A Short Glossary of Astronomical Terms
by Martin J. Powell
C/1995 O1
- an official designation by the International Astronomical Union for newly
discovered comets. The prefix C/ is used for a comet which has an orbit
of longer than 200 years, or which has a parabolic or hyberbolic orbit
(i.e. it will never return to the solar system). The year of the comet's
discovery is then given (in this case 1995). This is followed by an upper
case letter identifying the half-month in which the discovery took place
(the letters I and Z are not used). Hence the letter 'A' refers to comets
discovered between January 1st and 15th, the letter
'B' refers to comets discovered between January 16th to 31st,
and so on. The final number represents the order of the discovery announcement
within the half-month. Hence Hale-Bopp has an orbit of greater than 200
years, and was the first comet to be discovered between July 16th
and July 31st 1995. Similarly, Comet Hyakutake received the
designation C/1996 B2. Comets with well computed orbits of less than 200
years (known as "periodic comets") have the prefix P/.
Astronomical Unit
(abbrev AU) - a standard unit for measuring distances within the
Solar System. By definition, 1 AU is equal to the average distance of the
Earth from the Sun during the course of a year, i.e. 92,955,806 miles (149,597,870
km). For example, Mercury orbits at an average distance of 0.387 AU and
Pluto at an average distance of 39.44 AU from the Sun.
Averted Vision
- the act of looking slightly to one side, rather than directly at, a faint
object. When looked at directly, a faint object often cannot be seen, however
it becomes visible when one looks slightly to one side of it. This is because
the most sensitive part of the retina is a circular area around its centre,
where the highest concentration of rods occurs.
Circumpolar
- an expression used for celestial bodies which are above the horizon throughout
the day, i.e. they do not rise or set. As the Earth rotates they appear
to prescribe a circle around the celestial pole. An object will be circumpolar
when its declination (q.v.) is greater than the value of the observer's
latitude subtracted from 90, i.e. the co-latitude. For example,
stars observed at the latitude of London (51°
.5 North) will be circumpolar if their declination is greater than (90
- 51.5) = +38°.5.
Co-latitude
- a value equal to (90 - observer's latitude). See also circumpolar.
Coma
- the principal part of most comets, consisting of a diffuse cloud of gas
and dust which surrounds the nucleus (q.v.)
Declination
- the angle North (+) or South (-) of the celestial equator, measured relative
to the Earth's centre. