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Glossary for Astronomy
by Peter Eyland
This is a glossary based on a number of presentations, given by Peter Eyland on Astronomy. These include talks to members of the general public, amateur astronomers, local groups (like schools, probus etc), Motorhomers at Rallies, and to travellers on Princess cruises. Updated 26 September 2020
Jump to Top ★ B ★ F ★ H ★ I ★ K ★ M ★ N ★ Q ★ S ★ End of Glossary ★
★ Absolute Magnitude How bright a star would appear, in magnitude, at a standard distance of 10 parsecs (32.6 light years) from Earth.
★ Accretion disc A flat sheet of gas and dust surrounding a newborn star, a black hole, or any massive object growing in size by gravitationally attracting material.
★ Active Galaxy Nuclei (AGN) A galaxy with unusually high energy output compared with a normal galaxy.
★ Albedo The ratio of the amount of light reflected from a Solar System object to that received by it. A perfectly reflecting object has an albedo of 1.0 , or 100%.
★ Algol A variable star of a class known as eclipsing variables. Algol’s brightness fluctuates every 69 hours as it is eclipsed by its fainter companion.
★ Alt-azimuth coordinates The angular height (altitude) of an object above or below the horizon and its angular direction (azimuth) from north measured towards the east.
★ Alt-azimuth mount A simple two-axis mount used for supporting & rotating a telescope or other instrument about two perpendicular axes: one vertical & one horizontal. The two axes of movement are altitude (up/down) or azimuth (left/right). An example is the simple "point & view" Dobsonian mount.
★ Altitude The angular elevation of an object above or below the horizon. This is measured in degrees.
★ Analemma Plotted positions of the Sun on the celestial sphere at the same time of day (at approximately 24 hour intervals) and from the same location on Earth on successive days through the calendar year. This appears as a figure eight. An analemma is a visual representation or illustration of the equation of time. Historically, Analemma is a sundial on top of a column. Measured at the same time each day, the shadow of the tip of the sundial traces out the yearly path of the sun on the ground. The shape is a figure of eight. This shape defines (expresses) the "equation of time". This is a complicated formula (that is not covered here).
★ Angular diameter The apparent diameter of an object measured in degrees or radians.
★ Angular separation The angular distance between two celestial bodies measured in degrees.
★ Aperture In optics, this is the size of the objective opening through which light enters. Together, aperture & focal length, determine the resolving power (when two objects can be seen separately) through the telescope. It can be the diameter of a telescope’s objective lens or mirror.
★ Aphelion The farthest distance from the Sun on a planet or comet’s orbit.
★ Apogee The point at which a body in orbit around the Earth reaches its farthest distance from the Earth. It is the opposite to Perigee.
★ Apparent magnitude The brightness of a star (or any celestial object) as seen from Earth.
★ Apparition A period when a planet or comet is visible in the night sky.
★ Arcminute A unit of angular measurement used in astronomy: 60 arcminutes is equal to 1 degree. The disc of the Moon is approximately 30 arcminutes across.
★ Arcsecond A unit of angular measurement used in astronomy: 60 arcseconds is equal to 1 arcminute.
★ Armillary Sphere An armillary sphere is a model of the celestial globe constructed from rings & hoops representing the equator, the tropics & the other celestial circles, & able to revolve on its axis. Used by Ptolemy & early medieval Muslim astronomers.
★ Asterism A recognizable pattern or shape created by a group of stars – smaller in area than a constellation. A good example is the Teapot, which is part of the larger constellation of Sagittarius.
★ Asteroid Usually with a diameter of less than 1,000 km orbiting the Sun, an asteroid is a lump of rock or ice drifting around in space. Some are huge and some are small. When a small one falls into the Earth’s atmosphere at high speed, it heats up and glows brightly. It is then called a meteor, or shooting star. If it hits the ground in one piece, it is called a meteorite.
★ Asteroid belt Also called the Main Belt. The large number of asteroids between the orbits of Mars and Jupiter.
★ Astrometry The study of the precise measurement of positions and motions of celestial bodies.
★ Astronomical Unit (au or ua) The average distance between Earth and the Sun – roughly 149.6 million km. See Units of Distance for a nice diagram.
★ Astrophotography Photography of the night sky and the objects in it.
★ Astrophysics Study of the dynamics, chemical properties, and evolution of celestial bodies.
★ Atmosphere The layer of gas enveloping a celestial object.
★ Aurora Curtains and arcs of light in the sky visible over mid-to-high latitudes. High velocity charged particles from the Sun are deflected by the Earth’s magnetic field and enter the Earth’s atmosphere and cause some of its gases to glow. in the Southern Hemisphere, they are called the Aurora Australis or the Southern Lights.
★ Averted vision A useful method for increasing the contrast of of faint targets by looking slightly to one side of the object.
★ Axis The imaginary line through the centre of a planet, star, or galaxy around which it rotates; also, a similar line through a telescope mount.
★ Azimuth Horizontal coordinate of an object’s position in the sky. Derived by drawing an imaginary vertical line from the object to the horizon below. The position is then expressed in degrees east from the north point.
★ Big Bang Theory One theory to describe the creation of the universe as we know it from a singularity where the laws of Physics break down. It is estimated 13.8 billion years ago. See nice diagram under Cosmology.
★ Binary star Two stars linked by mutual gravity and revolving around a common centre of mass, as opposed to the chance alignment of two stars as seen from Earth.
★ Binoculars Two identical telescopes, one for each eye, are mounted side-by-side and aligned to point accurately in the same direction.
★ Black hole A massive object so dense that no light or other radiation can escape from it. They can only be seen by what happens around it.
★ Blue Shift When a star is moving towards an observer, its spectral lines are shifted towards the blue end of its spectrum (shorter wavelengths).
★ Bolide A very bright, long-duration fireball that explodes or appears to fragment. They are quiet rare.
★ Brightness see Magnitude.
★ Catadioptric telescope A telescope (such as a Maksutov or Schmidt-Cassegrain) that uses a combination of mirrors and lenses to form an image. "Cata" means down, "di" means two, so this means light travels twice down the barrel of the telescope. It combines both mirrors & corrector lenses to form an image. The double travel of light down the barrel means they are shorter, & they can also have a greater degree of error correction & a wider aberration-free field of view.
★ CCD (charge-coupled device) A light-sensitive electronic detector that collects light in a grid pattern and can record an image more conveniently than photographic film.
★ Celestial Referring to the sky and/or heavens.
★ Celestial equator The imaginary line encircling the sky midway between the two celestial poles.
★ Celestial poles The imaginary points on the sky where Earth’s rotation axis, extended infinitely, would touch the imaginary celestial sphere. NCP = North Celestial Pole and SCP = South Celestial Pole.
