Astronomy Glossary
Click on a letter of the alphabet to jump to the terms beginning with that letter.
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
A
Absolute Magnitude: The absolute magnitude of a star is the apparent magnitude which we would see it at if it was 10 parsecs from Earth.
Absolute Zero: Absolute zero is the coldest it is possible for anything to get, and is equal to 0 K, or -273 degrees Celsius.
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.
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 arcsecond resolution it would not be able to see any of the detail in any object that takes up less than 1 arcsecond of sky. Any object larger than 1 arcsecond is then said to be resolvable.
Angular Size: The angular size of an object is how big the object appears in the sky.
Apparent Magnitude: The apparent magnitude of a star is a classification astronomers use that tells how bright a star appears in the sky. The scale for magnitude is backwards, so brighter stars are more negative.
Arcminute: An arcminute is one 60th of a degree. Astronomers use arcminutes and arcseconds to define the angular resolution of images and the angular size of objects.
Arcsecond: An arcsecond is one 60th of an arcminute. Astronomers use arcminutes and arcseconds to define the angular resolution of images and the angular size of objects.
Array: An array is a line of objects.
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.
AU (Astronomical Unit): One AU is the average distance between the Earth and the Sun. 1 AU = 149.6 million km.
Autumnal Equinox: The autumnal equinox is the equinox which occurs on the 21st of September.
B
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.
Binary Star: A 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.
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.
Blueshift: Blueshift is the shortening of wavelength of light, due to the emitter traveling towards the observer.
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.
C
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.
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.
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.
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.
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.
D
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.
Dark Nebulae: Unlike the diffuse nebulae, dark nebulae have no internal or near by 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.
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.
Double Stars: Double stars are two stars which look like they are near each other, but are not close enough to feel each others gravitational forces. This is normally because they are different distance from Earth, and they only look close because of an optical effect.
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.
E
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.
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 energy it has (higher energy = shorter wavelength).
Electromagnetic Spectrum: The electromagnetic (EM) spectrum is the spectrum containing all of the different types of electromagnetic radiation.
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 (ie 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.
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.
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.
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).
Event Horizon: The event horizon of a black hole is the distance from the physical black hole from which no information (ie light, sound, etc) can escape, as the gravitational pull is too great.
Extrasolar: Extrasolar simply means outside of our solar system. That is, something not gravitationally bound to the Sun.
Exoplanet: "Exoplanet" is short for "extrasolar planet"; a planet existing in a different solar system to our own.
F
Filter: Filters are generally by astronomers when observing to reduce the range of wavelengths they look at.
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.
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.
G
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.
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 nanometres.
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.
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.
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.
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.
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.
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.
H
Halley’s Comet: Halley’s Comet is one of the most famous comets. It has an orbital period of around 76 years, so not many people get to see it twice. The earliest recording of its appearance was in 240 BC in China, but it was named after the first person to realize it was periodic, Edmund Halley.
It was last seen in 1986, and its next appearance is due in 2061, so we don’t have too long to wait!
Heliocentric: Heliocentric means Sun-centered. For example, our solar system is heliocentric.
Hertzsprung-Russell Diagram: The Hertzsprung-Russell diagram is a tool used by astronomers to classify the stars they are observing, and help determine what they are at in their evolution.
The Hertzsprung-Russell diagram is named after two astronomers, Ejnar Hertzsprung and Henry Norris Russell, who, in 1911 and 1913 respectively and independently, examine different samples of stars and plotted their absolute magnitude (luminosity) against their spectral class/colour (temperature) and found that they sat on an almost straight line.
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.
I
Ion: An ion is an atom which is missing one of its electrons. This happens by giving the atom a lot of energy, generally from a photon, and leaves the atoms with a negative charge.
Ionosphere: The ionosphere is the very upper region of the Earth’s atmosphere, which is constantly bombarded by solar wind, electrons and radiation, causing the atoms in this region to become ionised.
Infrared (IR) Radiation: IR radiation is the electromagnetic radiation with the next longest wavelength after visible light. As the name suggests, infrared radiation sit off the red edge of visible light. The Earth's atmosphere prevents most of the IR radiation from reaching the surface, so IR observing has to be done at high altitudes or using space-based telescopes.
The IR band sits between 0.001 and 1 millimetre.
Interferometer/Interferometry: Interferometers work by using signals received at multiple telescopes looking at the same object. All telescopes will receive the same information from the source, but they will have different pollutants (ie light, local radio waves, etc). By correlating, the signals the observers are able to get rid of all the uncommon elements (ie pollutants), and get a much sharper image than they could ever receive just using the one telescope.
