle, introduced in the Peccei-Quinn Theory, that is also a candidate for dark matter.


Baryogenesis – In the beginning of the universe, particles and anti-particles were created in equal numbers. Since particles and their anti-particles annihilate each other, some mechanism must have caused more particles than anti-particles to remain in the Universe. The creation of more ordinary matter than anti-matter is known as baryogenesis.

Baryon – Baryons are particles made of three quarks. While three quarks can be combined to make many different types of particles, the most common baryons occurring naturally in the Universe are protons and neutrons.

Baryonic matter – matter that is built out of groupings of three quarks, called baryons. The most common and stable baryons are the proton and neutron.

Big Bang – This famous theory suggests that the universe was not always as it is today, and that at one point everything was compressed down into an extremely small point of unimaginable heat and density. Then, in the most violent and energetic event ever to occur, the universe began to expand and evolve into its present state. This rapid expansion was aptly named the “big bang.”

The term, big bang, was coined by an English astronomer, Sir Fred Hoyle, to mock the theory. He was a proponent of the steady state theory, which suggests that the universe looks, on the whole, the same at all times and places. In order to explain Hubble’s observations, this theory was modified to include the spontaneous creation of matter, which would keep the density of the universe at a constant value. The steady state theory had too many holes in it to be viable and was eventually dropped. However, the moniker, big bang, to describe the hot, dense beginning of the universe stuck and is now a part of astronomical vocabulary.

Binary – The word binary simply means there are two of something. When applied to a star system, it means that instead of having a single star, two stars orbit their common center of gravity.

Binary System – This is a system of two stars, very close to each other, that orbit around their common center of mass. They are actually quite common.

Binding energy – the energy it takes to break apart the nucleus of an atom into its constituent parts. Since energy is conserved, this amount of energy is also released when those parts fuse together to form a heavy nucleus. The graph below is called the curve of binding energy. It is a plot of the binding energy of all atomic nuclei (vertical axis) vs. their mass (horizontal axis). Notice that the curve rises steeply to a peak value, then drops slowly as mass increases. The peak of the curve is at Fe56 (Iron with mass 56). Stars produce energy by combining light elements, to the left of the iron peak, into heavier elements. This process is called nuclear fusion. The difference in binding energy between the initial and final nuclei is released as heat. Fusion beyond the iron peak does not release energy. Instead it requires that energy be input. That is why stars are not able to continue stable nuclear fusion of elements heavier than iron. Those elements are produced by different processes, typically accompanying supernova explosions.

Binding Energy Chart

Black Hole – A region of space within which the force of gravity (space-time curvature) is so strong that nothing, not even light, can escape from it.

Blazar – is an active galaxy with jets, one of which is pointed directly at the observer. Blazars are the most common source of high-energy gamma-ray emission.

Brightness – The amount of light an observer sees from an object like a star. Brightness is measured in watts per distance squared (W/d2).

Brown dwarf – are cosmic objects that are too small to be stars and too large to be planets. They have the same composition as stars but because of their low mass are unable to sustain nuclear fusion (if they ever manage to get fusion started) at their cores.


Catalogue – Since there are so many astronomical objects in the sky, the scientists who discover them organize them into catalogues. In many cases, the only name an astronomical object has is its catalogue number. Some objects appear in more than one catalogue and have more than one name.

Cen A – Centaurus A, also known as NGC 5128, is a lenticular galaxy in the constellation Centaurus. The “A” in Cen A means it is the brightest radio source within that constellation. Cen A is a radio galaxy, and at only 11 million light years from Earth it is the closest active galaxy to us.

Chandrasekhar mass limit – The upper mass limit of a white dwarf, approximately 1.4 solar masses. If a white dwarf exceeds this limit it will explode in a Type 1A supernova and may collapse to a neutron star.

Chronometer – A watch that has been specifically designed to keep very accurate time.

Climate Change – is the change in a planet’s weather patterns over long periods of time ranging from decades to hundreds of thousands of years.

Collimated – Means that all the energy moves in a confined column of light, with the rays of energy traveling nearly parallel to one another. This means they behave less like the spherically dispersed light from a light bulb, which fades quickly as you get further away, and more like the directed light generated by a laser.

Coma Cluster – A galaxy cluster located in the Coma Berenices constellation. The cluster contains over 1,000 galaxies and is 320 million light years from Earth.

Comets – are dusty bodies of ice that orbit a star. We typically imagine comets with their characteristic tails; but the tails only form when their orbits bring them close to a star. Comets have three distinct tails: one caused by dust pushed out by radiation pressure from the star, the ion tail caused by solar winds melting the frozen ice and gas and pushing it back, and a tail of sodium escaping from the dust. The sodium tail is not visible to the naked eye. These tails point in slightly different directions but always away from the star.

Compile – translate source code written by a computer programmer into code that computer hardware can understand.

Compiler – a software program that translates source code into machine code.

Conditional statement – Simply put, a conditional statement checks to see if a particular condition is true or false. It then executes a certain block of computer code if the condition is true, and a different block if it is false.

Conductive plasma – Plasma is a gas in which the electrons have been stripped off the atoms. The resulting positively charged particles are called ions, and they are just atoms that have lost some electrons. Plasma also contains negatively charged particles, which are the electrons that have been stripped away. The process of stripping electrons from atoms is called ionization. Ionization happens very easily in stars because they are so hot. The temperature variations within stars, along with their rotation, causes the plasma to move. Since the plasma contains charged particles, under the right conditions this motion can create an electric current. The current, in turn, gives the star a magnetic field.

