Glossary

the visual magnitude an observer would record if an asteroid were placed one astronomical unit (au) away, and 1 au from the Sun and as if viewing the asteroid from a place on Earth as if the asteroid is at opposition and on the ecliptic

the magnitude a star would have if it was placed at a distance of 10 parsecs from Earth. By considering stars at a fixed distance, astronomers can compare the real (intrinsic) brightnesses of different stars.

A disk of gas and dust that accumulates around a center of gravitational attraction, such as a newly-forming star.

small Solar System bodies that have asteroid-like orbits but can sometimes display tails, like comets.

a process of creating colors by mixing wavelengths of light

The ratio of the amount of light reflected from a surface to the amount of incident light. Example: an object with an albedo of .15 reflects 15/100 (15%) of the amount of light that arrives at the surface of an object.

a time-sensitive, computer-generated message that identifies a change in an image compared to the previous exposure made of the same section of sky

• Exploring the Observable Universe

a measure of the amount of light received by Earth from a star or other object—that is, how bright an object appears in the sky

a stony or metallic object orbiting the Sun that is smaller than a major planet, but that shows no evidence of an atmosphere or of other types of activity associated with comets

the average Earth-Sun distance, about 150 million kilometers

a line drawn on a scatter plot to represent the trend of the data. This line may pass through some or all of the points and error bars

the theory of cosmology in which the expansion of the Universe began with a primeval explosion (of space, time, matter, and energy)

two stars that revolve around each other

a filter that transmits light over a large range of wavelengths

a yellow supergiant pulsating star; these stars vary periodically in brightness over a well-defined period of 1-100 days

the upper limit to the mass of a white dwarf (equal to 1.4 times the mass of the Sun)

a technique for creating a color image that matches the filter that transmits light with the shortest wavelengths to a color in the visible spectrum that also has a short wavelength. Subsequent filters that transmit light of increasing wavelengths are assigned to visible light colors of increasing wavelengths.

the difference between the magnitudes of a star (or other object) measured through two different filters—for example, blue minus visual (B–V) magnitudes

to assign or add color to a greyscale image

a small body of icy and dusty material that revolves around the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a cloud of tenuous gas (a coma) around its solid nucleus, and often a tail

a color-coded map that indicates how relative numbers of objects (such as stars or galaxies) are distributed in an area

cells in the retina of the human eye that respond differently to light of different wavelengths, and are responsible for enabling color vision

one of the 88 sections into which astronomers divide the sky, each named after a prominent star pattern within it

in massive stars, when core fusion ceases, gravity causes the core to collapse, producing neutrons and an outgoing shockwave

the succession of methods by which astronomers determine the distances to celestial objects

microwave radiation coming from all directions that is the redshifted afterglow of the Big Bang

a principle that states that the distribution of matter across very large distances is the same everywhere in the Universe and that the Universe looks the same in all directions

observed spectral lines are shifted to longer (redder) wavelengths, caused solely by the expansion of the Universe

the energy that is causing the expansion of the Universe to accelerate; its existence is inferred from observations of distant supernovae and its nature is not yet understood

the equivalent of latitude on a star chart, denoting how far (in degrees) an object in the sky lies north or south of the celestial equator.

a long-lasting observation of a particular region of the sky intended to reveal faint objects by collecting the light they emit for a long time. The 'deeper' the observation is (i.e. longer exposure time), the fainter the objects that become visible on the images.

a gas that resists further compression because no two electrons can be in the same place at the same time doing the same thing (the Pauli Exclusion principle)

an equation that uses the difference between the apparent magnitude and absolute magnitude of a star to measure its distance

a measure of how elliptical an orbit is. A perfect circle has an eccentricity of 0, and an extremely elliptical orbit has an eccentricity of just under 1.