An increased distance between the telescopes provides for less common pollutants, and provides a much clearer image. However, as the distance between increases the correlation becomes a more difficult task.
ISM (Inter Stellar Medium): The Inter Stellar Medium (ISM) is the name for the matter which exists between stars. This consists of a range of atoms and molecules. On a larger scale the ISM mainly consists of hydrogen and dust.
J
Jovian: The term Jovian means pertaining to Jupiter, but has been expanded to include all the Jupiter-like planets, or gas giants. The Jovian planets in our solar system are Saturn, Jupiter, Neptune and Uranus.
Jovian planets are characterized by their gas rich composition, strong magnetic fields and the rings which they have around them. The rings are generally composed of ice and ice covered rocks.
K
Kuiper Belt: The Kuiper Belt consists of more than 70,000 small asteroids which orbit out past Neptune and Pluto. The Kuiper Belt also likely the home to most short period comets (like Halley’s Comet).
L
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.
Lightyear: A lightyear is the distance which light travels in one year. 1 ly = 63240 AU = 9461 billion km!
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 dependant on temperature, in that a hotter star is more luminous.
M
Magnitude: The magnitude of an object is a measure of the brightness of the object. Magnitudes are often given as apparent or absolute magnitudes.
Main Sequence: Astronomers generally class stars by where they are placed on the Hertzsprung-Russell diagram. The majority of these sit on a diagonal line, which is called the main sequence. Main sequence stars are characterized by the fact that they have Hydrogen fusion reaction going on in their cores. Most stars spend almost 90% of their lifetime on the main sequence.
Matrix: A matrix is a set of arrays which have been placed next to each other to for a grid.
For example:
Array: Matrix:
X XXXX
X XXXX
X XXXX
X XXXX
Meteor: A meteor is a meteoroid that has entered the Earth atmosphere. Most meteors will burn up before they get to the Earth's surface.
Meteoroid: Meteoroids are small pieces of rock and metal which travel through space.
Moon: The term moon actually refers to any natural satellite of a planet, with substantial mass. Mercury and Venus are the only planets which don’t have a moon.
The proper name for Earth’s moon is Luna.
N
Nanometre: One nanometre is one billionth of a metre.
Nebula: Nebulae are clouds of gas which are generally extended spatially. There are four main types of nebulae: Diffuse Nebulae, Planetary Nebulae, Dark Nebulae and Supernova Remnants.
The term nebula was initially used in pre-telescopic times to describe all non-stellar objects in the sky. The majority of these were later shown to be open clusters, globular clusters and galaxies. The classification has now evolved and nebulae are now classed as pre-stellar or star forming (diffuse and dark nebulae) and post-stellar (planetary nebulae and supernova remnants).
Neutron: Along with protons and electrons, neutrons are one of the building blocks of atoms. Unlike the other two, neutrons have no electric charge.
Neutron Star: A neutron star is the core of massive star, which remains after a supernova explosion. These stars generally contain around one and a half times the mass of our Sun, with a radius of less than 10km. Neutron stars are normally observed in radio, X-ray and gamma-ray bands.
O
Occultation: The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star. A commonly known occultation is the eclipse of the sun by the moon. It can also refer to the observational technique for determining the position or radiant structure of a celestial source being occulted.
Open Star Clusters: An open star cluster is a group of young stars which were formed in a similar region of space and feel each others gravitational forces.
P
Parabola: A parabola is a form of curve, in which all points on the curve have the same focus. A parabola has an eccentricity equal to 1.
Parabolic Orbit: A parabolic orbit is one in which the object escapes the gravitational force of the central body, by following a parabola.
Parabolic Surface: A parabolic surface is a curved surface which takes all incoming light and reflects it to one single point, called the focus point. Parabolic curves are generally used in the construction of radio telescope dishes.
Parsecs: A parsec is a measure of distance astronomers use. 1 parsec = 3.26 lightyears, = 206265 AU.
Photon: Photons are the most basic component of electromagnetic radiation, and are considered to contain no size, mass or electric charge.
Planetary Nebulae: Towards the end of the lifetime of a star with similar mass to that of the Sun, it will eject the outer shell of gasses. These gasses are then heated by the remaining solar core, and then emit light. As the gas will continue to expand, planetary nebulae are only short lived.
This process is one way to make the ISM more metal rich.
Planetesimal: A planetesimal is a small object which orbits the Sun. They are thought to be some of the earliest inhabitants of our solar system, and are possibly the building blocks for the planets. Planetesimals are sometimes called planetoids.