Cosmic Microwave Background (CMB) – This is the radiation left over from the big bang. It was produced early in the age of the universe, when the average density and temperature were much higher than today. The expansion of the universe has cooled the radiation to its current temperature of about 2.7 kelvin (The SI unit of temperature is the kelvin, symbol K. The name is to honor the 19th Century Scottish physicist William Thomson, who is more commonly known as Lord Kelvin).

Countdown – A series of time-sensitive procedures that are carried out leading up to the launch of a mission. While the last few seconds of the countdown are the most iconic, countdowns for spaceflight missions often range between 72 and 96 hours. T minus indicates the time remaining until launch, after launch, the clock starts counting up and mission time is measured as T plus.

Crab Nebula – A supernova remnant (expanding cloud left after a supernova explosion) in the constellation Taurus. The supernova itself was recorded by Chinese and Arab astronomers in 1054 A.D.


Data type – lets the computer know what type of data to expect, whether they are numbers, or sets of numbers or if they are words.

Dark energy – a hypothesized exotic form of energy that makes up the majority of the universe. This form of energy is causing the expansion of the universe to accelerate.

Dark matter – a hypothesized, exotic form of matter that does not emit nor absorb electromagnetic energy (light) and is, currently, only detectable by the gravitational influence it has on surrounding matter and energy.

Dark Side – The part of the Moon the sun is NOT currently shining on.

Data – is the factual information a scientist collects that is related to the hypothesis he or she is testing. Data can be direct measurements of properties, for example, the height of plants one month after seeds have been planted. Data can also be observations of patterns, for example, the behavior of animals when they encounter a predator. In order to determine whether a particular measurement or observation is a rule rather than an exception, a scientist will often repeat measurements or observations to gather additional data.

Density – is a measure of how much matter is packed into a certain amount of space. Denser objects have more material packed into a given space than lower density objects.

Detector – a device, or devices, used to detect photons, or in some cases other particles like protons, electrons, etc.

Deuterium – is an isotope, or a variation, of the hydrogen atom with a neutron in its nucleus. Most hydrogen atoms (~99%) have only a proton in its nucleus. Deuterium, which is about 1% of naturally occurring hydrogen, has a proton and an neutron.

Diffuse X-ray Emission – A source of x-rays that is not a point source, but rather generating x-rays from a large region or area.

Direct Evidence – Uses measurable phenomena (data) to support a hypothesis. If an archeologist, say, were to discover a clay jar from an ancient civilization that would be direct evidence that the civilization knew how to make ceramics.

Doppler Effect – The Doppler Effect is the apparent change in the wavelength and frequency of sound or light depending on whether the source is moving towards or away from you. The faster you or the source is moving, the more profound the impact. You may have experienced an example of this with sound waves if you have ever heard a higher pitch sound from a car moving towards you, and a lower pitch noise when it is moving away.

Example of Doppler Effect.

Dwarf Galaxy – A galaxy that contains somewhere around several billion stars, as opposed to the hundreds of billions of stars found in a galaxy like our own.

Dwarf (star) – Is a low-mass main sequence star that is not quite as luminous or hot as more massive stars. Because of their low masses these types of stars can steadily burn (fuse) hydrogen for several billions of years. Our own Sun is a dwarf star.

Dyson Sphere – A hypothetical monumental construction where a hollow sphere is constructed around a star to harness all of its radiant energy.


Edwin Hubble – An American astronomer, Hubble (November 20, 1889 – September 28, 1953) profoundly changed our understanding of the nature of the universe by demonstrating the existence of other galaxies beyond the Milky Way. The Hubble Space Telescope is named after Edwin Hubble.

Einstein Cross – Also known as G2237+0305, is an example of gravitational lensing, where a foreground galaxy has produced four images of the same quasar that lies directly behind the galaxy.

Einstein’s Field Equation – describes gravity as the curvature of space-time dependent on the mass that exists in it.

Electromagnetic Force – The electromagnetic (or EM) force is one of the four known universal forces, along with gravity and the strong and weak nuclear forces. The EM force holds all the molecules and cells in your body together and is the result of interactions between charged particles (protons and electrons) within the atoms and molecules.

Electromagnetic (EM) Spectrum – This is the continuum of waves of light, which range from very low frequency and low energy radio waves to very high frequency and high energy gamma rays. The kind of light we are familiar with is visible light, which is a tiny sliver of the EM spectrum.

Ellipse – is a closed shape with two focal points such that the sum of the distances from the edge to the points is constant.

f1 and f2 are the two focal points, or foci of this ellipse. The sum of line segments a and b is equal to the sum of line segments c and d.

Elliptical Galaxies – These galaxies range in shape from nearly spherical to flattened disks. They are characterized by a much older population of stars. They also have very low rates of star formation, meaning very few stars are being born here. Ellipticals contain little or no cool gas or dust.

Equation – An equation is a mathematical way of saying that two expressions are equal. Newton’s Second Law of Motion, F = ma, is an equation.

Equatorial radius – the radius of the Sun measured from its center to its equator. This is not the same distance as measuring from the center to the pole. Since the Sun is spinning, it is slightly flattened, causing the equator to bulge slightly outward and the poles to move slightly inward. The Sun’s shape is sort of like the shape you get when you hold a spherical balloon on opposite sides and then squeeze it slightly inward.

Evolutionary computing – a form of computer programming that mimics the adaptive abilities of species on a genetic level. The technique is used in problem solving and data correlation.


Far Side – The side of the Moon that always faces away from the earth. Features on this side are named mainly after Russian scientists since they were first explored by Russian probes.