★ Celestial sphere The imaginary sphere enveloping the Earth, upon which the stars, galaxies, and other celestial objects all appear to lie. See Declination, and Right Ascension. The First Point in Aries 
is considered to be the celestial prime meridian from which Right Ascension, in hours, is calculated.
★ Chromosphere In the Sun, the thin layer of atmosphere lying above the photosphere (visible surface), and below the corona. Usually can only be seen during an eclipse.
★ Circumpolar Objects located near either the celestial north or south pole which never set when viewed from a given location.
★ Collimation The procedure of aligning a telescope’s optics.
★ Colour index It is a numerical measure of the colour (i.e. temperature) of a star. The difference in the magnitudes of a celestial object measured at two different wavelengths.
★ Coma The head of a comet, consisting of a cloud of gas and dust that forms around its nucleus as the comet heats up on its approach to the Sun.
★ Comet A small body composed of ices and dust that orbits the Sun on an elongated path. When near the Sun, evaporating ices produce a large coma and long gas and dust tails.
★ Conjunction The moment when two celestial bodies appear near one another in the sky.
★ Constellation One of the 88 precise geometric boundaries that encompass all of the stars within them. This was made official by the International Astronomical Union (IAU) in 1933.
Here are four easy to recognise constellations in the Southern Sky. They include the orientation you would likely see them in the sky (say on 11 October 2017). The images were taken from an astronomy app, and enhanced by Peter by adding the colours and the magnitude of the stars. Notice that doubles, optical doubles and multiples are shown.
First Example: Crux
Second Example:Scorpius
Third Example: Sagittarius
Fourth Example: Orion
★ Corona The high-temperature outermost atmosphere of the Sun, visible from Earth only during a total solar eclipse.
★ Cosmology The study of the large-scale structure and evolution of the whole Universe. It includes how the universe was formed, how it developed and how it will eventually end. The following diagram represents this, and shows the seven stages. See also Big Bang Theory.
★ Cosmic Microwave Background Radiation In 1965 Penzias & Wilson investigated radio waves from various parts of the universe with an old radar horn. They found microwave signals coming uniformly from everywhere in the sky. They initially measured the signals to have a peak wavelength of 73 mm. This corresponded to a Universe with an average temperature of 4.2 Kelvin (-268.95°C). Later revised to a peak wavelength of 1.063mm & an average temperature of 2.7 Kelvin (-270.45° C). This is is used by Cosmologists to determine early features of the Cosmos, and supports the Big Bang Theory.
★ Crescent The phase of a moon or planet when it appears less than a half disc. For example, the Moon is a crescent between Last and First quarter.
★ Culmination The instant when a celestial body crosses your meridian, ( i.e. the north/south line that passes though your zenith). An object culminates when it reaches its highest point above the observer’s horizon. (See nice image under local meridian).
★ Dark adaption The process by which the human eye increases sensitivity under conditions of low (or no) illumination.
★ Declination The angular distance of a celestial object north or south of the celestial equator. It corresponds to latitude on Earth. Declination is measured in degrees from +90° to -90°. Directly out from the earth’s equator is the celestial equator which is a declination of zero degrees. Directly below the South Pole is the -90° declination. See nice diagram under Celestial Sphere.
★ Deep space Anywhere outside the Solar System.
★ Deep-sky object A celestial object located beyond the Solar System.
★ Digital Setting Circles (DSCs) These are little computers that show you where to point your telescope to locate your astronomical target. You input the target’s coordinates (or find it in the DSC’s catalogue) and the DSC will then show you a readout of numbers that represent the angular distances to your target, your goal being to move your telescope until those numbers are as close to zero as possible. At zero (or close to), your scope will be pointing at your target object. Think of them as a GPS for your telescope. These devices are suitable for most telescopes, to convert them into "push-to" telescopes.
★ Diurnal motion The daily motion of the sky produced by rotation of the Earth, causing the rising and setting of the Sun, Moon, planets and stars.
★ Dobsonian mount A simple type of alt-azimuth mount designed by John Dobson.
★ Doppler Shift This is used in Cosmology & Spectroscopy (see image below).
★ Double star Two stars that appear close together in the sky. Optical doubles are chance alignments of the stars as seen from Earth, while the stars in a binary or multiple system are actually linked by mutual gravity.
★ Dust lane A trail of thin gas and dust among the stars of the galaxy or between galaxies.
★ Dust tail The curved, yellowish tail that streams out behind a comet. See also Ion tail.
★ Dwarf Planet There are five dwarf planets, as defined by the International Astronomical Union (IAU). They are Ceres, Make Make, Pluto, Haumea & Eris. They revolve around the sun, have a generally spherical shape but are surrounded by asteroids or minor planets. See image below. 
★ Dwarf star A star that lies on the Main Sequence on the Hertzsprung-Russell Diagram.
★ Earthshine The faint, grey-blue light seen on the portion of the Moon’s surface not illuminated by the Sun.
★ Eccentricity A measure of how long or thin an ellipse is. The closer the eccentricity is to zero, the more circular the orbit.
★ Eclipse When one celestial body passes in front of another, dimming or obscuring its light. Common eclipses include a solar eclipse or a lunar eclipse. This is not to be confused with a transit (small body moving in front of a larger body) or an occultation (one body going behind another body). See images under Lunar Eclipse and Solar Eclipse.
★ Eclipse of the Moon When the Moon passes into the shadow of the Earth. It is a total eclipse when the Moon is immersed in the umbral shadow, partial if only partly covered by the umbra, and penumbral if the Moon passes only through the penumbra of the Earth’s shadow. See image under Lunar Eclipse.
★ Eclipse of the Sun When the Moon passes in front of the Sun. It is total when the Moon has a larger angular diameter than the Sun and completely covers the disc, annular if smaller (leaving a ring of sunlight surrounding the Moon), and partial if only partly covered. See image under Solar Eclipse.
★ Ecliptic This is the apparent annual path of the sun through the year. It is named as the path of the eclipses. It also is where the dozen common constellations called the zodiac are located across the sky. (Note: the ecliptic line is really useful to find the moon and planets, which apparently align in the sky approximately along this line, due to the almost flat plane of the solar system).
★ Egress The finish of a satellite or shadow transit.
★ Elevation see Altitude.
★ Ellipse The oval, closed path followed by a celestial object moving under the influence of gravity: e.g. a planet around the Sun.
★ Elongation The angular separation between Mercury or Venus and the Sun; also used for satellites and their planet. Most often seen as maximum elongations, describing the greatest separations.
★ Emission nebula A cloud of gas glowing as the gas re-emits energy absorbed from a nearby hot star.
★ End of the Universe In Cosmology, there are a number of scenarios for the end of the Universe. (See image below).
