A B C D E F G H I L M N P R S T W Y Z
A B C D E F G H I L M N P R S T W Y Z
Absolute Magnitude: The brightness of a star, as viewed from Earth, if that star were a standard distance from Earth; in modern astronomy, the distance is 10 parsecs. Absolute magnitude is related to the logarithm of the star's luminosity.
Altitude: The degree measurement of a star's location up from the local horizon, at a given observing location, date, and time.
Apparent Magnitude: The brightness of a star as seen from Earth. Apparent magnitude is dependent on a star's luminosity and its distance from Earth.
Astronomical Unit (AU): The average distance between the Earth and the Sun. (1 AU = 1.49x10^13 centimeters)
Azimuth: The degree measurement of a star's local compass direction, where East is considered to be zero degrees, at a given observing location and time.
Binary system: A system where two stars are close enough in space to be gravitationally bound to one another, and orbit each other in space as do the Earth and the Sun. (Illustration)
Black Hole: A massive (over 5 solor masses) star which, in its death throes, collapsed so completely in on itself that nothing that gets close enough to it can escape - not evey light. Since we cannot directly see these condensed, collapsed stars, we call them "black" holes.
Blue Giant: A very hot, large star, which has left the main sequence behind.
Brightness: see Magnitude
Cluster: A large group of stars that formed close together in space, at essentially the same time. They are gravitationally bound and move together, but do not actually orbit each other.
Color: In astronomy, generally denotes the measured intensity difference in a star's spectra between one wavelength and a second wavelength; in StarCalc, the blue (B) and visual (V) bands, centered at 410 nanometers and 650 nanometers, respectively.
Constellation: A group of stars which may or may not be physically close together in space, but were seen by the ancients to form recognizable pictures and patterns when viewed from Earth.
Declination: The latitude location of a star in the equatorial sky grid system.
Deep Sky object: Any object which is not a star but is present in space - i.e., other galaxies, nebula of various types, star clusters, etc.
Electromagnetic Radiation: Any kind of photon emission, the best known being commonly called visible light, but also extending to radio, microwave, infrared, ultraviolet, x-ray, and gamma ray, etc. emissions.
Epoch: The year upon which a set of coordinates are based. Equatorial coordinates do change slowly, over the course of many years, so for very precise work it is necessary to state when your coordinates are from. Usually expressed as J2000, J1950, etc.
Equatorial Grid: The fixed coordinate system for stellar locations, expressed as declination (equivalent to latitude) and right ascension (equivalent to longitude). (Illustration)
Frequency: The number of full wave cycles that pass by a specific point during a specific time period. Usually expressed in hertz, which is (number of waves)/second.
Galaxy: A large collection of stars, all orbiting a common point in space, typically on the order of 10 kpc in radius. Most galaxies can be grouped into the categories, based on their shapes, of elliptical, spiral, or irregular.
Grid: see Horizontal or Equatorial
Hertzsprung-Russel
Catalog: The listing of all stars originally included in the Hertzsprung
-Russel
diagram.
HR Diagram: see Hertzsprung-Russel Diagram
Hertzsprung-Russel Diagram: Hertzsprung and Russel's graph of stellar temperature versus luminosity - demonstrates the fundamental physical relationship between these two quantities for stars.
Horizontal Grid: A coordinate system centered on your local observing time and location - stars move through the coordinates over the course of the night. (Illustration)
Intensity: The amount of light coming from a source - how many photons of a particular wavelength and frequency there are.
Luminosity: The actual output of energy per unit of surface area for a star or other object. It is usually expressed as Joules/meter^2 or ergs/centimeter^2.
Main Sequence: The section of the H-R Diagram where a star spends the majority of its life time, and which contains the majority of stars.
Magnitude: A log scale system that expresses the brightness of a star or other source in the sky. There are two basic types of magnitude measurements: apparent and absolute.
Meridian: The arc running from north to south across the sky, which divides the local sky grid into East and West. (Illustration)
Messier Catalog: Catalog of deep sky objects developed by Charles Messier, based out of Cambridge, England in the early 1900s. One of the first catalogs of deep sky objects. Objects present in Messier's catalog are denoted by an M - M1, M27, etc.
Nebula: A low-density cloud of gas and dust in interstellar space. There are various kinds of nebula, their types being based on how they formed.
New General Catalog: A more complete catalog of deep sky objects, done in the mid 1900s. Objects are denoted by NGC - NGC 6828, NGC 7027, for example.
Normalized: When a particular point-value in a set of data has been designated at zero, and all other points adjusted accordingly. (For instance, assume we had a set of numbers 3-4-5-3-2-6, and we declared that 2 would be our "normalized zero point". The set would then change to: 1-2-3-1-0-4. This is useful for comparing objects of different brightnesses, etc.)
Observing Location/time: The physical location and time from which you will be doing your observing.
Optical Double: Two stars which look very close together in the sky from Earth, but are actually very far away from each other in space. (Illustration)
Parallax: The apparent motion of a star in the sky, over the course of the year, which results from the Earth's motion around the Sun. (Illustration)
Parsec (pc): Distance measurement for interstellar distances; equal to 3.086x10^18 centimeters, or 206,265 AU. Also kilo-parsecs (kpc=10^3 pc), mega-parsecs (Mpc=10^6 pc), etc.
Period: The amount of time required for one object to orbit another - for instance, the period of the Earth around the sun is 365.25 days.
Planet: A gravitationally bound, comparatively dense sphere of dust, gas, or rock, which orbits a star.
Precession: The rotation of the direction of Earth's orbital tilt, occuring over a period of some 26 million years. (Illustration)
Red Giant: A less massive, cool, but very large, star, which has left the main-sequence.
Right Ascension: The longitudinal coordinate of a star, in the equatorial system. (Illustration)
Sidereal Time: The time as determined by the stars. The sidereal time is determined by the right ascension that is currently located directly at the meridian of the local sky grid.
Spectrum: The pattern of the intensity of absorption and emission lines in the light coming from a particular star.
Speed of Light: The absolute speed limit of the Universe - nothing can move faster than the speed of light. Usually denoted as c, its value is 2.99x10^8 meters/second.
Star: A sphere of gas which has reached an internal temperature such that nuclear fusion can (or at one point in time did) occur.
Supergiant: The largest type of star. A supergiant is an exceptionally massive star with a very short life-span. These can come in any star color (blue, yellow, orange, or red).
Supernova: The explosive death of the largest stars - those with masses more than 8 times that of our sun.
Temperature: A measure of the average energy of motion in each particle (protons, electrons, atoms, molecules, etc.)
Velocity: The speed at which an object moves in a particular direction.
Visual Magnitude: A star's magnitude in the visual band, about 400-700 nanometers.
Wavelength: The distance from one wave crest to the next - for visual light, usually expressed in nanometers, 10^-9 meters.
Yellow Giant: A mid-temperature, mid-mass star which has left the main sequence and become very large.
Zenith: The highest point on the local sky grid - located directly 90 degrees overhead.