(Computer) Function – small sections of computer instructions that perform a particular task. Source code is broken up into functions so that programs can easily use the same code over and over without having to enter that code into the program many times.

Fuse – Fuse is a verb which means to merge two things into one. In the process called nuclear fusion, light atoms, for example hydrogen, are forced together to form slightly heavier atoms, like helium. Helium in turn can be fused into the heavier atom carbon, and so on. This process releases large amounts of energy and is what actually powers stars. However, fusion requires very high temperatures. That is why a tube of hydrogen or helium gas in a laboratory will not spontaneously undergo nuclear fusion.


G2 star – Our Sun is a type G main sequence star, but more specifically, it is a type G2V. The “G” means that it’s a yellow star. The “2″ means that it is about 2/10ths of the way between a yellow “G” and an orange “K.” These arbitrary letter designations are historical in origin and are based on atomic absorption lines in the star’s spectrum. The “V” is a Roman numeral 5. It means the star is a main sequence star, often known as a dwarf star.

G2V – Our Sun is a type G main sequence star, but more specifically, it is a type G2V. The “G” means that it’s a yellow star. The “2″ means that it is about 2/10ths of the way between a yellow “G” and an orange “K.” These arbitrary letter designations are historical in origin and are based on atomic absorption lines in the star’s spectrum. The “V” is a Roman numeral 5. It means the star is a main sequence star, often known as a dwarf star.

Galaxy Cluster – Thousands of nearby galaxies that are gravitationally bound together into a large group. Clusters contain at least as much mass in hot x-ray emitting gas as they do in galaxies. However, they are dominated by dark matter, which comprises 80% to 90% of their total mass.

Gamma ray – The very highest energy end of the electromagnetic spectrum, with the shortest wavelengths. Gamma-rays typically have wavelengths a few hundred times shorter than low-energy x-rays, and are usually shorter than a few hundred picometers (pm, 10-12m).

Gamma-ray burst (GRB) – a brief, but very energetic burst of the most powerful form of electromagnetic radiation.

General Theory of Relativity – Commonly called General Relativity, this theory postulates that mass and energy curve and distort spacetime. The severity of the curvature and distortion caused by the mass/energy is the strength of the gravitational attraction. General Relativity expands upon Einstein’s earlier Special Theory of Relativity.

Generation ship – is a theoretical space craft that could be constructed with the intent of sustaining several generations of people through the duration of its voyage.

GLAST – The Gamma-ray Large Area Space Telescope (GLAST) is an Earth-orbiting satellite launched on June 11, 2008 to explore the gamma- ray sky. Its called Gamma-Ray because it measures the highest energy light, known as gamma rays; Large Area because the telescope can see almost one-fifth of the entire sky at once; and space telescope because, well, it’s in space! Update: GLAST was renamed to Fermi Gamma-ray Space Telescope 60 days after it launch.

Gluon – The subatomic particle which carries the strong nuclear force. “Gluon” is a fitting name for the particle that glues atomic nuclei together, overcoming the repulsion from the positively charged protons in nuclei.

GPS – The Global Positioning System consists of a network of Earth-orbiting satellites that are used to triangulate the position of a receiver (generally a radio or microwave receiver). The receiver uses signals emitted by the satellites to compute its position on Earth, typically to accuracies of meters or even less.

Gravitational lensing – The gravitational bending of light due to the gravity of a foreground object.

Gravitational Wave – Objects that undergo a change of mass, and/or have asymmetrical rotations are hypothesized to affect spacetime in such a way as to generate ripples in spacetime. The ripples, or gravitational waves, are produced when the objects change their physical configuration, i.e. their size, shape or extent.

Gravity – is the universal force of attraction between all matter.

Great Red Spot – is a giant hurricane that has existed on Jupiter for at least 300 years. It is not known whether it will ever disappear.

Greenhouse Effect – The process by which a planet’s atmosphere prevents radiant energy from its surface from escaping into space, thereby increasing the surface temperature of the planet. The effect is caused by the trapping of infrared radiation by gases such as carbon dioxide, methane and water vapor in the planet’s atmosphere. On Earth, sunlight warms the planet surface, and the surface then re-emits the solar energy in the form of infrared light. Greenhouse gases prevent a portion of the infrared from escaping to space.


Habitable – a place that is suitable for life to exist.

Habitable zone – The region of space in a solar system that scientists believe would be suitable for life.

Hack – In computer science, hacking means to take advantage of a weakness in a computer system to bypass its normal operations. Sometimes hacking can be beneficial by making the system do more than it was designed to or it can be harmful if it stops the system from doing what it was meant to do.

Hadron – Baryons and mesons together are categorized as hadrons. These are particles made of quarks.

Helium-3 – He-3, which is used in nuclear fusion research, is rare on Earth. He-3 has 2 protons and 1 neutron. Some scientists have speculated that it could be used as a source of fuel for future spaceships.

Helium-4 – The most common form of helium, making up 9,999 of every 10,000 helium atoms; it has two protons and two neutrons in its nucleus. A bare He-4 nucleus (stripped of its two electrons) is sometimes called an alpha-particle.

Higgs Boson – According to the Standard Model of Particle Physics (the most complete and accurate model we have of elementary particles), the Higgs boson is responsible for giving the other particles mass. The Higgs is the only particle predicted by the Standard Model that has not yet been detected.

Homogeneous – Looks the same from every location.

Hydroponics – Growing plants with their roots suspended in a nutrient rich solution rather than soil.