★ Ephemeris (plural ephemerides)- this gives the measured positions of all the objects in the sky. This includes stars, galaxies, planets, moons, and even man made satellites. Note: An ephemeris is different to a planisphere because it gives precise co-ordinates which change every 50 years. It is different to a star atlas which gives representations of surrounding constellations.
★ Epoch A date chosen as a reference point for observations. Epoch 2000.0 is currently used for all coordinate data and is compatible with modern star atlases & ephemerides.
★ Equation of Time The difference between apparent and mean solar time. This is the East or West Component of the Analemma.
★ Equatorial mount A telescope mount with one axis parallel to Earth’s rotational axis, so the motion of the heavens can be followed with a single movement. Equatorial mounts are for telescopes & cameras that compensate for Earth’s rotation by having one rotational axis parallel to the Earth’s axis of rotation. One rotational motion, by hand or motor, will keep the object centred in the viewpoint, which is useful in longer exposure photography.
★ Equinox The two times of the year when the Sun crosses the celestial equator; vernal or spring equinox occurs on or about September 22nd, and autumnal equinox on about March 20th. Is an equinox a day, or a moment in time? It is both. Firstly, as the seasons go, an equinox is either of the two times in the year when day & night are of equal length. This happens around 21 March & 23 September each year. Also an equinox is either of the two points in the sky where the sun’s annual path (ecliptic) & the celestial equator intersect. This is gradually changing as the Earth slowly wobbles on its axis. This is the spring equinox.
This is the path of the sun over the year.
★ Event horizon. The boundary of a "point of no return" near a black hole. When crossed, towards a black hole, not even light can return. See Black Hole.
★ Exit pupil The diameter of the exit pupil can be calculated by dividing the focal length of the eyepiece by the focal ratio (f-number) of the telescope. More simply, it is the diameter of the objective lens (or telescope’s aperture size) divided by the magnification of the eyepiece.
★ Exoplanets are planets that revolve around another star (other than our sun). The first confirmed detection of an exoplanet occurred in 1992. As of 1 September 2018, there are 3,823 confirmed planets in 2,860 systems, with 632 systems having more than one planet. As of 26 September 2020 there are 3730 exoplanets confirmed by NASA.
Exoplanets are deep space objects, they can be seen by larger space telescopes like Kepler, Hubble, Chandra, Spitzer and the soon to be lauched James Webb Space Telescope (JWST).
★ Eye relief of the telescope The distance from the eyepiece to the eye, within which the user’s eye can obtain the full viewing angle. If the viewer’s eye is outside this distance, a reduced field of view is obtained. Or in other words, the distance where a person’s eye can be placed, so that the size of the telescope’s "exit pupil" is approximately the same size as the person’s pupil.
★ Eyepiece A group of small lenses used as a single unit to magnify the image produced by a telescope’s objective, or main, lens or mirror.
Notes on different eyepieces and AFOV (see Apparent Field of View)
Plössl eyepieces are generally designed to have an AFOV of 50°,
DeLites use 62°,
Panoptics use 68°,
Delos use 72°
and Naglers use 82°.
★ Facula A large, bright, irregular patch of slightly hotter material marking an active region in the photosphere of the Sun.
★ Field of view (FOV) Amount of sky visible through the eyepiece of an optical instrument, such as a pair of binoculars or a telescope. There are a couple of different "fields of view" to keep in mind, namely the True Field of View (TFOV) and the Apparent Field of View (AFOV).
1) The True Field of View (aka TFOV) through your telescope can be measured by timing a star, near the celestial equator, as it drifts across the eyepiece from one side to the other. In simple terms, the stars make one complete rotation (360°) in a day (24h). A star on the celestial equator will then appear to move at a speed of (360°/24h) which is 1/4 degree/minute.
For Example: Mintaka, aka Delta Orionis, is a convenient star close to the celestial equator as circled below in Orion’s belt. (The Arabic word Mintaka means a belt. The central star, Alnilam, means a belt of pearls and Alnitak, the eastern star, signifies a band of material around the waist.)
If it took 260 seconds for Mintaka to cross the eyepiece view, the true field of view is given by first converting to minutes (260/60 = 4.33 minutes). Then multiplying by 1/4 degree/minute (i.e. divide by 4), to get 1.08 degrees. The True Field of View would then be just over 1 degree.
2) The Apparent field of view (aka AFOV)
As shown below, there is a metal "ring" inside the barrel of eyepieces which creates a circular hole that is looked through. This is called a "field stop".
The diameter of this circle divided by the eyepiece focal length is an angle in radians and called the apparent field of view. It differs with each manufacturer’s eyepiece design, but sets what can be seen through the eyepiece by itself.
Changing to a second eyepiece with the same apparent field of view, but a shorter focal length, will increase the telescope magnification by spreading an image wider. The result would be that the outer parts of the first image can no longer be seen, i.e. the actual visible area is reduced. The inverse relationships, shown below, between the True Field of View and the magnification are approximate and may overestimate the TFOV.
Notes on different eyepieces and AFOV
Plössl eyepieces are generally designed to have an AFOV of 50°,
DeLites use 62°,
Panoptics use 68°,
Delos use 72°
and Naglers use 82°.
★ Finderscope A small, low-power telescope (or spy glass) attached to and aligned to the main telescope. Its wider field of view makes it useful for locating celestial objects.
★ Fireball Any meteor brighter than Venus (about magnitude -4). See also Bolide.
★ First Point in Aries The First Point in Aries is considered to be the celestial prime meridian from which Right Ascension in hours is calculated. See nice diagram under Celestial Sphere.
★ Focal length The distance, in millimetres, from the optical centre of an objective lens or mirror to the point where light rays converge to form a focused image.
★ Focal ratio (or f-number) A telescope’s focal length divided by its aperture (e.g. 600mm / 75mm = F/8).
Or putting it another way, the focal ratio is the ratio of the focal length of the objective to the aperture (f/a). It tells how much longer the focal length is compared to the aperture.
In the following examples:
a 10" Dobson has an aperture size of 254mm and a focal length of 1200mm
an 8" Celestron Schmidt Cassegrain Telescope (SCT) has an aperture size of 203.2mm and a focal length of 2032mm
For the Dobson (f/a) = 1200/254 = 4.7x and expressed as (f/5)
For the Celestron (f/a) = 2032/203.2 = 10x and expressed as (f/10)
For the same aperture size and eyepiece focal length:
a shorter focal length (e.g. f/5) takes in a wider field of view at prime focus and gives shorter exposure times for photography.
a longer focal length telescope (e.g. f/10) takes in a narrower field of view at prime focus and gives longer exposure times (less chance of aberrations).