Prism: A prism is an optical device, which uses refraction to split a single beam of light into all the separate colours. This enables astronomers to look at where there might be any absorption or emission from a star. Prisms are usually in a triangle shape, to create the correct angles to separate the different colours.
Proton: The proton is the positively charged building block of atoms. Atoms are defined by the number of protons they have.
Protostar: Protostars are gravitationally bound balls of gas which have yet to commence Hydrogen fusion. Protostars are the stage before a star becomes classified as a star.
Pulsar: A pulsar is a spinning neutron star, which sends beams of radiation out from its magnetic poles. If the beam path crosses our line of view then we see a sudden flash of radiation. Pulsars have amazingly regular periods, which range from days to thousandths of seconds. Pulsars were discovered in radio wavelengths, but have been observed in optical, X-ray and gamma-ray bands.
Q
QSO (Quasi-Stellar Object): QSO is a newer term for quasars, but includes quasar-like objects with no radio emission.
Quasar: Quasars are a distant, star-sized radiation source, which emit excessively in the UV band. Quasars have typically been detected by their radio emission, but have since been reclassified as QSOs to include not found with radio emission.
R
Radar: A method of detecting distant objects and determining their position, velocity, or other characteristics by analysis of very high frequency radio waves reflected from their surfaces, and the equipment used in such detection.
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 millimetre.
Radio Quiet Zones: Radio Quiet Zones are areas in which the radio emissions from local sources (ie radio and television stations and military communications) are limited to certain frequencies. This enables the RFI caused by controllable local sources to minimized, and increase the quality of observing. The establishment of Radio Quiet Zones is an important part of the construction of new radio telescope, such as those at the Mileura site.
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.
Refraction: Refraction is what happens to light when it passes from one medium to another (ie going from air into glass or water). If the densities of the two medium differ by a large amount the light is bent.
The best example of refraction is to look at a spoon in a glass of water. It appears to look bent. This is because the light is not coming directly to your eyes; it has to go through the water, then the glass and then the air (3 different densities).
Resolve: An object is said to be resolvable if you are able to see detail with its structure.
For example, the Moon is resolvable (or can be resolved) with the naked eye, as we are able to the craters on its surface.
RFI (Radio Frequency Interference): RFI is an unfortunate by-product of technological advancements and population increases. RFI is simply radio signals from any terrestrial object or emitter. These can come from sources such as radio and television stations, to the computers running the telescopes. RFI mitigation (reduction) is an important part of radio astronomy as the signals received from stars and galaxies are often much smaller than the noise that pollutes the observations. To combat this, radio telescopes are often built in Radio Quiet Zones and have the observatories shielded so they emit no radiation. However, the ever increasing population and ever advancing technologies make RFI mitigation more important for observers as we advance towards the SKA and beyond.
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.
S
Satellite: A satellite is any object which is in a stable orbit around a planet or moon. Moons are themselves considered natural satellites of their host planets.
Semi-Major Axis: The semi-major axis is the longest distance from the centre to the outside of an ellipse. This is half the distance across the longest part of the ellipse.
Semi-Minor Axis: The semi-minor axis is the shortest distance from the centre to the outside of an ellipse. This is half the distance across the shortest part of the ellipse.
Shell: Outside of the core of a star, there are discrete layers of gas which are heated and mixed to different levels. One can think of a star as like an onion, with many layers which hotter as peel towards to central core.
Sidereal Time: The sidereal time is simply the time measured with respect to the stars, not the Sun. One sidereal day is equal to 23 hours and 56 minutes, which is how long is takes the Earth to rotate once, so the sidereal time can be approximated to the right ascension which is directly overhead.
Sinusoidal Wave: Sinusoidal waves are waves which have consistent and smooth undulations and periods.
Solar Cell: A semiconductor device that converts the energy of sunlight into electric energy. Also called photovoltaic cell.
Solar Flare: A solar flare is an enormous explosion in the solar atmosphere. It results in sudden bursts of particle acceleration, heating of plasma to tens of millions of degrees, and the eruption of large amounts of solar mass. Flares are believed to result from the abrupt release of the energy stored in magnetic fields in the zone around sunspots.
Solar Wind: Solar wind is the continuous stream of ions that the Sun emits into space. Because the Sun is rotating the solar wind is given off in a kind of pinwheel shape, and takes 5 days to get to Earth.
Solstice: Solstices are the days when the Sun reaches its most northern and southern points. These occur on December 21 for the southern solstice and June 21 for the northern solstice. Depending on whether you’re in the northern or southern hemisphere these are considered to be the summer and winter solstices, and mark the longest and shortest periods of daylight respectively.