the apparent annual path of the Sun projected on a map of the stars

radiation consisting of waves propagated through regularly varying electric and magnetic fields and traveling at the speed of light

a spherical or egg-shaped galaxy that contains no young stars and does not have spiral arms

the point where an earthquake originates

the minimum speed an object must reach to permanently break away from the gravity of another object, and also the minimum speed at which a projectile approaching Earth or the Moon will hit (e.g., the escape velocity for Earth is 11 kilometers per second)

a value created by dividing amounts of light energy collected through two different filters

a star in the late stages of its life that is undergoing helium fusion; giants are typically much larger than main sequence stars with similar temperatures

a spherical group of hundreds of thousands of stars that are gravitationally interacting, located in the center or halo of a galaxy

a black and white image showing up to 256 shades of grey

a plot of luminosity vs. surface temperature for a group of stars

a plot that displays a representation of the distribution of numerical data; although it looks like a bar chart, a histogram’s rectangles (called bins) represent ranges of data

a constant whose value is equal to the slope of a Hubble plot; it indicates the rate of the expansion of the Universe.

a graph of galaxy distance vs. recessional velocity

a rule that the radial velocities of distant galaxies are proportional to their distances from Earth

in star interiors, the balance between the gravitational force (due to the mass of the star) and the gas pressure (due to energy generation in the core of the star)

an electronic device that converts an optical image into an electronic signal; used in digital cameras and imaging devices to convert the light received on the camera or imaging device lens into a digital image

the angle between the plane of an object's orbit and the plane of the Earth's orbit (the ecliptic)

having no end, e.g., the Universe is infinite

electromagnetic radiation of wavelength 103-106 nanometers; longer than the longest (red) wavelengths that can be perceived by the human eye, but shorter than radio wavelengths

a narrow, almost vertical region in the Hertzsprung-Russell diagram which contains many different types of variable stars more massive than the Sun; these stars have left the main sequence, and they experience internal instabilities that cause them to pulsate in size and vary in luminosity

the international organization responsible for defining astronomical constants and names

(J) SI unit of power, equal to one kilogram-meter per second squared

the energy an object has due to its motion, which depends on the object's mass and speed

a region of space beyond Neptune that is dynamically stable (like the asteroid belt); the source region for most short-period comets

the galaxy supercluster that is home to the Milky Way galaxy and approximately 100,000 other nearby galaxies

The structure of the Universe when viewed at very large scales; galaxy clusters are concentrated into bubble-like structures (sometimes called walls, filaments or sheets) around areas of very low galaxy concentration (voids)

a plot showing how an object's brightness changes over time

the distance light travels in one year: 9.46×10^12km or about 19 trillion miles

the group of more than 50 nearby galaxies to which the Milky Way galaxy belongs

the time elapsed between when we detect light here on Earth and when it was originally emitted by its source

the rate at which a star or other object emits electromagnetic energy into space, i.e., the total power output of an object

• Coloring the Universe

the design and use of computer algorithms which can learn and make predictions based on data. In the case of astronomy, machine learning helps identify objects or categorizes them according to their characteristics.

two smaller galaxies that orbit the Milky Way galaxy, easily visible in the night sky from the southern hemisphere

an asteroid occupying the area between the orbits of Mars and Jupiter, where the orbits are generally the most stable, from 2.2 to 3.3 au from the Sun

the area on an H-R diagram that contains the majority of stars, running diagonally from the upper left to the lower right of the diagram

the estimated number of years that a star will remain on the main sequence, which represents about 90% of its entire life span

the topmost area of the main sequence in an H-R diagram that appears to connect with the area of giant stars

the minimum orbit approach distance between an asteroid and a planet. The MOID is considered to be the limit at which the planet’s gravitational field can change the path of the asteroid and potentially cause it to impact the planet (for Earth, the MOID is 0.05 AU)

a comet or asteroid whose path comes within 1.3 au of the Sun

a cloud of interstellar gas or dust; the term is most often used for clouds that that glow in visible or infrared light

• Coloring the Universe

a type of chart that relates the quantities of three variables to each other by means of a number of scales

a nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus with the release of energy

all light that can be seen from Earth, which is limited not by technology, but by the travel time of light since the beginning of the cosmological expansion.

the large spherical region around the Sun from which most long period comets come, extending from about 1000 to 100,000 au

a loose cluster of gravitationally-bound stars, containing between a few dozen and a few thousand stars, located in the spiral arms or disk of a galaxy