★ Fraunhofer lines The dark (absorption) lines in the Sun’s (or other star’s) spectrum caused by specific transitions of electrons within atoms. See also Spectrum for a nice diagram with Fraunhofer lines on it.
★ Galactic equator The great circle along the line of the Milky Way, marking the central plane of our galaxy.
★ Galactic (or open) star cluster A group of some few hundred stars bound by gravity and moving through space together.
★ Galaxy A huge gathering of stars, gas, and dust, bound by gravity and having a mass ranging from 100,000 to 10 trillion times that of the Sun. There are spiral, elliptical, and irregular types of galaxies.
★ Galilean satellites Named after their discoverer, Galileo Galilei. The four brightest satellites of Jupiter: Io, Europa, Ganymede, and Callisto. Also known as the Jovian satellites.
★ Gamma rays High-energy rays emitted from energetic sources such as quasars.
★ Geocentric As viewed or measured from the centre of the Earth.
★ Geocentric universe Under Ptolemy, Aristotle & other ancient civilisations, the cosmos was described with the Earth at the centre. The Sun, Moon, planets & stars all rotated around the Earth. Epicycles were used to show the retrograde movement of planets.
★ Gibbous The phase of a moon or planet when it appears greater than a half disc, but less than a full disc. For example, the Moon is gibbous between First and Last quarter.
★ Globular star cluster A tightly packed, spherical grouping of up to a million older stars.
★ Graben Fault A depressed block of crust bordered by parallel faults formed when a block of ground slips down between the fractures in surrounding rocks. These are seen as a feature on the moon. (see image below)
★ Granulation Mottling of the Sun’s photosphere caused by upwelling cells of convecting gas breaking down into smaller cells.
★ Gravitational Lensing A galaxy or other massive object standing between Earth and a more distant object. Its gravity bends the light from the distant object and creates distorted or multiple images of it. See the following two images.
★ Gravity The tendency of objects with mass to accelerate toward each other. It is the result of space being bent by mass.
★ Heliocentric As viewed or measured from the centre of the Sun.
★ Heliocentric universe Developed under Nicolaus Copernicus & published in 1543, it had the sun at the centre of the Universe, motionless with the Earth & all the other planets rotating around our Sun.
★ Heliosheath The region of space where the solar wind meets the interstellar medium. Expected to be found somewhere beyond Pluto in the Kuiper Belt.
★ Hertzsprung-Russell (HR) Diagram A graph which shows every star in the sky (or in a galaxy or a cluster) whose horizontal axis gives star colour (or temperature) and whose vertical axis gives star luminosity (or absolute magnitude). It was developed independently by two astronomers: Ejnar Hertzsprung and Henry Norris Russell, in the early 1900s.
★ Hour angle The angular measure of the distance of an object from the local meridian. (See nice image under local meridian).
★ Hubble’s Original Data Edwin Hubble found that most of the galaxies were receding from us, and the further away the faster they were going. This is information was used by him to determine that the Universe had a beginning. (See image below of Hubble’s original data.)
★ Hubble Telescope This is a famous space telescope launched into low earth orbit in 1990, named in honour of the man Edwin Hubble. He never used it. It shows breathtaking images that are much clearer than any photographs taken on earth, because it is above earth’s atmosphere.
★ International Astronomical Union (IAU) In 1933 the IAU divided the sky into 88 official constellations surrounding recognized asterisms, with precise geometric boundaries that encompass all of the stars within them. They also voted to demote Pluto from a planet to become a "dwarf planet".
★ Inclination The angle that the plane of the orbit of one astronomical body makes with the plane of the orbit of another. Usually the reference is to the ecliptic.
★ Inferior conjunction A conjunction where two celestial bodies appear close to one another on the same side of the Sun.
★ Inferior (or Interior) planet A planet orbiting the Sun inside Earth’s orbit i.e. Mercury and Venus.
★ Infrared (IR) That portion of the electromagnetic spectrum with wavelengths a little longer than the reddest light visible. It extends from 750 nm (nanometers) to 1.4 μm (micrometers).
★ Ingress The start of a satellite or shadow transit.
★ Ion tail The straight, bluish tail of ionised gas that streams out behind a comet during its time near the Sun. See also Dust tail.
★ Julian date Named after Julius Scaliger (not Julius Caesar). It is the number of days since noon on 1st January 4713 B.C. It is useful for astronomical observations as it saves confusion with other calendars. The starting date chosen was arbitrary, but far enough back in time for there to be no astronomical records prior to then. It is not related to the Julian calendar (which was named after Julius Caesar).
★ Julian Year 365.25 days It is the basis for the definition of the Light Year (ly) as a unit of distance.
★ Kepler’s three laws
★ Kuiper Belt A region of our Solar System, filled with asteroids beyond Neptune. there are a number of trans-Neptunian objects (TNOs) out beyond Neptune, including the dwarf planet Pluto (among others).
★ Kuiper Belt Object (KBO) an object in the Kuiper Belt. For example "New Horizons" is heading towards 2014MU69 which is a KBO. It plans to fly by this object on 1 January 2019.
★ Large Magellanic Cloud (LMC) Satellite galaxy to our own Milky way, appearing to the unaided eye as a large nebulous patch situated in the constellation of Dorada. From mid-southern latitudes the LMC is circumpolar.
★ Lifecycle of Stars See image below.
★ Lifetime of stars The luminosity of a star is a measure of how quickly a star is using up its fuel. Therefore, the lifetime of a star can be estimated by dividing how much fuel it has (its mass) by its luminosity (mass/time).
The lifetime of our Sun has been calculated to be 10 Billion years and with its known mass, this this provides a standard reference.
Hydrostatic equilibrium is maintained when a star’s luminosity increases with its mass. It is found empirically (by observation rather than calculation) that:
A normal star is less than or equal to 10 Solar masses.
Stars that end up more than fifty times the mass of our Sun are very rare. In our galaxy, only a handful are these super-massive stars. They die in giant supernova explosions within a million years of their birth. If the Eddington limit is reached, a strong stellar wind will push away any in-falling matter rather than accreting it.
★ Light curve The change in brightness of a variable star as plotted over time on a graph.
★ Light pollution The glow from artificial city lights, which overpowers the dim glow of fainter targets in the sky.
★ Light year (ly) The distance that light travels in a vacuum during one Julian year: approximately 9.46 trillion km. It is used as a measurement of astronomical distance (not time). It is the distance that light travels in 365.25 days. In scientific notation this is written as 9.461x1012 kilometres. See Units of Distance for a nice diagram.
★ LIGO Acronym for the Laser Interferometer Gravitational Wave Observatory. There are two in the USA, one in Washington (State) and one in Louisiana, separated by 3002 km. They are run by Caltech and MIT.