Spectral Class: Before the advent of modern telescopes, and exact ways of measuring the luminosity of stars, they were classified by their colour. This was largely dependant on the way it looked through a telescope, as opposed to its physical properties. Spectral classes have been replaced by temperature and colour classifications.
Spectrograph: A spectrograph is a device which separates a single beam into a spectrum, and records the intensity for each wavelength. Spectrographs are the main tool used when observing for the purpose of spectography. The wavelength which is recorded can be set by the observer, so they can either obtain a broad picture of the spectrum, or a fine detail picture of a small section.
Spectography: Spectography is the study of looking at the spectrum of a star to determine is chemical composition, age and/or mass.
Spectrum: A spectrum is what we see when we look at a large range of wavelengths at once. This is normally done by using some form of splitting prism, place between the eyepiece of the telescope and the recording device.
Sun Spots: A relatively dark area on the surface of the sun consisting of a dark central umbra surrounded by a penumbra, which is intermediate in brightness between the umbra and the surrounding photosphere. Sunspots are often nearly circular with a typical dimension of 20 000 km. The strongest solar magnetic fields, up to 4000 gauss, are found within the umbra. Sunspots usually occur in pairs with opposite magnetic polarities.
Supernova: There are two types of supernovae: Type I and Type II. Both of these have very different causes, but end with a very violet but beautiful explosion.
Type I: In falling matter on a white dwarf can cause the star to explode and eject the excess matter back into the interstellar medium.
Type II: At the end of a massive (more than 3 times bigger than our Sun) stars life time, after the giant phase, it will explode with a flash up to 10 billion times brighter than our Sun. This flash expels most of the outer shell of the star, leaving a small core.
Supernova Remnants: In the event of both a Type I and Type II supernova, the matter which is ejected into space is lit up by process. As supernova remnants expand they will eventually fade and recombine with the ISM.
This process is one way to make the ISM more metal rich.
Syzygy: The alignment of three celestial bodies in the same gravitational system along a straight line. The word is usually used in context with the Sun, Earth, and the Moon or a planet, where the latter is in conjunction or opposition. Solar and lunar eclipses occur at times of syzygy, as do transits and occultations. The term is also applied to each instance of New Moon or Full Moon when Sun and Moon are in conjunction or opposition, even though they are not precisely on one line with the Earth.
T
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.
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.
U
UV (UltraViolet) Radiation: UV light is the form of radiation which has the next shortest wavelength after visible light, and sits off to the blue side. Observing in UV bands with ground-based telescopes is not easy because of the ozone layer which keeps us from getting sun burnt.
UV light has wavelengths from 20 to 400 nanometres.
Universal Time (UT): The universal time (UT) is the same as Greenwich Mean Time (GMT), which is the local time in the time zone of Greenwich, England.
V
Vernal Equinox: The vernal equinox is the equinox which occurs on the 21st of March.
Visible Spectrum: The visible spectrum is the section of the electromagnetic spectrum which corresponds to the light which we can see with the naked eye.
The visible spectrum has a wavelength range from 400 to 700 nanometres.
Visual Binary: Visual binaries are binary stars which are far enough apart to be seen as separate objects.
VLBI (Very Long Baseline Interferometry): VLBI involves the same principals as standard interferomtry, but over a much larger scale. The main problem with VLBI is the correlation of the data, so instead the data is collected at each of the individual telescopes, and the correlation is done after observing.
VLT (Very Large Telescope): The VLT consists of a system of four separate optical telescopes (the Antu telescope, the Kueyen telescope, the Melipal telescope, and the Yepun telescope) organized in an array formation. Each telescope has an 8.2 m aperture. VLT is located at the Paranal Observatory on Cerro Paranal, a 2,635 m high mountain in the Atacama desert in northern Chile.
W
Wavelength: All electromagnetic radiation can be thought of a sinusoidal wave traveling through space, which each of the different bands having a unique wavelength. This is the length between two consecutive peaks or troughs.
White Dwarf: A white dwarf is a star which gone through the majority of its stellar evolution, and has collapsed to a small star. They are about one hundredth the size of our Sun, but with a similar mass. To put it in perspective, white dwarfs have a density about one million times that of water!
X
X-Rays: X-rays are the second shortest wavelength form of electromagnetic radiation, and sit between gamma-rays and UV rays. The Earths atmosphere blocks out the majority of X-ray radiation, so telescopes need to be space-based to observe in this band.
X-rays have wavelengths between 0.01 and 20 nanometres.