Hydrostatic equilibrium – Hydrostatic equilibrium in stars is the state in which the inward force of gravity is balanced by the outward force of the pressure inside the star, thus making the star stable against collapse or expansion. The pressure is due to the hot gas and the radiation, and each contributes different amounts to the total pressure depending on both the type of star and on what region of the star is being considered.

Hypergiant – These are the biggest stars in the Universe, more than 100 times the mass of our Sun and with up to millions of times its brightness. These stars are so bright that they burn through their fuel quickly, causing them to have short lifespans of only a million years or so. In comparison, our Sun’s lifespan is about 10 billion years.

Hypernova – The explosion of an extremely massive star at the end of its lifetime. The explosion is triggered when the star exhausts the nuclear fuel in its core and can no longer support itself against gravity. Hyperrnovae can liberate hundreds of times the energy of a supernova.

Hypernova remnant – The expanding cloud of material ejected by the star during a hypernova explosion.

Hypothesis – A hypothesis is a testable prediction about how something works based on the observations you have initially made.


Ice core – is a cylindrical section of ice retrieved from snow packs or glaciers that have formed over several hundreds or thousands of years.

Indirect Evidence – The use of logical and rational outcomes from other direct evidence in order to conclude that there is support for a hypothesis. Our archeologist might use that same clay jar as indirect evidence that the civilization used jars to store food.

Infrared – This band of the electromagnetic spectrum has wavelengths in the micron (μm, 10-6m) range. They are correspondingly more energetic than microwaves and radio.

Initial (default) value – The starting value for a variable. Without a starting value, a computer program may be behave in unexpected ways when it comes time to use a variable that has no default value.

Inner solar system – Is the region of space that is smaller than the radius of Jupiter’s orbit around the sun. It contains the asteroid belt as well as the terrestrial planets, Mercury, Venus, Earth and Mars.

Intermediate, Black Hole – Black hole with mass on the order of tens or hundreds of solar masses that are typically observed near, but not at, the centers of distant galaxies.

Intrinsic – In science, the intrinsic value or property of an object does not change based on outside factors. For example, your mass, or the amount of stuff of which you are made, does not change whether you are standing on Earth or the Moon. Mass is an intrinsic property of yours. Your weight, which is related to your mass by the force of gravity will be different whether you measure it on Earth or on the Moon.

Input – The values that are fed into a computer program from a user or perhaps a sensor, for example, a temperature sensor.

Ionized – is the term for an atom which has lost one or more electrons causing it to become positively charged.

Ions – An atom or molecule that has a net positive or negative electric charge due to the loss or gain of electrons.

Irregular Galaxies – These are the galaxies that do not fit into any of the other three categories.

Isotope – Isotopes of a given atom differ in the number of neutrons in their nucleus. Using carbon as an example, the most common isotope has six protons and six neutrons in the nucleus. But there is a different isotope that has six protons (that’s what makes it carbon) and seven neutrons. Both isotopes behave the same in chemical reactions. Other elements can also have different numbers of neutrons, and thus will have different isotopes. The chemical behavior of different isotopes of a given atom is always the same.

Isotropic – Looks the same in every direction.


(Active galactic) Jets – thin, highly focused beams of matter and energy emitted from the nuclei of some active galaxies. Jets can be hundreds of thousands of light years long.

Jovian – or Jupiter-like planets do not have solid surfaces as they are composed primarily of helium and hydrogen. They typically have radii larger than 10,000 km (6,213.7 miles) with a mass over 1 x 1025 kg. Gaseous planets also have rings and many moons.


Kelvin – is the standard unit that scientists use to measure temperature. The temperature at which water freezes on the surface of Earth is 273 Kelvin, and the temperature that water boils is 373 Kelvins.

Kepler’s 3rd law of planetary motion – The square of the period (P) of an object’s orbit is proportional to the cube of the semi-major axis (a) of that orbit. Mathematically, this can be represented as P2 ∝ a3. Kepler discovered this law by studying the motions of the planets around the Sun, but it is a fundamental law that can be derived from Newton’s Law of Gravitation.

keV – Kilo Electron Volts. The electron volt (eV) is a measurement of energy; it is the energy gained by an electron (or proton) dropping through a potential of one volt. This is a tiny amount of energy, appropriate for describing the energies of atomic and subatomic particles. To give an example, the energy of visible light photons is about 1 eV. Adding the prefix “kilo” means we are talking about one thousand electron volts (keV). It takes about 6 x 1015 keV to equal 1 joule.

Kilogram – is the standard unit for measuring mass.

Kilometer – is a unit for measuring length. The average length between Earth’s center and its surface is about 6,000 Kilometers.


Laboratory – The location where a scientist conducts research. It is often, but not always, a room with specialized equipment and safety precautions. Often abbreviated as “lab.”

Laser Inertial Fusion Energy (LIFE) – is a fusion energy concept for delivering energy to the electrical grid.

Lenticular Galaxies – These galaxies are flattened disks like the spiral galaxy. They do not, however, have the spiraling arms, and have much lower rates of new star formation. Similar to ellipticals, lenticular galaxies have little or no cool gas or dust.

Lepton – One of the 3 most fundamental building blocks of the Universe, along with quarks and bosons. The most common lepton is the electron.

Life Cycle Of Stars – The cycle in which stars are born as protostars, grow old by expending their nuclear fuel, and eventually end their lives as black holes, white dwarfs and neutron stars. Stars expel material that will eventually form new stars; sometimes slowly over the long course of their lives, and sometimes quickly and violently in the form of supernovas.

