What does THAT mean? - an astronomical glossary
Terms included on this page:
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- Aperture ..... The aperture of a telescope is the diameter of its main lens or mirror, and it determines 2 things: how faint an object you can see, and how much fine detail you can see. In one sense, a telescope is just a "light bucket": it gathers more light than your eyes can, and the larger the aperture, the more light it gathers. (This is one reason why professional astronomers build bigger and bigger telescopes, allowing them to observe extremely faint objects, millions or billions of light years away.) The other other function of large aperture - seeing fine detail - is called resolving power. For example, with a small telescope, no matter how many times it magnifies, you will only see a certain amount of detail. You might be able to see the main 2 cloud belts on Jupiter, or the basic shape of Saturn's rings. A larger telescope will have greater resolving power, and more detail of Jupiter's clouds, and of Saturn's ring system, will be visible.
To complicate matters, how faint an object you can see is not necessarily determined solely by the aperture. In particular, if you are observing an extended or diffuse object, such as a galaxy, nebula, planet, or even the moon, the telescope magnification also affects the situation. In effect, if you use a high magnifying power, the light from such an object is "spread out" over a greater area, and therefore becomes fainter. So, if you want to find faint objects, use a wide aperture telescope and a low power eyepiece (and a dark sky!). For observing stars, this effect does not happen, because no matter how much you magnify a star, it doesn't get any bigger - it remains a point of light. Therefore, the light is not spread out, and there is no practical difference in star brightness at low or high magnifying power. However, it may be easier to see fainter stars at high power, because any background light - from city lights, for example - will be dimmed just as the light from nebulae etc would be.
- Arcminute, arcsecond ..... When referring to the size of a celestial object, it is often the apparent size in degrees, or fractions of a degree, that is used. For example, something that filled the sky from horizon to horizon would be 180 degrees across. The Moon is approximately half a degree across, but this can also be expressed in minutes of arc. Degrees can be divided into 60 minutes of arc, and each minute can be further divided into 60 seconds of arc; any further division is into decimal fractions of seconds. These terms are often shortened to arcminutes and arcseconds.
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This picture of the rocky asteroid 951 Gaspra was taken by NASA's Galileo spacecraft. Image Credit: NASA
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- AU - Astronomical Unit ..... In simple terms, an Astronomical Unit is the average distance from the Earth to the Sun - approximately 150 million kilometres (93 million miles). For a more precise definition, see this NASA page.
- Binoculars ..... Binoculars are basically two low-power telescopes mounted side by side, and as such, they are well worth considering, when thinking of buying your first telescope. The are described by their aperture and their magnifying power. For example, a good binocular for astronomy would be a 7x50; that is, it magnifies the view by 7 times, and has objective lenses 50 mm in diameter. Such a binocular will be fairly easy to hand-hold, and will have a good light grasp to see fairly faint objects.
To get an idea of how useful a binocular is for astronomy, divide the second number by the first; this gives the size of the binocular's exit pupil, in millimetres. If the exit pupil is the same size as the pupil of the observer's eye, then the image will be as bright as possible. For a 7x50 then, divide 50 by 7, and the result is about 7, and this is the maximum size of the eye's pupil when wide open. In other words, a 7x50 binocular will take full advantage of your night vision. If you use an 8x30 binocular, the exit pupil is just 3.75 mm - half the diameter of your eye's pupil when fully open. This won't matter if you are looking at the moon, but if you want to see faint objects such as nebulae, you need the exit pupil to be as wide as possible (although if it is wider than the eye's pupil, there is no further advantage). For more on buying and using binoculars, see So you want to buy a telescope...?
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Globular cluster M72. NASA image.
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- Colour of stars ..... At first glance, most stars just look like white points of light, without any appreciable colour. A few stars have a distinct red colour though, for example Antares, Arcturus and Betelgeuse. The colours of the stars are generally very subtle when observed visually, but photography will show that they have a range of colours such as blue, yellow, orange and red.* The colour indicates how hot a star is, with blue stars being the hottest, down through yellow and orange, to red. Our Sun is a yellow star, with a surface temperature of around 6,000 degrees C. Blue stars may be hotter than 10,000 degrees, and red stars are around 3,000 degrees.