Y
Z
Zenith: The zenith is the point directly overhead.
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
A
Absolute Magnitude: The absolute magnitude of a star is the apparent magnitude which we would see it at if it was 10 parsecs from Earth.
Absolute Zero: Absolute zero is the coldest it is possible for anything to get, and is equal to 0 K, or -273 degrees Celsius.
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.
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 arcsecond resolution it would not be able to see any of the detail in any object that takes up less than 1 arcsecond of sky. Any object larger than 1 arcsecond is then said to be resolvable.
Angular Size: The angular size of an object is how big the object appears in the sky.
Apparent Magnitude: The apparent magnitude of a star is a classification astronomers use that tells how bright a star appears in the sky. The scale for magnitude is backwards, so brighter stars are more negative.
Arcminute: An arcminute is one 60th of a degree. Astronomers use arcminutes and arcseconds to define the angular resolution of images and the angular size of objects.
Arcsecond: An arcsecond is one 60th of an arcminute. Astronomers use arcminutes and arcseconds to define the angular resolution of images and the angular size of objects.
Array: An array is a line of objects.
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.
AU (Astronomical Unit): One AU is the average distance between the Earth and the Sun. 1 AU = 149.6 million km.
Autumnal Equinox: The autumnal equinox is the equinox which occurs on the 21st of September.
B
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.
Binary Star: A 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.
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.
Blueshift: Blueshift is the shortening of wavelength of light, due to the emitter traveling towards the observer.
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.
C
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.
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.
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.
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.
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.
D
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.
Dark Nebulae: Unlike the diffuse nebulae, dark nebulae have no internal or near by 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.
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.
Double Stars: Double stars are two stars which look like they are near each other, but are not close enough to feel each others gravitational forces. This is normally because they are different distance from Earth, and they only look close because of an optical effect.
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.
E
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.
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 energy it has (higher energy = shorter wavelength).
Electromagnetic Spectrum: The electromagnetic (EM) spectrum is the spectrum containing all of the different types of electromagnetic radiation.
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 (ie 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.
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.
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.
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).
Event Horizon: The event horizon of a black hole is the distance from the physical black hole from which no information (ie light, sound, etc) can escape, as the gravitational pull is too great.
Extrasolar: Extrasolar simply means outside of our solar system. That is, something not gravitationally bound to the Sun.
Exoplanet: "Exoplanet" is short for "extrasolar planet"; a planet existing in a different solar system to our own.
F
Filter: Filters are generally by astronomers when observing to reduce the range of wavelengths they look at.
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.
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.
G
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.
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 nanometres.
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.
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.
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.
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.
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.
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.
H
Halley’s Comet: Halley’s Comet is one of the most famous comets. It has an orbital period of around 76 years, so not many people get to see it twice. The earliest recording of its appearance was in 240 BC in China, but it was named after the first person to realize it was periodic, Edmund Halley.
It was last seen in 1986, and its next appearance is due in 2061, so we don’t have too long to wait!
Heliocentric: Heliocentric means Sun-centered. For example, our solar system is heliocentric.
Hertzsprung-Russell Diagram: The Hertzsprung-Russell diagram is a tool used by astronomers to classify the stars they are observing, and help determine what they are at in their evolution.
The Hertzsprung-Russell diagram is named after two astronomers, Ejnar Hertzsprung and Henry Norris Russell, who, in 1911 and 1913 respectively and independently, examine different samples of stars and plotted their absolute magnitude (luminosity) against their spectral class/colour (temperature) and found that they sat on an almost straight line.
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.
I
Ion: An ion is an atom which is missing one of its electrons. This happens by giving the atom a lot of energy, generally from a photon, and leaves the atoms with a negative charge.
Ionosphere: The ionosphere is the very upper region of the Earth’s atmosphere, which is constantly bombarded by solar wind, electrons and radiation, causing the atoms in this region to become ionised.
Infrared (IR) Radiation: IR radiation is the electromagnetic radiation with the next longest wavelength after visible light. As the name suggests, infrared radiation sit off the red edge of visible light. The Earth's atmosphere prevents most of the IR radiation from reaching the surface, so IR observing has to be done at high altitudes or using space-based telescopes.
The IR band sits between 0.001 and 1 millimetre.
Interferometer/Interferometry: Interferometers work by using signals received at multiple telescopes looking at the same object. All telescopes will receive the same information from the source, but they will have different pollutants (ie light, local radio waves, etc). By correlating, the signals the observers are able to get rid of all the uncommon elements (ie pollutants), and get a much sharper image than they could ever receive just using the one telescope.