Light-year – The distance that light travels in one year, which is about 10 trillion kilometers. Since light travels so fast, it covers a lot of distance in a small amount of time and because the universe is so large it makes it easy for astronomers to use light-years as a measuring stick.

Light Side – The part of the Moon the sun is currently shining on. The side of the Moon that receives sunlight changes as the Moon orbits around the earth.

Loop – is a block of computer code that can repeat for a fixed number of times, or until some condition is met, or forever.

Luminosity – The amount of energy an object, like a star, radiates per unit time. This is usually measured in watts, just like a light bulb.


Machine code – The code, readable by computer hardware that is created after source code is compiled.

Magnetosheath – The region in space where a planet’s magnetic field interacts with the charged particles of the solar wind.

Magnitude – Astronomers measure how bright an object is in terms of its apparent magnitude. Counter intuitively, the brighter an object is, the lower its magnitude. A first magnitude star is about 2.5 times as bright as a second magnitude star, and so on. The brightest object in the sky is, of course, the Sun; with a magnitude of -26.73. The full moon is -12.6, and with the naked eye we can see all the way down to about a magnitude of 6.

Main sequence – is the name given to a region on a graph, known as the Hertzsprung–Russell (HR) diagram, of stellar color versus brightness. While on the main sequence, which runs from hotter and brighter stars (upper left) to cooler and dimmer ones (lower right), a star generates energy by nuclear fusion in its core. In the Sun these fusion reactions convert hydrogen to helium.

HR Diagram
Hertzsprung–Russell (HR) diagram

Mare – Maria is latin for “seas”, mare is singular. The dark regions that you can see on the Moon are the maria.

MASER – Stands for Microwave Amplification by Stimulated Emission of Radiation. It’s like a laser, but in microwave light instead of higher energy optical light.

MeV – Mega Electron Volts. The electron volt (eV) is a measurement of energy; One eV is the energy gained by an electron (or proton) dropping through an electrical potential of one volt. This is a tiny amount of energy, appropriate for describing the energies of atomic and subatomic processes. To give an example, the energy of visible light photons is about 1 eV, as is the typical energy of outer electrons in atoms. Adding the prefix “Mega” means we are talking about one million electron volts (MeV).

Meson – Mesons are particles made up of a quark and an anti-quark pair. Because mesons are unstable they exist only for a short time before decaying into other particles. Mesons, like exotic baryons, are often created in particle accelerators.

Mass – is a measurement of how much stuff, or “matter,” an object is made.

Meteor – A meteor is the flash of light we see when a particle or chunk of solid matter falls into our atmosphere and disintegrates.

Meteor Shower – When the Earth passes through a cloud of dust, we see large numbers of meteors in a relatively short time. This is called a meteor shower. During its peak, observers might notice one or two meteors a minute from the Perseids.

Microorganism – As the name implies, a microorganism is an organism that is microscopic in size, usually too small to be seen with unaided human eye.

Microwave – The energy of microwaves is a bit higher than radio waves. Their wavelengths are therefore shorter, and are typically measured in centimeters (cm or 10-2m).

Missing mass – The phrase used by Fritz Zwicky to refer to the material needed to gravitationally bind galaxy clusters. We now call this material dark matter.

Molecular cloud – Also sometimes referred to as a stellar nursery if new stars are being formed within the cloud. Molecular clouds are giant regions of diffuse gases that can be as big as 326 light years (3.1 x 1015 km) across. They are composed mostly of Hydrogen and Helium, with a few other elements dispersed throughout. Internal gravitation in denser regions of the cloud can trigger the collapse of parts of the cloud, and when this happens, new stars can be formed.

Moon – is a celestial body that orbits a planet or smaller body.

M87 – The 87th object in the Messier Catalogue is a very large elliptical galaxy some 55 million light years away at the center of the Virgo Cluster of galaxies (our local supercluster). M87 is an active galaxy; its strong nuclear radio emission makes it a radio galaxy. It is also one of the brightest radio objects in the sky.


Near Side – The side of the Moon that always faces toward the earth. Features on this side are named mainly after the European astronomers who first observed them with telescopes.

Neutrino – A fundamental particle that has no charge, almost zero mass and that travels at nearly the speed of light. Neutrinos can pass through most matter without interacting with it.

Neutron – is one of the particles that makes up the nucleus (center) of atoms.

Neutron star – The collapsed core of a massive star, made up mostly of neutrons. It has a very small size, with a diameter of about 10 km, the same size as a small city. However, the mass of a neutron star is very large for its size, at least 1.4 times the mass of our Sun. With so much mass stuffed into such a small volume, the density of a neutron star is around a billion tons per teaspoon.

Newton’s Law of Universal Gravitation – Two objects with masses m1 and m2 that have a distance r between their centers attract each other with a force equal to (G*m1*m2)/r2. The Universal Gravitational Constant, G, is equal to 6.672×10-11Nm2/kg2 in SI units.

Nuclear fusion – is the process by which lighter elements like hydrogen and helium fuse together to make heavier elements like lithium, carbon, oxygen, etc.

Nuclear Radiation – The particles emitted by the decay of unstable nuclei. This type of radiation typically consists of neutrons, electrons/positrons (historically called beta rays) and/or alpha particles (now known to be helium nuclei with mass 4). High energy photons (gamma rays) are also frequently produced – these are particles of very energetic light.

Nucleosynthesis – is the name given to the process that created atomic nuclei containing more than just a single proton.

Numerical operations – Mathematical operations like adding, subtracting, multiplying or dividing numbers.


