Glossary Section :-

 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


Aurora :-   An Aurora is a natural light display in the sky, particularly in the polar regions, caused by the collision of charged particles directed by the Earth's magnetic field. An aurora is usually observed at night and typically occurs in the ionosphere.


Asteroid :-   Asteroids are large rocky objects. Some of these can actually be large enough to be considered small planets. Asteroids are mostly found in an asteroid belt between Mars and Jupiter, or in the Kuiper Belt, outside the orbit of Neptune and Pluto.

      Asteroid Itokawa


Absorption :-   Absorption is the act of matter receiving energy from light or other radiation. The wavelength of light which is absorbed by the matter is determined by the internal structure of the atoms or molecules.


This leads to the classic black lines seen in spectrographs of stars or nebulae from which astronomers can determine which elements are present and can also determine the redshift of an object if the lines are patterns are shifted to higher or lower frequencies.


Angular Resolution :-   The angular resolution of a telescope or image is how much detail it can see on the sky. For example, if a telescope had a 1 arc second resolution it would not be able to see any of the detail in any object that takes up less than 1 arc second of sky. Any object larger than 1 arc second is then said to be resolvable.


Arc second :-   An arc second is one 60th of an arc minute. Astronomers use arc minutes and arc seconds to define the angular resolution of images and the angular size of objects.


Arc minute :-     An arc minute is one 60th of a degree. Astronomers use arc minutes and arc seconds to define the angular resolution of images and the angular size of objects.


Angular Size :-    The angular size of an object is how big the object appears in the sky. Measured in degrees, arc minutes and arc seconds


AU (Astronomical Unit) :-   One AU is the average distance between the Earth and the Sun. 1 AU = 149.6 million km=92.956 million miles.


Apparent Magnitude :-   The apparent magnitude of a star is a classification astronomer’s use that tells how bright a star appears in the sky. The scale for magnitude is backwards, so brighter stars are more negative.


Absolute Magnitude :-   The absolute magnitude of a star is the apparent magnitude which we would see it as if it was 10 parsecs from Earth.


Brown Dwarf :-   A brown dwarf is a star which has not collected enough mass to begin Hydrogen fusion reactions in its core. They are called “brown” as they do not shine very brightly.


Blueshift  :-   Blueshift is the shortening of wavelength of light, due to the emitter traveling towards the observer.


Black Hole :-   A black hole is one of the most exotic objects in space, as it has no real spatial dimension, and radiates no light, but contains a super-massive amount of matter. Black holes are sometimes created at the end of a super-massive stars lifetime. Because of the massive amount of matter, the gravitational forces are so strong that even light can’t escape from its pull.


Binary Star :-   binary star system is one where there are two stars which are gravitationally bound to each other. You are able to see some of these systems with the naked eye, or through a telescope, and these are called visual binaries. Some stars appear near each other in the sky, but are not actually near each other, these are called double stars.


Band :-   Astronomers use the term “band” to describe regions of the electromagnetic spectrum. For example the wavelength range covered by visible light is called the optical band. Larger bands are generally divided again into smaller regions which are created using filters.


Correlation :-     Correlation of data is the act of essentially lining up different signals so that the data received is aligned correctly. Correlation is used in multi-telescope and multi-receiver configuration to ensure that any difference in path length from the receiver to the astronomer is corrected for.
A good analogy is looking at two telescopes (A and B) spaced by 10m, with telescope A 10m from the control room. The signals from telescopes A and B have to travel 10 and 20m, respectively, to get to the control room. So, to make sure we get the correct pairings of data we need to “delay” the signal from telescope A by 10m. This is essentially what happens in correlation.


Core :-     The core of a star is defined as the central section where the initial hydrogen fusion process takes place. Outside of the core, the gravitational forces are balanced by the radiation from the fusion process, and thus don’t become compact enough to start fusion itself. This makes the core and outer shells distinctly different.




Comet :-   Comets are small icy objects which orbit the Sun. The tails of comets are made by the Sun's heat forcing some of the ice and gas off the surface. These tails can be up to 250 million km long, and always point away from the Sun.