An increased distance between the telescopes provides for less common pollutants, and provides a much clearer image. However, as the distance between increases the correlation becomes a more difficult task.
ISM (Inter Stellar Medium): The Inter Stellar Medium (ISM) is the name for the matter which exists between stars. This consists of a range of atoms and molecules. On a larger scale the ISM mainly consists of hydrogen and dust.
J
Jovian: The term Jovian means pertaining to Jupiter, but has been expanded to include all the Jupiter-like planets, or gas giants. The Jovian planets in our solar system are Saturn, Jupiter, Neptune and Uranus.
Jovian planets are characterized by their gas rich composition, strong magnetic fields and the rings which they have around them. The rings are generally composed of ice and ice covered rocks.
K
Kuiper Belt: The Kuiper Belt consists of more than 70,000 small asteroids which orbit out past Neptune and Pluto. The Kuiper Belt also likely the home to most short period comets (like Halley’s Comet).
L
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.
Lightyear: A lightyear is the distance which light travels in one year. 1 ly = 63240 AU = 9461 billion km!
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 dependant on temperature, in that a hotter star is more luminous.
M
Magnitude: The magnitude of an object is a measure of the brightness of the object. Magnitudes are often given as apparent or absolute magnitudes.
Main Sequence: Astronomers generally class stars by where they are placed on the Hertzsprung-Russell diagram. The majority of these sit on a diagonal line, which is called the main sequence. Main sequence stars are characterized by the fact that they have Hydrogen fusion reaction going on in their cores. Most stars spend almost 90% of their lifetime on the main sequence.
Matrix: A matrix is a set of arrays which have been placed next to each other to for a grid.
For example:
Array: Matrix:
X XXXX
X XXXX
X XXXX
X XXXX
Meteor: A meteor is a meteoroid that has entered the Earth atmosphere. Most meteors will burn up before they get to the Earth's surface.
Meteoroid: Meteoroids are small pieces of rock and metal which travel through space.
Moon: The term moon actually refers to any natural satellite of a planet, with substantial mass. Mercury and Venus are the only planets which don’t have a moon.
The proper name for Earth’s moon is Luna.
N
Nanometre: One nanometre is one billionth of a metre.
Nebula: Nebulae are clouds of gas which are generally extended spatially. There are four main types of nebulae: Diffuse Nebulae, Planetary Nebulae, Dark Nebulae and Supernova Remnants.
The term nebula was initially used in pre-telescopic times to describe all non-stellar objects in the sky. The majority of these were later shown to be open clusters, globular clusters and galaxies. The classification has now evolved and nebulae are now classed as pre-stellar or star forming (diffuse and dark nebulae) and post-stellar (planetary nebulae and supernova remnants).
Neutron: Along with protons and electrons, neutrons are one of the building blocks of atoms. Unlike the other two, neutrons have no electric charge.
Neutron Star: A neutron star is the core of massive star, which remains after a supernova explosion. These stars generally contain around one and a half times the mass of our Sun, with a radius of less than 10km. Neutron stars are normally observed in radio, X-ray and gamma-ray bands.
O
Occultation: The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star. A commonly known occultation is the eclipse of the sun by the moon. It can also refer to the observational technique for determining the position or radiant structure of a celestial source being occulted.
Open Star Clusters: An open star cluster is a group of young stars which were formed in a similar region of space and feel each others gravitational forces.
P
Parabola: A parabola is a form of curve, in which all points on the curve have the same focus. A parabola has an eccentricity equal to 1.
Parabolic Orbit: A parabolic orbit is one in which the object escapes the gravitational force of the central body, by following a parabola.
Parabolic Surface: A parabolic surface is a curved surface which takes all incoming light and reflects it to one single point, called the focus point. Parabolic curves are generally used in the construction of radio telescope dishes.
Parsecs: A parsec is a measure of distance astronomers use. 1 parsec = 3.26 lightyears, = 206265 AU.
Photon: Photons are the most basic component of electromagnetic radiation, and are considered to contain no size, mass or electric charge.
Planetary Nebulae: Towards the end of the lifetime of a star with similar mass to that of the Sun, it will eject the outer shell of gasses. These gasses are then heated by the remaining solar core, and then emit light. As the gas will continue to expand, planetary nebulae are only short lived.
This process is one way to make the ISM more metal rich.
Planetesimal: A planetesimal is a small object which orbits the Sun. They are thought to be some of the earliest inhabitants of our solar system, and are possibly the building blocks for the planets. Planetesimals are sometimes called planetoids.