Optical – The band of energy we can see with our eyes. It is intermediate in terms of energy and wavelength, with wavelengths somewhat shorter than infrared (approximately 400 – 750 nm) and energies somewhat higher.

Orbit – is the path followed by a moon, planet or artificial satellite as it travels around another body in space.

Orbital Period – In a binary system, the time it takes for the two stars to complete one orbit around their mutual center of gravity.

Output – The result of a computer program that is displayed to a user, or transmitted to another computer program or a device, which in turn does something with the result.


Parameter – a special type of variable that may be required as the input for a function. If the variable being passed to the function does not match the data type that the function expects, the program will produce an error.

Parsec – is another measuring stick used by astronomers. One parsec equals about 3.3 lightyears.

Particle accelerator – These devices accelerate charged particles to speeds near the speed of light. By colliding the particles together and studying what comes out of the collisions, scientists are able to learn about the fundamental properties of matter.

Photon – A photon is a particle, or “quantum” of electromagnetic radiation, or light. While light is often thought of as a wave, it also exhibits features that are particle-like.

Photon Epoch – The time near the beginning of the Universe which was dominated by photons and lasted from approximately 10 seconds to 3 x 105 years.

Planetesimal – objects that are large enough to gravitationally attract gas, dust and other planetesimals, thus forming larger bodies, and eventually protoplanets. Planetesimals range in size from about 1 km to the size of the Moon.

Planck Epoch – refers to the period from the beginning of the Universe to the Planck Time. This epoch only lasted 10-43 seconds.

Planck Time – Planck Time is the unit of time that is determined by unit, or dimensional, analysis using fundamental constants such the speed of light, gravitational constant and the value of (Pi). It is approximately equal to 5.4 x 10-44 seconds.

Plasma – Plasma is a gas composed of unbound electrons and positively charged atomic nuclei.

Positive feedback loop – When an action results in the amplification of that action, it is said to be in a positive feedback loop. See Episode 101 notes for an example.

Positron – is the anti-particle of the electron. It has all the same characteristics as an electron except for its charge, which is positive.

Potential energy – The energy that is stored in objects due to is position in a force field. For example, the atoms in a molecular cloud have gravitational potential energy that is turned into heat energy, which is just the energy of the random motions of the particles as the cloud collapses towards its gravitational center.

Primordial, Black Hole – Very tiny, theoretical black holes that may have formed during the Big Bang. Originally predicted by Prof. Stephen Hawking, these types of black holes have very short lifetimes and have not yet been detected.

Programming – is the process by which computers are given instructions to perform a particular task.

Programming language – Just as there are many human languages, computers also have many different languages, and each language has its own rules that a computer programmer must know and understand in order to write programs in that language.

Proton – Protons are positively charged and are one of two types of particles found in the nuclei of atoms (the other being neutrons). The type of element (e.g. Hydrogen, Helium, etc.) is determined by the number of protons in its nucleus. For example, all hydrogen atoms have one proton in their nucleus, helium atoms have two protons, lithium atoms have three protons, and so on. A proton is made up of two Up quarks and one Down quark. Note: A hydrogen atom is made up of one proton, zero neutrons, and one electron.

Proportional counters – Detectors in which high-energy photons (such as X-rays) ionize gas, producing a number of charged particles that is proportional to the energy of the incoming photon. The photon’s energy can then be determined by reading out the resultant pulse of charge.

Protoplanetary disk – A disk of swirling gas and dust surrounding a young star that may eventually form into the planets, asteroids and debris of a solar system.

Protoplanets – Regions of dust and gas in a protoplanetry disk that are condensing under their own gravity but are still too unstable to be considered fully formed planets. Protoplanets are at least as large as the Moon. Collisions between protoplanets can eventually form planets.

Pulsar – A type of spinning neutron star that emits a beam of light like a lighthouse. Since it only points at us some of the time, it looks to us like it is pulsing on and off, hence the name pulsar.

Period – Periods apply to things that repeat, and measure how long it takes them to repeat. For instance, the period of the Earth to turn around its axis is 24 hours, while the period for the Earth to travel around the sun is 365 days.


Quark – One of the most basic subatomic building blocks of matter. Quarks combine to form protons and neutrons, as well as other more exotic particles.

Quark Epoch – The period in the early Universe where matter existed, but only as a hot dense “soup” of quarks, gluons and a few other elementary particles.

Quasar – A quasar (quasi-stellar radio source) is an active galaxy so distant, it appears star-like.


Radiation Belt – Regions around planets that contain high-energy charged particles which are trapped within the planet’s magnetic field.

Radio – is the name given to the lowest energy region of the electromagnetic spectrum. Radio waves have wavelengths of meters (m), or even kilometers (km, 103m).

Radioactive – describes the tendency of unstable atomic nuclei to break apart into stable ones by emitting particles and (electromagnetic) radiation.

Radioactivity – The natural or artificial process that causes the nucleus of a stable atom to become unstable thereby breaking apart to become a new element.

Radio lobe – A large glowing cloud of matter located at the ends of the jets in some active galaxies. Lobes are formed when the matter in the jet is slowed by collisions with intergalactic material. The lobes emit most strongly in the radio part of the spectrum, but they also emit at other wavelengths.

Red Giant – At the end of its life, a dwarf star will turn into a red giant, growing to several times its normal size and with a relatively low surface temperature. The low surface temperature makes it appear red, thus the name, red giant.