* If you place a camera on a tripod, and give a time exposure of, say, 10 minutes or more, the stars will appear as short streaks on the image. The colours of some of the stars will be visible, but the brighter ones may be overexposed, and just appear as white. An interesting trick used by David Malin gets around this. By changing the focus of the lens every few minutes (on the same exposure), you will start out with sharp streaks, which will gradually become larger blobs with each change of focus. This will spread out the light of the brighter stars, and the colour will become more apparent. Alternatively, you could take several successive exposures, each with the lens defocused a bit more than the last.
- Declination ..... When astronomers need to define the position of an object in the sky, they often use a coordinate system equivalent to longitude and latitude. The celestial terms are Right Ascension (RA) and Declination (Dec), where RA is measured from west to east around the sky, starting at the First point of Aries - the point where the Sun crosses the celestial equator at the March equinox, on its journey north - and Dec is the distance north or south of the celestial equator. RA is measured in hours, minutes and seconds (it is effectively a measure of time elapsing as the sky revolves overhead) from 0 to 24, and Dec is measured in degrees, minutes and seconds of arc, where the equator is at 0 degrees, and the celestial poles are 90 degrees north and south.
- Earthshine ..... Earthshine is a phenomenon visible when the Moon is just a thin crescent. At that time, the Earth would be almost 'full' if viewed from the Moon, and this bright light is reflected into the dark portion of the moon, showing as a faint glow. Sometimes called "the old Moon in the new Moon's arms", it gradually fades as the Moon's phase grows towards first quarter, but reappears after last quarter, as the phase moves towards new again.
- Ecliptic ..... The ecliptic is the path followed by the Sun throughout the year, across the background stars. During its travels, it passes through 12 constellations of the Zodiac. The Moon and planets also move approximately along this path, although due to their individually varying orbital inclinations, they can be found in a broad band above and below the actual path of the Sun itself.
- Galaxy ..... A galaxy is an entire star system, generally containing millions or billions of stars (plus any planets around those stars), nebulae, clusters, black holes, comets, etc. There are different types of galaxy: spiral, barred spiral, elliptical and irregular. Galaxies may be vary large or very small. Our own Milky Way galaxy is a fairly large spiral, while the nearby Large and Small Magellanic Clouds (LMC & SMC) are small and generally irregular (although the LMC shows some sign of having a vague barred spiral structure). The spiral galaxies usually have 2 or more spiral arms, which contain most of the star-forming regions - the gaseous nebulae, along with many younger, hot blue stars. A barred spiral usually has some sort of straight bar going through the nucleus, connecting the inner portion of the spiral arms. The nucleus of a spiral galaxy tends to be composed of older yellowish stars, with much less nebulosity. The elliptical galaxies have no appreciable spiral structure, and are believed to be very old, having used up their supply of gas in forming stars. The irregular galaxies are just that - irregular accumulations of stars and nebulae.
- Inferior & superior conjunction ..... As planets move around the sky during the year, they eventually pass close to the Sun; this is called a conjunction. There are 2 kinds of such conjunctions: when a planet passes between Earth and Sun, it is called inferior conjunction; when a planet passes around the far side of the Sun, it is called superior conjunction. All of the planets can reach superior conjunction, but only Mercury and Venus can reach inferior conjunction, as they are the only two planets with orbits inside that of the Earth. At superior conjunction, the planet will appear "full", while planets at inferior conjunction become very thin crescents.
- Jupiter's moons ..... Jupiter has dozens of moons, most of them extremely small and faint. Four of them though, are substantial bodies that are easily visible with the slightest optical aid. Also known as the Galilean satellites, they were first discovered by Galileo in January 1610, when he pointed his primitive telescope at Jupiter. He first thought they were stars close to Jupiter, but after a few weeks, realised that they were moons, orbiting Jupiter, as our Moon orbits the Earth. It is interesting to follow the motions of these moons as they orbit the giant planet. The four bodies - Io, Europa, Ganymede and Callisto - have very different orbital periods because they are at varying distance from Jupiter. Io, the closet, orbits in less than 2 days, while Callisto takes nearly 17 days. Observing over a few hours therefore, will show a change in the position of Io and Europa. Sometimes, one or more of the moons will be hidden, as its orbit takes it behind the planet.