Prism: A prism is an optical device, which uses refraction to split a single beam of light into all the separate colours. This enables astronomers to look at where there might be any absorption or emission from a star. Prisms are usually in a triangle shape, to create the correct angles to separate the different colours.
Proton: The proton is the positively charged building block of atoms. Atoms are defined by the number of protons they have.
Protostar: Protostars are gravitationally bound balls of gas which have yet to commence Hydrogen fusion. Protostars are the stage before a star becomes classified as a star.
Pulsar: A pulsar is a spinning neutron star, which sends beams of radiation out from its magnetic poles. If the beam path crosses our line of view then we see a sudden flash of radiation. Pulsars have amazingly regular periods, which range from days to thousandths of seconds. Pulsars were discovered in radio wavelengths, but have been observed in optical, X-ray and gamma-ray bands.
Q
QSO (Quasi-Stellar Object): QSO is a newer term for quasars, but includes quasar-like objects with no radio emission.
Quasar: Quasars are a distant, star-sized radiation source, which emit excessively in the UV band. Quasars have typically been detected by their radio emission, but have since been reclassified as QSOs to include not found with radio emission.
R
Radar: A method of detecting distant objects and determining their position, velocity, or other characteristics by analysis of very high frequency radio waves reflected from their surfaces, and the equipment used in such detection.
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 millimetre.
Radio Quiet Zones: Radio Quiet Zones are areas in which the radio emissions from local sources (ie radio and television stations and military communications) are limited to certain frequencies. This enables the RFI caused by controllable local sources to minimized, and increase the quality of observing. The establishment of Radio Quiet Zones is an important part of the construction of new radio telescope, such as those at the Mileura site.
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.
Refraction: Refraction is what happens to light when it passes from one medium to another (ie going from air into glass or water). If the densities of the two medium differ by a large amount the light is bent.
The best example of refraction is to look at a spoon in a glass of water. It appears to look bent. This is because the light is not coming directly to your eyes; it has to go through the water, then the glass and then the air (3 different densities).
Resolve: An object is said to be resolvable if you are able to see detail with its structure.
For example, the Moon is resolvable (or can be resolved) with the naked eye, as we are able to the craters on its surface.
RFI (Radio Frequency Interference): RFI is an unfortunate by-product of technological advancements and population increases. RFI is simply radio signals from any terrestrial object or emitter. These can come from sources such as radio and television stations, to the computers running the telescopes. RFI mitigation (reduction) is an important part of radio astronomy as the signals received from stars and galaxies are often much smaller than the noise that pollutes the observations. To combat this, radio telescopes are often built in Radio Quiet Zones and have the observatories shielded so they emit no radiation. However, the ever increasing population and ever advancing technologies make RFI mitigation more important for observers as we advance towards the SKA and beyond.
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.
S
Satellite: A satellite is any object which is in a stable orbit around a planet or moon. Moons are themselves considered natural satellites of their host planets.
Semi-Major Axis: The semi-major axis is the longest distance from the centre to the outside of an ellipse. This is half the distance across the longest part of the ellipse.
Semi-Minor Axis: The semi-minor axis is the shortest distance from the centre to the outside of an ellipse. This is half the distance across the shortest part of the ellipse.
Shell: Outside of the core of a star, there are discrete layers of gas which are heated and mixed to different levels. One can think of a star as like an onion, with many layers which hotter as peel towards to central core.
Sidereal Time: The sidereal time is simply the time measured with respect to the stars, not the Sun. One sidereal day is equal to 23 hours and 56 minutes, which is how long is takes the Earth to rotate once, so the sidereal time can be approximated to the right ascension which is directly overhead.
Sinusoidal Wave: Sinusoidal waves are waves which have consistent and smooth undulations and periods.
Solar Cell: A semiconductor device that converts the energy of sunlight into electric energy. Also called photovoltaic cell.
Solar Flare: A solar flare is an enormous explosion in the solar atmosphere. It results in sudden bursts of particle acceleration, heating of plasma to tens of millions of degrees, and the eruption of large amounts of solar mass. Flares are believed to result from the abrupt release of the energy stored in magnetic fields in the zone around sunspots.
Solar Wind: Solar wind is the continuous stream of ions that the Sun emits into space. Because the Sun is rotating the solar wind is given off in a kind of pinwheel shape, and takes 5 days to get to Earth.
Solstice: Solstices are the days when the Sun reaches its most northern and southern points. These occur on December 21 for the southern solstice and June 21 for the northern solstice. Depending on whether you’re in the northern or southern hemisphere these are considered to be the summer and winter solstices, and mark the longest and shortest periods of daylight respectively.