★ Limb The edge of a celestial body such as the Sun, the Moon, or the planets.
★ Local Group A group of roughly 30 nearby galaxies, including the Milky Way.
★ Local Fluff A dense warm gas cloud, approx 10 ly (light years) across that protects our Sun from deadly radiation.
★ Local Bubble A cloud of low density gas 1630 ly (light years) across surrounding the Local Fluff.
★ Local Meridian is an imaginary line that runs from north to south and goes straight over your head through what we call zenith, a point in the sky directly above you. Also the local meridian defines midday, as it signifies the highest point the Sun and other celestial bodies will reach in their daily arc across the sky. Mariners practicing celestial navigation would take sun-angle observations when the Sun reached the local Meridian to determine their latitude.
★ Luminosity The total intrinsic brightness of a star or galaxy. Measured in Watt /square metre..
★ Lunar Relating to the Moon.
★ Lunar Eclipse This is where the moon goes dark. Occurs when the Moon passes directly behind the Earth and into the Earth’s shadow (Umbria). See image below.
★ Lunation The period of time between two consecutive New Moons.
★ MACHOs Acronym for Massive Compact Compact Halo Objects – supposed dark, massive objects surrounding our galaxy.
★ Magnetic Field Some objects in Space have a magnetic field. The earth’s magnetic field gives us a compass heading on earth, pointing North. Also the earth’s magnetic field protects us from the Solar wind, and enables life to exist here.
★ Magnification The ratio between the apparent size of an object (or its size in an image) and its true size. Calculated by dividing the telescope’s focal length by the focal length of an eyepiece (e.g. 600mm / 25mm = 24 times magnification).
Here is a better explanation.
A simple telescope has a one objective lens and one eyepiece lens, as shown below. The prime focus is where the intermediate image forms. It forms where the objective focal length (ƒo) coincides with the eyepiece focal length (at ƒe).
A telescope makes the stars seem further apart by increasing all the angles. This also makes objects seem bigger. From geometry below, the blue right-angled triangles will enable the magnification to be calculated. The blue triangles have the same intermediate image heights and the tangents are as given in the diagram below.
Therefore, Magnification is the objective focal length divided by the eyepiece focal length.
★ Magnitude An approximately logarithmic unit used to measure the optical brightness of celestial objects. Numerically lower magnitudes are brighter than numerically higher magnitudes. From one magnitude to the next the ratio of brightness is the 5th root of 100, or approximately 2.5 times, so magnitude 1 is 100 times brighter than magnitude 6.
★ Main Belt This is a region in the Solar System, containing many asteroids and minor planets. It lies between the orbit of Mars and Jupiter.
★ Main Sequence From the Hertzsprung-Russell diagram, that explains the lifecycle of stars formation. The band on the HR diagram where stars lie for much of their life.
★ Mare A plain of congealed lava on the surface of the Moon, darker than the surrounding areas.
★ Meridian An imaginary line on the sky that runs due north and south and passes through your zenith. It is the great circle passing through the north point on the horizon, the zenith, celestial pole, & nadir at the observer’s location. (See nice image under Local Meridian).
★ Meteor A small particle of space debris entering Earth’s atmosphere at high speed that is heated to incandescence by friction with the air molecules. We see these as "shooting stars".
★ Meteor shower A group of meteors that appear to originate from a small region of the sky (the radiant). For example the Orionids seem to be coming out or the constellation Orion.
★ Meteor swarm (or stream) Meteoroids grouped in a localized region of an orbit around the Sun (the source of meteor showers).
★ Meteorite A meteor that survives its trip through the atmosphere and reaches the ground. We may see evidence of these scattered through "impact craters".
★ Meteoroid A small, solid particle moving in orbit about the Sun.
★ Messier Object Charles Messier made a list of over 100 astronomical objects that were fuzzy like comets were, but were definitely not comets. He did not know really what they were, but his list is invaluable to us today.
★ Milky Way The spiral galaxy in which our Solar System lies.
The image below, is an artistic representation of what the Milky Way galaxy looks like from the outside. It is sort of flat like a pancake, with a bulge at the centre. There is a "bulgy" centre bar and spiral arms radiating out from it. Our Sun is in one of the spiral arms out near the edge (labelled in blue at the top). As we look into the "pancake" from near the edge, it appears like a "milky" band in the sky. This is what we see stretched out, in a band, above us at night. In other directions we can look outside our galaxy. This is where we see fewer stars.
★ Minor planet A small rocky object which revolves around the Sun. Most lie between the orbits of Mars and Jupiter (i.e. in the Main belt) & beyond Neptune (i.e. in the Kuiper belt).
★ Mnemonic Is a set of letters to easily remember something. The following are examples:
★ Moon. A natural satellite that orbits a planet. Earth has one moon, which shows different phases during the month. (See image under next listing – moon phases).
Because the moon and the Earth are dancing around each other we can generally only see one side of the moon towards us. The moon does not produce it’s own light, but only the reflected light from the sun. See below for the comparative sizes of several moons.
★ Moon Phases The phase of something tells you how far you are through
a complete cycle. A Lunar, or
Synodical month is the number of days between seeing the same phase. When measured accurately, a lunar
month is 29.53 days long. Thinking in
terms of the Moon’s movement around the Earth, just before a new Moon appears, the Moon appears dark because it is between the Earth and
the Sun (as shown below) and no illuminated part of the Moon is seen from
Earth. If the Moon orbited in the
same plane as the Earth around the Sun, the Sun would be eclipsed every month,
but the Moon’s orbit is in a slightly inclined plane, making eclipses of the
Sun rare.
Note: the Moon orbits the Earth in 27.322 days and the difference between this and a Synodical moon is due to the Earth moving in its orbit.
★ Multiple star system Three or more stars linked by mutual gravity and revolving around a common centre of mass.
★ Nadir The point directly below the observer, opposite the zenith. Nadir is in the direction in which gravity pulls. (See nice image under Local Meridian).
★ Nebula (plural nebulae) It is a gas cloud formed of small molecules or dust particles. It can be dark if it is cold, or glow if it is hot.
★ Nebulosity The faint presence of gas appearing around a star or galaxy.
★ Neutron star A massive star’s collapsed remnant consisting almost wholly of very densely packed neutrons. Also called a Pulsar.
★ Newtonian telescope A common type of reflector telescope designed by Sir Isaac Newton.
★ Newton’s Law of Gravitation Every mass in the universe is attracted to every other mass, with a force that is directly proportional to the product of their masses & inversely proportional to the square of the distance between them. It is useful because when one astronomical object revolves in orbit around another, gravity provides a force towards the central mass that enables the object to move in an elliptical or circular orbit.