Red Shift – This is the name given to the apparent change in the wavelength of light due to the Doppler Effect. Scientists know what the regular spectrum of a galaxy should look like (based on the spectrum of light emitted from known elements). If the light waves from a galaxy appears to have shifted towards higher frequency (blue), it is moving towards us, and if it has shifted towards a lower frequency (red), that means the object is moving away.

Redshift (cosmological) – This is the name given to the apparent change in the wavelength of light due to the expansion of the universe. Scientists know what the spectrum of a galaxy should look like (based on the laboratory spectra of light emitted from known elements). In all but a few nearby galaxies, the familiar spectral lines are shifted to longer wavelengths than would be measured in a laboratory. Red light is at the long wavelength end of the visible spectrum, and blue light is at the short wavelength end, so shifts to longer wavelengths are called redshifts. The cosmological redshift is denoted by the letter z, and it is defined such that the universe has expanded by an amount 1+z over the time the light has traveled to us. So an object with redshift z=1 is seen when the universe was half its present size (it is twice as big as when the light was emitted), if z=2 the universe is three times bigger than when the light was emitted, if z=3 the universe is four times bigger, and so on.

Relativistic speed – a speed near to the speed of light. At such high speeds objects show effects predicted by Einstein’s Theory of Special Relativity.

Return value – this is the final result of a function that is returned to the function that initiated it.


Sandbox – In computer science, sandbox means to isolate a computer program such that it is unable to affect any other part of the computer, software or hardware.

Satellite – is a natural or man-made object that orbits a planet or other object.

Scientific Method – A general systematic procedure for learning about the world. It consists of gathering information (data), and testing ideas through experiments and careful observations.

Scintillation – When high energy light, such as gamma rays, strikes certain substances, such as sodium iodide crystals, it produces a flash of lower energy light. Physicists call this process scintillation.

Seyfert galaxy – A spiral galaxy (named for astronomer Carl Seyfert) with an extremely bright nucleus that emits strong emission lines. In Type I Seyferts the inner disk is tipped to our line-of-sight, allowing us to see broad emission lines produced in regions where the gas moves very fast, 1000 km/s or more. In Type II Seyferts the angle is such that we do not see down to these fast-moving regions, so we see only relatively narrow emission lines, about 200 km/s in width.

Singularity – In this context, singularity is a place in space where mass density and gravitational force become infinite.

Sodium Iodide Crystals – These specially treated crystals, made from sodium and iodine atoms, are used as part of gamma-ray detectors. The gamma rays produce flashes of optical light when they pass through the crystals, and the optical flashes are detected by photomultipliers – devices that convert visible light into electric current.

Sol – This is one of the proper names for the Sun. Sol was the god of the sun in Roman mythology. It is from this root that words like “Solar” are derived.

Solar masses – This is the mass of our Sun, or about 2 x 1030 kilograms.

Solar wind – is the term given to the stream of charged particles that are ejected from the Sun’s atmosphere. One of the effects of the interaction of solar wind and the Earth’s atmosphere is a creation of beautiful light patterns in sky known as auroras.

Solar/star system – is defined as a system of celestial objects such as planets and asteroids orbiting one or more stars.

Source code – the set of computer instructions that makes up computer programs.

Space station – an artificial satellite orbiting a star, planet, or moon that is capable of supporting several one or more crew members.

Spacetime – The three physical dimensions (length, width, and height), plus the dimension of time is spacetime. An object moving around in a volume over a period of time is moving through spacetime.

Spectrometer – An instrument that measures the intensity of light over a given range of wavelengths. In astronomy, spectrometers are especially useful in determining what elements make up a given source of electromagnetic energy. A prism is a very simple type of optical spectrometer.

Spectroscopy – is the scientific technique in which the intensity of light at different colors or wavelengths is measured. Comparing the measurements at different wavelengths can help to determine which elements are present in the light source.

Spectrum – The distribution of how much light an object emits (its brightness) as a function of wavelength or frequency.

Spiral Galaxy – These galaxies are flattened disks with long spiraling arms that extend out from a central core or bulge. In the arms are large amounts of interstellar matter which lead to high rates of new star formation.

Stellar, Black Hole – Black hole formed from collapse of a dying star with a mass at least ten times that of our Sun. Scientists have observed more than a dozen stellar black holes in our Milky Way galaxy, and have also detected them in other galaxies that are nearby.

Stellar Classification – This is a system in which stars are given a classification of O, B, A, F, G, A, K, or M based on their surface temperature, with O being the hottest and M being the coolest. Our Sun is a type G, with a surface temperature of about 6000K. Officially, this is known as the Morgan-Keenan spectral classification system.

Stellar Density – The number of stars that exist within a given region of space.

Stellar Flare – An explosion in the atmosphere of a star caused by a sudden release of magnetic energy. When this occurs in our own sun we call it a Solar Flare.

Stellar (star) formation – the process by which stars are formed.

Stellar nurseries – Are giant molecular clouds in interstellar space where the abundance of hydrogen molecules (H2) is such that parts of the clouds collapse under their own gravity to form new stars.

Strong and Weak Nuclear Force – the strong and weak nuclear forces are short-range forces that act only inside atomic nuclei. The strong force holds the nuclei together. The weak force is responsible for radioactivity and the emission of particles from nuclei.

Supergiant (star) – is the most massive type of star. A supergiant may have a mass between 10 and 70 times that of the Sun, or even more. Its size (diameter) will typically be several times larger than that of our own Sun.

Supermassive, Black Hole – Black hole with mass on the order of millions or billions of solar masses. There is scientific evidence for a 4-million solar mass supermassive black hole at the center of our Milky Way galaxy, as well as much larger black holes at the cores of distant galaxies.