Colour :-   Astronomers will often refer to the colour of a star. They are talking about the difference between that stars absolute magnitude when observed through two different filters, or in two different spectral classes.


CCD (Charge-Coupled Device) :-   A CCD is the part of a digital camera which detects the light and transfers it into digitized data. It works by having a matrix of light sensitive elements which count the number of light photons which hit it in the time the shutter is open.
The quality, and value, of the CCDs used in astronomy are much more advanced than those in our digital camera at home, but they work in the same way.


Celestial Equator :-   The celestial equator is the line which projects from the Earth equator onto the celestial sphere. So, if you were standing on the equator, the celestial equator would be directly above you.


Celestial Sphere :-   The celestial sphere is the imaginary sphere all heavenly objects would sit on if space was a flat sphere around the Earth. Astronomers use the celestial sphere to map the position of stars in the sky.


Dust :-   Dust is what astronomers call more complex molecule that exists in the ISM. It is much unlike the dust we deal with everyday as it is made of mostly carbon- and hydrogen-based molecules. If you were to look at a spiral galaxy side on, you would a dark cloudy region in the stars; this is interstellar dust.


Double Stars :-   Double stars are two stars which look like they are near each other, but are not close enough to feel each other’s gravitational forces. This is normally because they are different distance from Earth, and they only look close because of an optical effect.


Declination :-   The declination of an object is how far above or below the celestial equator it is. Declination is often coupled with right ascension to give the location of an object.


Diffuse Nebulae :-   Diffuse nebulae are thin extended clouds of gas and dust. Such nebulae will have enough gas to create several hundred to several thousand stars. The light which we see in them is either created by the radiation from stars within the nebulae being absorbed and re-emitted or reflected.


Dark Nebulae :-   Like the diffuse nebulae, dark nebulae have no internal or nearby star to be illuminated by. As such, these show up as dark dust clouds in front of other objects. These are normally very dense, as all the light coming from behind the nebula is absorbed, with none or very little allowed to pass through. Due to their detection method, a large number of Dark Nebulae exist which have not been discovered. Dark matter anyone???


Dark Matter :-   Dark matter is an unknown material, which makes up around 90% of the mass in the universe. It is called dark matter as it doesn’t interact with light in any of the ways other matter does.
Dark matter was discovered by looking at how galaxies and galaxy clusters moved through space. Astronomers noticed that there was not enough mass in the stars and galaxies to produce the motion which they were seeing. After observing this with all systems, not just a few isolated ones, it has become common theory due to the lack of anything better. However there is a lot of cold interstellar and intergalactic hydrogen out there that gives off very little radiation that we can detect...


Exoplanet :-   Exoplanet" is short for "extra solar planet"; a planet existing in a different solar system to our own.


Extrasolar :-   Extrasolar simply means outside of our solar system. That is, something not gravitationally bound to the Sun.


Event Horizon :-   The event horizon of a black hole is the distance from the physical black hole from which no information (i.e. light, sound, etc) can escape, as the gravitational pull is too great.


Equinox: :-   An equinox is a day in which both the night and day are of equal length. This occurs when the Sun is directly over the equator. Each year there are two equinoxes: the vernal equinox (March 21st), and the autumnal equinox (September 21st).


Emission :-   Emission is the act of matter letting out energy in the form of radiation. The wavelength of light which is emitted is determined by the internal structure of the atoms or molecules.


Elliptical Orbit :-   An elliptical orbit is one where the orbit is not perfectly circular, but has been “flattened” in one direction. This creates two foci, as opposed to the one of a perfectly circular orbit, and the central mass will be sitting in one of these.


Electrons :-   Electrons are one of the three components of atoms, along with neutrons and protons. Electrons have a negative charge, which is used to repel them from another electron if they are too close. While the number of protons determines the element, there needs to be the same number of electrons to make the atoms neutral (i.e. not charged).


Ellipse:-   An ellipse is essentially a flattened circle. It has a circular shape, but has an elongated shape on two of the sides.