★ Night vision And how do I keep it working well at night? A human has eyes that have cone receptors (for colour vision), rod receptors (for black & white vision) & visual purple. Cones are active in the daytime. Rods activate in the night when the cones switch off, & are enhanced when visual purple is present. Visual purple is a chemical that takes about twenty minutes to fully accumulate.
This is why astronomers use red light when they are observing, because it does not disturb the visual purple. White light is frowned upon by astronomers, because if your eyes see white light, the visual purple is rapidly dissipated, & it takes another twenty minutes to accumulate again. Suggest you use red cellophane to cover any white lights from torches or from the car.
★ NGC object This stands for New General Catalogue (compiled by Dreyer). It lists 7,840 objects. Dreyer also published two Index catalogues (IC numbers).
★ Node One of two points (ascending and descending) at which an orbit passes through a reference plane (usually the ecliptic).
★ Nova A white dwarf star in a binary system that brightens suddenly by several magnitudes as gas pulled away from its companion star explodes in a thermonuclear reaction.
★ Object An observable item in the sky, a planet, star, nebula, galaxy, comet, moon, satellite, meteor etc.
★ Objective The main light-gathering optical element in a telescope; it may be a lens or a mirror.
★ Oblateness The ratio of a planet’s polar to its equatorial diameter.
★ Obliquity The degree of inclination (or tilt) of a planet’s equator to its orbital plane.
★ Occultation The covering up of one celestial object by a larger one, such as the Moon passing in front of a star or planet, as seen from Earth. From the Latin word "occult" meaning hidden.
★ Oort cloud This is a distant region in our Solar System, with is the long term home of many comets. It is the limit of Solar gravity, from our Sun.
★ Open star cluster A group of tens or hundreds of young stars bound by gravity and moving through space together; normally closer to Earth than a globular cluster.
★ Opposition Point in a planet’s orbit when it appears opposite the Sun in the sky.
★ Optical double Two stars that appear close together in the sky but are actually chance alignments as seen from Earth. The have no interaction (like gravity, or movement around each other) because they are not really close to each other.
★ Orbit Path followed by any celestial object moving under the control of another object’s gravity.
★ Parallax The apparent change in position of a nearby object relative to a more distant background when viewed from different points; used in astronomy to determine the distances to nearby stars.
★ Parsec (pc) A unit of distance used by astronomers which is equal to 3.26 light years. It is defined as the distance to a celestial body whose parallax is one arcsecond. See diagram below. See also Units of Distance.
★ Particles of Nature We are familiar with the concept of atoms which have a central nucleus made up of protons and neutrons around which electrons are in orbit. Today Physicists generally agree that the basic building blocks are of two types of particles : Namely Leptons and Quarks. The permanent universe is made up of only the particles listed below. The remainder are essentially alternatively heavier duplicates of the previous particles and have finite lifetimes.
| Leptons | Quarks |
| electron | up quark |
| electron neutrino | down quark |
| muon | charm quark |
| muon neutrino | strange quark |
| tauon | top quark |
| tauon neutrino | bottom quark |
Note: Three is the magic number to define a Proton or an electron.
Protons are made of two "up quarks" and one "down quark" whereas neutrons are made up of two "down quarks" and one "up quark". When quarks were named, it was from a poem, "three quarks for Master Mark", which seemed appropriate and it also is the sound of a seagull.
★ Penumbra The outer part of an eclipse shadow; also, the lighter area surrounding the centre of a sunspot.
★ Perigee The point at which a body in orbit around the Earth most closely approaches the Earth. It is the opposite of Apogee.
★ Perihelion A planet or comet’s closest approach to the Sun.
★ Period The length of time for an object to go through a cycle, such as a planet around the Sun or a variable star to fluctuate in brightness.
★ Perturbation Changes in the orbit of a celestial body caused by the gravitational field of another body.
★ Phase The phase of something tells you how far you are through a complete cycle. The varying illuminated part of the Moon or a planet caused by the relative locations of the Moon or planet, Earth, and the Sun. For example the planet Venus shows phases, which are easily seen through a telescope.
★ Photometer An instrument that measures the luminous intensity of a source of light.
★ Photosphere The visible surface of the Sun.
★ Planets revolve around the Sun. They do not emit light, but only reflect light from the Sun. The early word for planet meant "wanderer" because at times they seemed to move backwards in the sky (see Retrograde motion)
★ Planetary nebula An expanding shell of gas ejected from a star. Thought to be the outer layers of a red giant during its latter stages of evolution, the core of which becomes a white dwarf. And why, if it has no planet inside, was it called a Planetary nebula? This is a misnomer. It was named this by William Herschel because it looks like a little planet within a large gas cloud. There is no actual planet, because it is a white dwarf star surrounded by the gas that blew off when it went Nova.
★ Planetesimal One of the small bodies that coalesced to form the planets.
★ Planisphere A handheld aid used to identify which constellations are visible to an observer on any particular date and time. This is a star chart analog computing device. It takes the form of two rotating discs on a common pivot. It can be adjusted to show the stars & constellations visible at any time & date. It shows an elliptical cut out to represent ONLY the stars visible on a given night from a given latitude.
★ Polar alignment Alignment of the polar axis of an equatorially mounted telescope to either celestial pole. Polar alignment of an equatorial mounted telescope in the Southern Hemisphere means pointing the mount’s polar axis at the southern celestial pole in the sky. The polar axis is therefor parallel to the north-south axis of the Earth, the telescope can then easily copy the "movement" of the stars across the sky.
★ Polar axis The line around which a celestial body rotates.
★ Precession A slow, periodic wobble in Earth’s axis caused by the gravitational pull of the Sun and Moon. A complete cycle of the axis takes 25,770 years. It is though the Earth is wobbling like a children’s top.
★ Primary star The brighter component in a double star.
★ Proper motion The small change in position of nearby stars due to motion across the line of sight (measured in arcseconds per year).
★ Protoplanet A stage in the formation of a planet which implies the body is nearly full-size.
★ Pulsar A rapidly spinning neutron star that flashes periodic bursts of radio energy. The first Pulsar was discovered in 1967 by Jocelyn Bell. Pulsars are also called Neutron stars.
★ Quadrant This is a hand held tool, popular with ancient mariners. A quadrant is like half of a standard protractor and used to measure angles up to 90 degrees.
★ Quadrature A configuration that two celestial bodies have apparent longitudes that differ by 90 degrees as viewed from a third body.
★ Quasar A compact object or radio source with a stellar appearance, the spectrum of which display a marked redshift. Believed to be the active cores of very distant galaxies.
★ Radians and how do I measure angles in the sky?
Measuring angles. The Babylonians measured angles by defining a full circle or rotation as 360 degrees. A degree was divided into 60 arc-minutes and an arc-minute into 60 arc-seconds.