Supernova (plural: supernovae) – The explosion of a star at the end of its lifetime, when it runs out of nuclear fuel in its coher type of supernova is due to an exploding white dwarf. White dwarfs are low-mass stars, but if they’re. Supernovae come in two types. One is called core collapse, and only massive stars can become core collapse supernovae. The ot acquire mass from a binary companion, they too can explode. Supernova explosions are so bright they can be easily observed in other galaxies.

Surface area – A 2-dimensional measurement that has both a length and a width. For instance, it could be the area on top of a table, in which case the surface area would be the product of the table’s length and width. Surface area can also be the total area covering a shape. As an example, the surface area of a cube is the length of one of its sides squared (that’s the area of a face of the cube) times 6 (since a cube has 6 faces).

Survey – In astronomy, a survey is a map that spans some region of the sky; some surveys even show the entire observable universe. Surveys can be taken in many different wavelengths, producing very different images of the Universe.

Swift – is a first-of-its-kind multi-wavelength satellite, launched in 2004, dedicated to the study of gamma-ray burst (GRB) science. Its three instruments work together to observe GRBs and afterglows in the gamma ray, X-ray, ultraviolet, and optical wavebands.

Swift-Tuttle – A near-Earth comet independently discovered in 1862 by American astronomers and comet hunters Lewis Swift and Horace Tuttle. It is the source of the Perseid meteors.


Terrae – Terrae is Latin for “lands”, terra is singular. The lighter regions on the Moon that you can see, also called the highlands, are the terrae.

Terrestrial – or rocky planets are made of rock and minerals, have solid surfaces, and typically have radii smaller than 10,000 km (6,213.7 miles) with a mass less than 1 x 1024 kg. They typically have only one or two moons, if any.

Tidal Force – the net force that results when the gravitational pull on one side of an object is greater than that on the other side.

Time dilation – is the name given to the phenomenon by which a clock closer to a massive object, like a black hole, will run slower than an identical clock that is farther away. This type of time dilation is predicted by Einstein’s General Theory of Relativity and has been tested successfully by flying a chronometer on an airplane and testing the effect against an identical clock that was left stationary on the ground.

Another type of time dilation occurs when a clock is moving with respect to an observer. In this case the observer will see the moving clock tick slower than the clock that is stationary with respect to the observer. This type of time dilation is predicted by Einstein’s Special Theory of Relativity and has also been tested with great accuracy.

Tides – Tides are the changes in sea level due to gravitational forces exerted primarily by the Moon, but also in part by the Sun.

Terraforming – A process of altering a planet or moon to make it habitable for humans. Terra is a Greek prefix meaning Earth, so terraforming a planet means to make it more like Earth.

Torus – Gas and dust outside the accretion disk in an active galaxy. The term refers to the gas orbiting the central black hole in a torus, or donut-shaped region.

Twin Quasar – Also known as QSO 0957+561 A/B, was the first gravitationally lensed object to be discovered (1979) and consists of a double imaged quasar formed by a foreground galaxy’s gravity which is nearly directly inline with QSO 0957+561 B and Earth.

Type 1A Supernova – A huge thermonuclear explosion resulting when certain white dwarf stars exceed the Chandrasekhar mass limit.


Ultraviolet – electromagnetic radiation beyond the blue/violet end of visible light. Ultraviolet radiation is more energetic than visible light but less than x-rays. Ultraviolet wavelengths are typically measured in nanometers (nm, 10-9m).

Unification of Active Galaxies – is the theory that active galaxies are essentially all the same and that the different observed features are due to the orientation of the galactic plane with respect to the viewer.


Variable – This indicates that a value can change. For instance, the brightness of a pulsar changes depending on whether or not its beam of light is pointing towards us when we are looking at it.

Variable (computer science) – In computer science, a variable is a placeholder for a value that can change. For example, if you are programming how a phone works, you want to make sure that it will be able to dial different phone numbers and be able to retrieve numbers from recent calls. These values (phone numbers) can be stored in variables until they are needed.


Whirlpool Galaxy – Discovered by Charles Messier in 1773, the Whirlpool Galaxy is one of the most striking examples of a grand spiral formation.

White Dwarf – After the outer layers of a red giant are expelled into space to form a planetary nebula, the core of the star that is left behind is so hot that it glows white. A white dwarf is about the size of Earth, but with nearly the mass of the sun, and therefore it is very dense.

Weakly Interacting Massive Particle (WIMP) – WIMPs are hypothesized sub-atomic particles that interact only through gravity and the weak force.

Wolter mirrors – Special mirrors that are able to reflect x-rays. (Video)

Wolter Telescope – X-ray telescopes that use Wolter mirrors to focus x-rays.

Worldship – is a theoretical space craft that serves as the home to the civilization that inhabits it. It differs from a generation ship in that residents of a worldship are not necessarily looking for any planet on which to settle.


XMM-Newton – is a joint European Space Agency (ESA)-NASA orbiting observatory, designed to observe high-energy X-rays emitted from exotic astronomical objects such as pulsars, black holes and active galaxies. It was launched on December 10, 1999 from the ESA base at Kourou, French Guiana and continues to make observations today.

X-ray – high energy electromagnetic radiation. X-rays are more energetic than ultraviolet light but less energetic than gamma-rays. Because of their high energies and short wavelengths, x-rays can easily pass through “soft” materials and can be used to image the insides of things, like the human body. Wavelengths of x-rays are shorter than about a nanometer.