Electromagnetic Spectrum :-   The electromagnetic (EM) spectrum is the spectrum containing all of the different types of electromagnetic radiation.


Electromagnetic Radiation :-   Electromagnetic (EM) radiation refers to all the different forms of “light” which can be created. EM radiation consists of packets of energy called photons, which carry energy but have no real size or mass. The amount of energy each photon has is inversely proportional to the wavelength it has (higher energy = shorter wavelength).


Eccentricity :-   Eccentricity is a measure of how non-circular an orbit is. Orbits with an eccentricity of 0 are perfectly circular. Orbits with an eccentricity between 0 and 1 are classed as elliptical. Eccentricities equal to 1 are parabolic, and orbits greater than 1 are considered hyperbolic.
If an object has an eccentricity greater than or equal to 1 then it is in a bound and completed orbit, and will escape the gravitational forces of the central body.


Fusion :-   Fusion is the process by which two or more atoms are joined together at the nuclear level to make one larger atom.
An example of this is two Hydrogen atoms making one Helium atom.


First Light :-   The term used to describe when a new telescope is tested and assorted adjustments carried out. As well as testing the telescope body's functions, adjustments are carried out to check equipment that traces the movements of celestial bodies, and that analyzes and records observation images. Adjustments have to be made over several months to make the telescope work as a single observation system.


Filter :-   Filters are generally by astronomers when observing to reduce the range of wavelengths they look at. For amateurs these are mostly optically flat disks of glass with exotic metal coatings either screwed in the back of an eyepiece or (if you have a fat wallet) held in a motorised filter wheel. There are also full aperture filters that go over the end of your telescope that enebale observations of the Sun such as a H alpha filter that only allows light generated from hydrogen through.


Gravitational Force :-    Gravitational force is the mutual pulling force that acts between two bodies. Gravity is proportional to mass, and inversely proportional to distance, so the heavier the body the greater the gravitational force and the farther you are from it the less you feel.
This is best seen by looking at the Sun, Earth and Moon: the Sun being the largest has the strongest gravitational force, and so the Earth and Moon orbit around it. But, with the Moon being so close to Earth, it feels Earth's pull much more and so it orbits Earth.
Gravitationally Bound: A system is gravitationally bound when the individual components are unable to escape the gravitational attraction of the system as a whole. An example of this is a satellite orbiting Earth: it is given the right amount of energy to keep circling Earth, but not enough to escape from its gravitational force.


GPS :-   GPS, which stands for Global Positioning System, is a radio navigation system that allows land, sea, and airborne users to determine their exact location, velocity, and time 24 hours a day, in all weather conditions, anywhere in the world.
24 GPS satellites (21 active, 3 spare) are in orbit at 10,600 miles above the earth. The satellites are spaced so that from any point on earth, four satellites will be above the horizon. Each satellite contains a computer, an atomic clock, and a radio. With an understanding of its own orbit and the clock, the satellite continually broadcasts its changing position and time. On the ground, any GPS receiver contains a computer that "triangulates" its own position by getting bearings from three of the four satellites. The result is provided in the form of a geographic position - longitude and latitude - to, for most receivers, within a few meters.
If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map. If a fourth satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position. If you are moving, your receiver may also be able to calculate your speed and direction of travel and give you estimated times of arrival to specified destinations.


Globular Cluster :-   Globular clusters are highly concentrated groups of between ten thousand and one millions stars. Globular clusters are gravitationally bound system, which are very old, containing some of the original star in our universe.


Geocentric :-   Geocentric means Earth-centered. So a geocentric radius is the distance from the Earth.


Geosynchronous Orbit :-   A direct, circular, low-inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface.


Gas Giants :-   Gas giants are the larger, gaseous planets in our solar system. These planets are all constructed similarly, with a solid core, liquid mantle and gas atmosphere. The gases in the atmosphere of these gas giants form the majority of the volume of the planet.
The gas giants, while still being considerably smaller than the Sun, are the largest planets in the solar system, and are much larger than the terrestrial planets.