The idea of a radian comes from defining an angle as the arc length divided by the radius.
The circumference of a circle is given by: 2 Π x radius.
The radian angle for a full circle is then the circumference/radius = 2Π x radius/radius
This means 360° = 2 Π radians
One radian is 360/(2 Π) = 57.3°
★ Radiant The point on the sky from where a shower of meteors appears to come.
★ Radiation Energy transmitted through space as waves (such as radio waves, infrared, gamma, light, and X-rays) or sometimes particles (such as protons and neutrons).
★ Radio Astronomy The study of celestial bodies by means of the radio waves that they emit and absorb naturally.
★ Radio Telescope An instrument for the detection, collection, and analysis of radio waves.
★ Red giant A large reddish star (such as Betelgeuse in Orion) in a late stage of its evolution. It is relatively cool and has a diameter perhaps 100 times its original size.
★ Redshift An apparent shift of spectral lines in the light of an object toward the red end of the spectrum, caused by the relative velocity lengthening the light’s wavelengths as the object and the viewer recede from each other.
★ Reflection nebula Cloud of dust or gas that reflects light from nearby stars.
★ Reflector A telescope that forms an image using mirrors. Reflectors use a combination of curved/flat mirrors to reflect the light into an eyepiece lens to form an image. They are often more portable than refractor telescopes, because an equivalent aperture refractor is much longer than a reflector.
★ Refractor A telescope that forms an image using an objective lens. Refractors use objective & eyepiece lenses, to form an image. An early form was a spy glass. They are usually long, so from a practical point of view, do not always travel well.
★ Resolution A measure for the separating capacity of a telescope, radio telescope or spectral apparatus. (See Resolving Power)
★ Resolving power The ability of a telescope to show two closely spaced objects as separate. Given by the Rayleigh Criterion (also Dawes Limit) (note: not discussed in lectures so not included here).
★ Retrograde Movement This is the apparent path or motion of a planet seen form Earth, where for a period of time, it appears to be moving backwards. For example, for Mars (or other Superior planet like Jupiter or Saturn). As Earth travels faster in its orbit around the sun, it occasionally appears to "catch up with" the slower Superior planet, which is in an orbit outside of Earth’s orbit. When viewed against the background of stars, Mars appears to move in a loop backwards for a short time prior to resuming its eastward journey. Mars retrogrades for two months, while the outer planets can retrograde for six months.
★ Revolution A planet or body in space revolves in its orbit around the Sun. The Earth takes 365.2422 days to revolve in it’s orbit around the Sun.
★ Right ascension Celestial coordinate analogous to longitude on Earth. It is similar to longitude on Earth, but think of it as projected above the Earth onto the "sphere of the stars". It is measured in hours, minutes & seconds up to 24h, (instead of plus or minus 180°). The zero point for longitude on the ground starts at Greenwich UK. However the zero point for right ascension starts at the March equinox when the Sun crosses the celestial equator. Right ascension is measured eastward along the celestial equator from this spot. The First Point in Aries is considered to be the celestial prime meridian from which Right Ascension in hours is calculated. See nice diagram under Celestial Sphere.
★ Rille A long, narrow, winding valley on the surface of the Moon.
★ Roche limit The closest one celestial object can get to another before the weaker object is broken apart by tidal effects.
★ Rotation A planet or body in space rotates about its North South Axis. For example 1, the Earth takes 24 hours to rotate. For example 2, the stars in the Southern Hemisphere appear to rotate around in the sky in a clockwise direction when looking south.
★ Satellite Any small object orbiting a larger one, although the term is most often used for rocky or artificial objects orbiting a planet.
★ Seasons - how do they come & go? for details see Peter’s webpage on calendars. http://www.insula.com.au/astro/PetersCalendars/calendarsweb.html
★ Secondary star The companion, or less bright, star in a double star.
★ Seeing A measure of the steadiness or how still the atmosphere is on a given night. Good seeing is essential to using high magnifications. Seeing depends on turbulence in the air (wind, rising heat), light pollution, & even particulates in the atmosphere, (smog, fog, dust, smoke or mists).
★ Sextant A sextant helps sailors in navigation by measuring the vertical angular distance between two visible objects (e.g. Sun and horizon). It is also useful for astronomers to measure the angle between stars.
★ Sidereal time A method of keeping time which uses the motions of the stars rather than the Sun. One sidereal day is equal to 23hrs 56min 4sec of normal solar time.
★ Skyglow The illumination of the night sky, usually by light from cities.
★ Small Magellanic Cloud (SMC) Satellite galaxy to our own Milky Way, appearing to the unaided eye as a nebulous patch in the constellation of Tucana. From mid-southern latitudes the SMC is circumpolar.
★ Solar A term to relate to our Sun.
★ Solar Day A complete Solar day occurs when the Sun has moved from one position in the sky, e.g. noon, through the night-time and back to that same initial position. The word "day" can describe the complete period or just the bright time.
★ Solar Eclipse A Solar eclipse occurs when the Moon crosses directly in front of the Sun, as seen from Earth and the Sun goes dark. The shadow of the Moon, rushes across the landscape towards the observer and for several minutes the sun’s rays are totally blocked by Moon shadow. There are Total, Annular and Hybrid Solar eclipses. Solar eclipses occur approximately once a year, somewhere on the Earth. Note: it is very important to wear solar glasses when observing the Sun, otherwise blindness may occur.
★ Solar Eclipse Totality This is where the entire surface of the Sun appears to have totally disappeared behind the "shadow" of the Moon. During totality, which may last a few minutes, you may see "the Diamond ring effect" or "Bailey’s Bead effect".
★ Solar filter A filter that reduces the Sun’s light to a level where you can view it with a telescope. It is very important to protect one’s eyes when observing the sun. Pirates wear patches, because they have burnt out their retinas, (damaged their eyes) by observing the sun through telescopes without solar filters. So be very careful - particularly with children.
★ Solar flare A sudden, violent release of pent-up magnetic energy in or near the Sun’s photosphere that often sends great gusts of radiation and charged particles into space.
★ Solar prominence A loop of cooler gas seen above the Sun’s surface, which sometime erupts outwards into space.
★ Solar System Our Sun with everything that orbits it: planets, moons, asteroids, comets, meteors, and other debris.
★ Solar wind A ceaseless, but variable, high-speed stream of charged particles (mostly protons and electrons) flowing out into space from the Sun.
★ Solstice The time when the Sun reaches its greatest northern or southern declination. In the southern hemisphere, around June 21st marks the shortest day of the year, and around December 21st marks the longest day.