Orbit size is the average distance from an object to the Sun. For the major planets in our Solar System the orbit sizes are:

A scale that rates the potential hazard for an asteroid impact. It uses data about the time and probability of impact along with the asteroid's estimated kinetic energy.

the shift in the apparent direction of a star due to Earth’s orbital motion

a unit of distance in astronomy, equal to 3.26 light-years

SI unit of pressure, equal to one newton per square meter

the time it takes for a star to undergo one complete cycle of brightness changes

to change the orbit of an asteroid by exerting gravitational influence

a technique for measuring redshifts of galaxies by measuring their brightnesses through a series of filters and comparing them to models

For more explanation, see https://en.wikipedia.org/wiki/Photometric_redshift

a term, derived from "picture element," that refers to one element of an image or a detector

classification for an asteroid with an Earth Minimum Orbit Intersection Distance (MOID) less than 0.05 au, and an absolute magnitude less than 22. This magnitude translates to an asteroid size of 140 m for the smallest PHAs.

a triangular piece of glass used to separate white light into spectral colors

a variable star that pulsates in size and luminosity

a method of dating geological specimens by determining the relative proportions of particular radioactive isotopes present in a sample

the speed at which an object appears to be receding from Earth, due to the expansion of spacetime

the stretching of light waves towards longer wavelengths caused by motion away from an observer

an image made by assigning colors to greyscale images based on the wavelength of light used to create the greyscale image, in chromatic order. For example, an x-ray (short wave) image might be assigned the color violet, and a radio (long wave) image assigned the color red.

a motion by which an object appears to move backwards in its orbit, due to the greater orbital speed of Earth relative to the object

an object with a retrograde orbit is orbiting in the opposite direction to most other Solar System objects

a number that estimates the amount of energy caused by an earthquake

the equivalent of longitude on a star map, measured in either degrees or hours and minutes. The zero point of right ascension is marked by the Sun’s position in the sky at the time of the March equinox.

energy caused by an earthquake

half the length of the major axis of an elliptical orbit

a sharp change in air pressure caused by an explosion or by an object moving at supersonic speed

An object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite (moon).

Spacetime is a mathematical model that combines time with the three dimensions of space. Time is the fourth dimension.

a technique for measuring redshifts of galaxies by measuring the shift in their spectral lines

a flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars winding out from its central bulge

an astronomical object that has a consistent absolute magnitude, used to determine distances in space

a group of stars (within a galaxy) held together by gravity; the stars in the group all formed at nearly the same time, from the same cloud, and have the same composition, differing only in mass

a formula used to compute the rate at which a blackbody radiates energy; the total rate of energy emission from a unit area of a blackbody is proportional to the fourth power of its absolute temperature: F = σT4

a process of creating colors by mixing pigments or dyes

a massive star in the late stages of its life that is undergoing fusion of elements heavier than hydrogen; these stars are the most luminous and occupy the top region of the Hertzsprung–Russell diagram.

a stellar explosion caused by the collapse of its core

bursts of gamma rays produced in Earth's atmosphere thought to be caused by intense electric fields produced above or inside thunderstorms

an icy object found in the outer Solar System with a mean orbital distance greater than that of Neptune

Cepheid stars with high metallicities and pulsation periods that are generally less than 10 days (also known as classical Cepheids)

Cepheid stars with low metallicities and pulsation periods between 10 and 100 days

a stellar explosion produced at the endpoint of the evolution of stars whose mass exceeds roughly eight times the mass of the Sun

an explosion of a white dwarf in a binary star system, as a result of the white dwarf accumulating a significant amount of matter from its companion

electromagnetic radiation of wavelengths 10-400 nanometers, shorter than the shortest visible wavelengths of violet

possible orbits generated from an asteroid's uncertainty region; these orbits put the asteroid on a projected path to intersect with Earth

spherical volumes in the Universe that have low concentrations of galaxies

a dense, low-mass stellar remnant that has ceased nuclear fusion and collapsed to a very small size

the range of infrared light passed by the y filter on the Rubin Observatory LSST Camera; from 970-1060 nanometers