- Light interference ..... Also known as light pollution, this is the bane of the astronomer's life! That great invention of Edison's - the electric light bulb (and its cousins in various forms) - is a curse to the modern astronomer. In our grandparents' time, before the growth of the modern city, it was possible for any person to see the Milky Way, if they cared to look upwards at night (at least, my father, not an astronomer by any stretch of the imagination, used to see it as a boy in urban central England in the 1920s). Nowadays, most people are just not aware of the pleasures of the night sky, except for the Moon and Venus, and the fact that there are a few stars up there. Artificial light reduces our ability to see fainter objects, and is especially troublesome if you want to do long-exposure photography anywhere near a sizeable town or city. We can use coloured filters to improve things to some extent, but that is far from an ideal solution. Best of all is to observe the sky from a dark location.
On a more personal level, if you go outside at night, it will take a while for your eyes to become "dark adapted"; it can take ten minutes or more to recover from bright artificial lights. One of the worst forms of light for ruining your night vision is fluorescent light. Energy efficient they may be, but don't expect to see much for several minutes, after you leave the glare of fluorescent light. TV and computer screens have a similar effect. Similarly, if you need to use a torch to see what you are doing, use one that has a red filter of some sort - even a few layers of red cellophane will do. Red light has minimal effect on night vision, so using a red torch while looking at star charts etc will not spoilt your dark adaption.
- Light year ..... A light year is a measure of distance, not time, as is often thought. It is simply the distance travelled by light in one year. As light travels at nearly 300,000 kilometres per second, and a year is roughly 31.5 million seconds, a light year is clearly a great distance. To state it in kilometres or miles is not especially helpful, as the numbers are so large, but just for the record, a light year equates to 9,405,579,686,455 km.
- Magnifying power ..... A telescope serves two functions: (1) to gather light, and (2) to bring objects closer, or magnify them. These two functions are connected in the design of the optics, and each has an effect on the other (the light gathering capacity is discussed above, in aperture). The power of a telescope is something that less scrupulous manufacturers and dealers may use to lure customers into buying a telescope. If you see a small telescope (say, 50-60 mm aperture) on a flimsy tripod, and the advertising says something like "Magnifies 300x!!! Brings galaxies closer than ever before!!!" walk away from it quickly; it will very likely be a disappointment.
There is more pleasure to be had in viewing the sky at 30-60x, than 300x. Read that again! At low powers, you can see the Moon as a whole, Jupiter and its 4 main moons, larger bright nebulae, etc. Unless you have a large aperture instrument, on a very stable mount, you will rarely want to use powers higher than 100-150x. We know that the stars appear to move across the sky; well, when you look at the sky through a telescope at 30x, the stars appear move 30x as fast! At 100x, they move 100x faster, and so on. This means that, unless you have a driven mount, so that the telescope moves to follow the stars, anything you are viewing will disappear from view after a while, and the higher the power, the sooner it will disappear. Using high powers then becomes very frustrating, and you will only be disappointed if you try.
It is much easier to find objects by using your lowest power to start with, moving to a higher power afterwards if conditions are good enough. If the atmosphere is unstable (called "bad seeing"), the image will wobble about, and this will only be magnified at higher powers. Also, if your telescope mount is not too stable, then the slightest touch may cause the whole instrument to shake, and this too, will be magnified through the eyepiece. As a rule of thumb, the highest power you should expect to be be able to use is 50x the aperture in inches (or 2x the aperture in millimetres). Therefore, if you have a 2 inch (50 mm) telescope, the highest power you would expect to use would be 100x. Even then, this applies to the best observing conditions - good seeing, good optics and mount, so if the conditions are somewhat worse, then you would only be able to use a lower power.
Yes, yes, but what do the numbers mean? The power of a telescope is determined by its focal length, and by the focal length of the eyepiece(s). If you buy a telescope that has 2 or more eyepieces, they should have the focal length printed on them. Commonly, these may be something like 20 mm, 12.5 mm, 6 mm. NOTE: the BIGGER the number, the LOWER the power. The focal length of the telescope itself may also be printed somewhere on the body, in the form "f=600 mm". To find the magification with each eyepiece, divide the telescope focal length by the eyepiece focal length. For example:
600 mm divided by 20 mm = 30x
600 mm divided by 12.5 mm = 48x
600 mm divided by 6 mm = 100x
Occasionally, a telescope will have an extra lens, possibly marked with "2x" or "3x". This is a Barlow lens, and is only used with an eyepiece, not on its own. It serves to double or treble the magnifying power of any eyepiece it is used with. Use it only if the seeing etc is good enough for the higher power.