★ South Celestial Pole (SCP) The stars in the Southern Hemisphere appear to rotate in a Clockwise direction around a point, looking south. This point is called the South Celestial Pole. In the Northern Hemisphere, there is a star called "Polaris" or the North Star located very near to the North Celestial Pole. In the Southern Hemisphere, however, there is no obvious star in this location. (Just an extremely faint one).
★ Spectral type A star’s spectral classification determined by its spectrum.
★ Spectrograph An instrument that breaks the light from a celestial object into its component colours for analysis.
★ Spectroscopy The analysis of light to determine, by study of the spectral lines, the chemical composition and conditions of the object producing it, as well as that object’s motion and velocity toward or away from the Earth.
★ Spectrum The light from an object spread out like a rainbow. As well as a continuous spectrum, a star normally shows a distinctive set of dark and bright lines which are characteristic of its composition. These dark lines are called Fraunhofer lines. Different elements can be determined by the lines from a spectrum from distant stars.
★ Speed of Gravitational Waves is the same as the speed of light in a vacuum.
★ Speed of Light in a vacuum is approx 3.00 x 108 m/s.
★ SKA Square Kilometre Array is being built to be the largest radio telescope distributed over two main sites, namely Western Australia and South Africa. It is currently being built to have up to a million dipole antennas arranged in an array in Western Australia. It already has several hundred dishes in South Africa. See images of dishes and dipoles.
The Australian dipole antennas receive very low frequencies, similar to those on which you receive FM radio stations. So they are like "the fish eye lenses" of a camera;
and in South Africa the dishes operate at higher frequencies, similar to those used for mobile phone signal transmission. So they are like "telephoto lens" of a camera.
★ Star A star is a massive luminous ball of ionised gas (mostly Hydrogen & Helium). Gravity pulls the gas together. Stars emit light and heat, but planets are much colder and we can only see them by the Sunlight reflected from them. Planets revolve around the Sun.
★ Star cloud An apparent grouping of large numbers of stars that looks like a separate dense concentration of stars against the Milky Way.
★ Star field A field of view including a scattering of visible stars.
★ Stella & Stellar The word stella (no "r") is the Latin word for star, so stellar (with the "r") is used to describe anything connected with stars. Also a "stellar" person or thing is considered to be very good.
★ Stellar Evolution - see Lifecycle of Stars
★ Sunspot A highly magnetised dark spot on the Sun’s surface, cooler than the area around it.
★ Supergiant star The largest type of star known.
★ Superior conjunction A conjunction where two celestial bodies appear close to one another on opposite sides of the Sun.
★ Superior planet A planet orbiting the Sun outside Earth’s orbit.
★ Supernova The massive explosion of a star during which it blows off most of its mass. The star may briefly equal an entire galaxy in brightness. Massive stars go supernova to form neutron stars or black holes. Normal stars (up to ten times our Sun) go Nova to form a Planetary Nebula.
★ Supernova remnant The shell of gaseous debris, rich in heavy elements, thrown off by a supernova.
★ Surface brightness Concept used when describing extended astronomical objects such as galaxies and nebulae, usually quoted in magnitudes per square arcsecond. If a galaxy is quoted as having a magnitude of 12.5, it means we see the same total amount of light from the galaxy as we would from a star with magnitude 12.5. However, while the star is so small it is effectively a point source in most observations, the galaxy may extend over several arcseconds or arcminutes.
★ Synodic period The period of a planet’s orbit with respect to the Earth.
★ Telescope A telescope is an optical instrument for making distant objects appear near and larger.
★ Telescope Mount A structure, either portable or permanent, on which to secure a telescope. there are a number of different types of mount e.g. Dobsonian mount, equatorial mount, forked equatorial mount, etc.
★ Terminator The line separating day and night on the disc of a moon or planet. (This is not a machine from the future, that has come back to kill Sarah Connor).
★ Telrad This is a cross hairs device useful to help find a distant star. It is a device attached to a telescope, instead of a finder scope. And do I need one as well on my telescope? Try one first, & see if you like them.
★ Transit The passage of Mercury or Venus in front of the Sun’s disc or the passage of a satellite or its shadow across the face of its planet.
★ Transit the meridian When a celestial object crosses your meridian, that is, a line between you and your zenith. (See nice image under local meridian).
★ Transparency A measure of the state of the atmosphere in allowing light to pass through it unaltered.
★ Twilight The period of time before sunrise and after sunset during which there is not complete darkness. (This is not a Movie series about Vampires). See below for other definitions of twilight.
★ Twilight – astronomical Astronomical twilight ends in the evening sky or begins in the morning sky when the Sun is 18 degrees below the horizon.
★ Twilight – civil Civil twilight ends or begins when the Sun is 6 degrees below the horizon.
★ Twilight – nautical Nautical twilight ends or begins when the Sun is 12 degrees below the horizon.
★ Ultraviolet (UV) The portion of the electromagnetic spectrum with wavelengths just shorter than the bluest light visible. This is from 400nm (nanometer) down to 10 nm (nanometer).
★ Umbra The dark inner part of an eclipse shadow. Also, the dark central part of a sunspot.
★ Units of Distance In astronomy, three common units are as follows: the Astronomical Unit (au or ua) or Light Year (ly) or Parsec (pc). (See image below).
★ Universe The entire make up of the Cosmos, including stars, planets, moons, comets, asteroids, nebula, galaxies, black holes, other clouds of dust, dark matter, dark energy. (i.e."Life, the Universe and Everything")
★ Universal Time (UT) A time system measured from the meridian of Greenwich in England
★ Variable star Any star, the brightness of which appears to change over periods ranging from minutes to years. Can include a Cepheid variable star.
In Cosmology, Hubble used data from Henrietta Leavitt & her work on Cepheid variable stars to calculate the distances of galaxies, using the absolute magnitude of Cepheid variable stars. (See image of Hubble and Henrietta Leavitt below).
★ Wavelength The distance between repeating phases of a wave pattern/ (e.g. distance between peaks or between troughs).
★ White dwarf The small and hot, but intrinsically faint, remnant left when a red giant star loses its outer layers as a planetary nebula.
★ "Wolf-Rayet" Stars In spectroscopy these are very hot and very bright stars.
★ X-ray Portion of the electromagnetic spectrum between UV and gamma rays. This is from 10 nm (nanometer) down to 0.01 nm (nanometer).
★ Zenith The Zenith is directly above the observer. (See nice image under Local Meridian).
★ Zenith Hourly Rate (ZHR) A general guide to the expected intensity of any given meteor shower. It is a theoretical rate, assuming the radiant is at the zenith with a sky limiting magnitude of 6.5.
★ Zodiac The 12 constellations straddling the ecliptic, through which the Sun, Moon, and planets appear to move during the year (astrologers for some reason ignore Ophiuchus, which lies between Scorpius and Sagittarius).
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