Gamma-Rays :-   Gamma-rays are the most energetic form of electromagnetic radiation, and have the shortest wavelength. Gamma-rays are one of the most difficult bands to observe at a ground-based telescope, as the Earth's atmosphere prevents most of them from reaching the ground.
Gamma-rays have wavelengths less than 0.01 nanometers.
Gamma Ray Bursts (GRBs): Gamma Ray Bursts are the most luminous objects currently known to exist in the universe. They are flashes of gamma rays which can last for seconds to hours, and usually exhibit an x-ray after glow.


Galaxy :-   A galaxy is a collection of stars and gas which are gravitationally bound to each other. Galaxies come in a variety of shapes and sizes, and generally hold billions of stars.
Galaxy Clusters: Galaxy clusters are groups of galaxies which interact with each other through gravitational forces.


Hyperbolic Orbit :-   A hyperbolic orbit is one in which the object escapes the gravitational force of the central body, by following a hyperbola.


Hyperbola :-   A hyperbola is a curve in which the focus is a line drawn through the centre of the curve.


Hydrogen Shell Burning :-   Hydrogen shell burning is the process where hydrogen fusion is taking place in the outer layers, or shells, of gas, as opposed to the core of a star.


Hyperbola :-   A hyperbola is a curve in which the focus is a line drawn through the centre of the curve.


Hyperbolic Orbit :-   A hyperbolic orbit is one in which the object escapes the gravitational force of the central body, by following a hyperbola.


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J :-


K :-


Lens :-   Lenses are optical devices designed to focus, broaden, scatter or split beams of light. Lenses are essential parts of spectrometers, and the focusing section of optical telescopes.


Light-year :-   A light-year is the distance which light travels in one year. 1 ly = 63240 AU = 9.46 trillion km=5.87 trillion miles!


Luminosity :-   The luminosity of a star is a measure of how much light comes from that star. In other words, it’s a way of putting a physical number to the magnitude of a star. Luminosity is dependent on temperature, in that a hotter star is more luminous.


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Parsec :-   A parsec is a measure of distance astronomer’s use. 1 parsec = 3.26 light-years, = 206265 AU.


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Radio Waves :-   Radio waves are the longest wavelength form of electromagnetic radiation, and have the least energy. Radio waves are created by moving charged particles back and forth. The Earth's atmosphere is almost completely transparent for most of the radio wave band (a few mm up to about 20m). Radio waves have a wavelength greater than one hundredth of a millimeter.


Redshift :-    Redshift is the lengthening of wavelength of light, due to the emitter traveling away from the observer. This will cause objects moving away from us to be redder than they actually are. Redshift can be used to determine how far away the object is, and how fast it’s moving away from us.


Right Ascension :-   Right ascension is the measurement used to determine how far around the celestial sphere an object is. Right ascension goes from 0 to 24, with 0 being the position of the Sun during the vernal equinox.


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Telescope :-   A Telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation (such as visible light).


The first known practical telescopes were invented in the Netherlands at the beginning of the 17th century. The word telescope can refer to a wide range of instruments detecting different regions of the electromagnetic spectrum.


The word "telescope" (from the Greek tele "far" and skopein "to look or see"; teleskopos "far-seeing") was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galilei's instruments presented at a banquet at the Accademia dei Lincei.


Temperature :-   Astronomers refer to temperature just like we do on Earth, but instead of dealing with measurements less than 50 degrees, like we see on the weather; they deal in thousands of degrees. Instead of Celsius or Fahrenheit, astronomers measure things in Kelvin (K). One Kelvin is equal to one degree Celsius, but they have different starting points, so that 0 K = -273 Celsius and this is classed as absolute zero.


Terrestrial Planets :-   Terrestrial planets are those like Earth, with a solid surface. The terrestrial planets in our solar system are Earth, Mars, Mercury and Venus. As well as the solid surface, except for Mercury, terrestrial planets have some form of atmosphere. The size of the terrestrial planets is much smaller than that of the gas giants.


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