- Magnitude ..... This term refers to the brightness of astronomical objects. When first used, the ancient astronomers decided there were 6 magnitudes of stars visible, with those of the first magnitude being the brightest, and 6 the faintest. The system is essentially still the same, but the numbers extend as far as required, to describe very bright or very faint objects. For example, Betelgeuse, the red star in Orion, and Aldebaran, the orange star in Taurus, are approximately magnitude 1, while sigma Octantis, the nearest thing we have to a southern "pole star", is barely visible at magnitude 5.5. Binoculars will show objects to around the 8th or 9th magnitude, and a 150 mm telescope will take you down to the 12th magnitude. The faintest objects observed are fainter than the 25th magnitude, while at the other end of the scale, Venus is around -4 (minus 4) and the full moon is around magnitude -12. Each magnitude step is 2.512 times brighter or fainter than the next, so that a star of magnitude 1 is 100 times brighter than one of magnitude 6. The brightest star, Sirius (-1.4), is 900 times brighter than the faintest star you can see with the naked eye. It might be worth noting that a star of magnitude 0 isn't invisible, with zero brightness, but is between magnitude 1 and -1!
- Messier ..... Charles Messier was a French astronomer who was interested in searching for comets. He was often stumbling across faint fuzzy objects which, although often similar to a faint comet, were actually nebulae, clusters, etc. He decided to catalogue these objects, so that any potential new comet could be checked against his list of known "non-comets". It is ironic therefore, that although he did indeed discover some comets, he is best remembered for his catalogue of "Messier" objects. Many of these are fairly easily observed with small instruments, although some will be a challenge under modern skies, brighter than Messier had to contend with.
- Meteor ..... A meteor is the visual appearance of a small object or particle of dust entering the Earth's atmosphere, burning up in the process. As well as the major and minor planets, the solar system contains a great number of smaller objects in orbit around the Sun. If one of these objects happens to stray into the Earth's orbit, and enters our atmosphere, it may then appear as a bright streak of light. Most meteors are little more than grains of sand, but some larger ones may occasionally be seen. Meteors vary in brightness from below naked eye visibility to fireballs as bright as the full Moon. Before a meteor enters the atmosphere, it is known as a meteoroid; if it reaches the ground, it is called a meteorite.
On any clear night, you should be able to see one meteor every 10-20 minutes, on average. These random, or sporadic meteors can appear (or not!) at any time, and in any direction; they are generally just stray particles of dust, following their own unique orbit through the solar system. In addition, at certain times of the year, meteor showers can be seen, when you may see many meteors per hour. These shower meteors, unlike their sporadic cousins, are left in the wake of comets, and appear when the Earth's orbit intersects that of a comet. As comets pass through the solar system, they shed particles of dust and other material. Because these particles have a common origin, having been once attached to the same parent object (the comet), they have similar orbits. Having similar orbits, they will then appear to be travelling in the same direction - or rather, they will appear to come from the same small region of the sky. Tracing the paths of shower meteors backwards, we can locate the radiant, the apparent region from which the meteors seem to originate. One of the most famous comets - comet Halley - produces 2 meteor showers each year: the Orionids, in October, and the eta Aquarids in May; they are so called because they appear to radiate from within the constellations of Orion and Aquarius. The 'eta' indicates that they radiate from a region near the star eta, in Aquarius, to differentiate that shower from others with radiants in the same constellation (delta Aquarids; iota Aquarids - which are themselves divided into northern and southern component streams).
The greater the number of particles in the comet's orbit, the more meteors will be seen in the shower. Some minor showers produce barely more than a handful of meteors per hour, while some might produce one every minute, at the peak of activity. The November Leonids can produce tens or hundreds of thousands of meteors per hour, at their peak, but only every 33 years or so. The reason for this is that the material is not spread evenly throughout the comet's orbit, but is a relatively young stream, and is concentrated close behind the comet itself. 1966 produced a spectacular shower - known as a storm - and hopes were high for a repeat in 1999 or 2000. As it turned out,there was enhanced activity for a few years around that time, but nothing approaching the numbers seen in 1966 and other peak years (not every 33rd year produces a great storm).
Occasionally, a very bright meteor will appear, brighter than the planet Venus, in which case it is known as a fireball, or bolide. Very few fireballs come from meteor showers, most being sporadic in nature. Some observers specialise in photograpic patrols, to record fireballs, in the hope of catching one large enough to fall to Earth. Very bright fireballs tend to move more slowly that normal meteors, lasting several seconds before they fade. See this film of a fireball recorded by a football fan, and this fireball in WA.