Spectral Class: Before the advent of modern telescopes, and exact ways of measuring the luminosity of stars, they were classified by their colour. This was largely dependant on the way it looked through a telescope, as opposed to its physical properties. Spectral classes have been replaced by temperature and colour classifications.
Spectrograph: A spectrograph is a device which separates a single beam into a spectrum, and records the intensity for each wavelength. Spectrographs are the main tool used when observing for the purpose of spectography. The wavelength which is recorded can be set by the observer, so they can either obtain a broad picture of the spectrum, or a fine detail picture of a small section.
Spectography: Spectography is the study of looking at the spectrum of a star to determine is chemical composition, age and/or mass.
Spectrum: A spectrum is what we see when we look at a large range of wavelengths at once. This is normally done by using some form of splitting prism, place between the eyepiece of the telescope and the recording device.
Sun Spots: A relatively dark area on the surface of the sun consisting of a dark central umbra surrounded by a penumbra, which is intermediate in brightness between the umbra and the surrounding photosphere. Sunspots are often nearly circular with a typical dimension of 20 000 km. The strongest solar magnetic fields, up to 4000 gauss, are found within the umbra. Sunspots usually occur in pairs with opposite magnetic polarities.
Supernova: There are two types of supernovae: Type I and Type II. Both of these have very different causes, but end with a very violet but beautiful explosion.
Type I: In falling matter on a white dwarf can cause the star to explode and eject the excess matter back into the interstellar medium.
Type II: At the end of a massive (more than 3 times bigger than our Sun) stars life time, after the giant phase, it will explode with a flash up to 10 billion times brighter than our Sun. This flash expels most of the outer shell of the star, leaving a small core.
Supernova Remnants: In the event of both a Type I and Type II supernova, the matter which is ejected into space is lit up by process. As supernova remnants expand they will eventually fade and recombine with the ISM.
This process is one way to make the ISM more metal rich.
Syzygy: The alignment of three celestial bodies in the same gravitational system along a straight line. The word is usually used in context with the Sun, Earth, and the Moon or a planet, where the latter is in conjunction or opposition. Solar and lunar eclipses occur at times of syzygy, as do transits and occultations. The term is also applied to each instance of New Moon or Full Moon when Sun and Moon are in conjunction or opposition, even though they are not precisely on one line with the Earth.
T
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.
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.
U
UV (UltraViolet) Radiation: UV light is the form of radiation which has the next shortest wavelength after visible light, and sits off to the blue side. Observing in UV bands with ground-based telescopes is not easy because of the ozone layer which keeps us from getting sun burnt.
UV light has wavelengths from 20 to 400 nanometres.
Universal Time (UT): The universal time (UT) is the same as Greenwich Mean Time (GMT), which is the local time in the time zone of Greenwich, England.
V
Vernal Equinox: The vernal equinox is the equinox which occurs on the 21st of March.
Visible Spectrum: The visible spectrum is the section of the electromagnetic spectrum which corresponds to the light which we can see with the naked eye.
The visible spectrum has a wavelength range from 400 to 700 nanometres.
Visual Binary: Visual binaries are binary stars which are far enough apart to be seen as separate objects.
VLBI (Very Long Baseline Interferometry): VLBI involves the same principals as standard interferomtry, but over a much larger scale. The main problem with VLBI is the correlation of the data, so instead the data is collected at each of the individual telescopes, and the correlation is done after observing.
VLT (Very Large Telescope): The VLT consists of a system of four separate optical telescopes (the Antu telescope, the Kueyen telescope, the Melipal telescope, and the Yepun telescope) organized in an array formation. Each telescope has an 8.2 m aperture. VLT is located at the Paranal Observatory on Cerro Paranal, a 2,635 m high mountain in the Atacama desert in northern Chile.
W
Wavelength: All electromagnetic radiation can be thought of a sinusoidal wave traveling through space, which each of the different bands having a unique wavelength. This is the length between two consecutive peaks or troughs.
White Dwarf: A white dwarf is a star which gone through the majority of its stellar evolution, and has collapsed to a small star. They are about one hundredth the size of our Sun, but with a similar mass. To put it in perspective, white dwarfs have a density about one million times that of water!
X
X-Rays: X-rays are the second shortest wavelength form of electromagnetic radiation, and sit between gamma-rays and UV rays. The Earths atmosphere blocks out the majority of X-ray radiation, so telescopes need to be space-based to observe in this band.
X-rays have wavelengths between 0.01 and 20 nanometres.
Y
Z
Zenith: The zenith is the point directly overhead.