- Milky Way ..... The Milky Way appears as an irregular, faint band of light across the sky. It is not easily seen from light polluted skies, but away from city lights, can be a magnificent sight. The Milky Way is the concentrated light from the millions of stars that make up most of our galaxy. If you imagine our galaxy as a disk with a bulge in the middle, we are somewhere out toward the edge of the disk. When we look at the sky "above" or "below" our galaxy, we see few stars, and can see out in deep space, to the galaxies beyond our own. When we look "through" our galaxy, either toward the centre or out to the edge, we are looking through our galaxy's spiral arms and so many more stars are visible. The Milky Way is brightest toward the galactic centre, in Sagittarius.
- Nebula ..... A nebula is a large cloud of gas, or gas and dust. Nebulae can be bright or dark, depending upon whether or not they are illuminated by starlight. The best examples of nebulae include that around the star eta Carinae, and the Orion nebula. These are bright, emission nebulae - that is, they emit their own light, due to the hot stars within them. This is the same principle as neon lights - a gas, energized by electric particles, starts to give out light. The colour of light is indicative of which gases are present, and how energetic the stars are within the nebula. The other kind of bright nebula is a reflection nebulae, and one of the best examples, though too faint to see visually, is in and around the Pleiades star cluster. A reflection nebula, as the name suggests, merely reflects starlight from the dusty material, so the colour of the nebula is dependent upon the colour of the stars that light it up. The other kind of nebula is a dark nebula, which becomes visible only when it blocks the light of more distant bright objects. One of the best examples in the Southern hemisphere is the Coalsack, near the Southern Cross. The Horsehead nebula in Orion is a swathe of dark nebulosity seen against bright emission nebula, with some nice examples of reflection nebula for good measure. Nebulae are important for the fact that they may be called "stellar nurseries", where the hydrogen gas condenses to form new stars. Indeed one of the most studied regions of the sky is the Orion nebula; the central regions are so dense that infrared observations are needed to see what is happening. As a result, it is clear that there are many regions where this star formation process can be observed, with young cool stars or "protostars" detectable only by the heat they are starting to emit. Eventually, there will be so many hot energetic stars in the nebula that most of the remaining gas and dust will be expelled, and the new stars will be visible with normal telescopes.
- Objective ..... A binocular or refracting telescope has essentially 2 lenses: an objective, and an eyepiece. The objective is the large lens at the front of the instrument, the eyepiece being a smaller lens at the rear.
- Perihelion ..... Any body in orbit around another moves in an elliptical path, rather than a circular one (although some orbits are very nearly circular, while others are highly elliptical); this means that the distance from the 'parent' body varies along the orbit. For any body orbiting the Sun, such as a planet, asteroid or comet, the point closest to the Sun is called perihelion, and the furthest point is aphelion. For orbits around the Earth, the corresponding terms are perigee and apogee.
- Right Ascension ..... (See Declination)
- Star fields ..... When looking at the Milky Way, some portions have such a density that they are referred to as star fields. Sweeping these areas with binoculars or a low powered telescope will often show many clusters and nebulae, as well as the general spectacle of stars too numerous to count.
- Telescope ..... A telescope at its simplest is two lenses separated by a long tube of some sort; this is called a refractor. The larger lens at the front - the objective - produces an image of anything it is pointed at. The smaller lens - the eyepiece - magnifies this image so that you can see the actual object in greater detail. The eyepiece acts in the same way as a hand-held magnifier, that you might use to see insects, fine print, or any other small object. It is fairly easy to make a basic telescope of this sort, but the image quality will be poor. To achieve higher quality, you need to use objective lenses and eyepieces that are composed of several elements, which then reduces the various problems of false colour, distortion, etc.
The other basic form of telescope is a reflector. As the name suggests, it uses one or more mirrors to focus the light, instead of lenses. One of the most common forms for the amateur is the Newtonian reflector, which has a large concave main mirror (the primary), and a smaller flat mirror (the secondary) angled at 45 degrees, which takes the focused light out of the side of the tube, and into an eyepiece. It is named after its designer, Isaac Newton.
There are several other forms of telescope, some of which use both lenses and mirrors to create the primary image, but most work to the same principle of collecting light, bringing it to a focus, then magnifying that focused image. The exceptions to this rule are telescopes that are used for photography, when the eyepiece is usually replaced with a camera of some sort, either using film or an electronic detector such as a CCD.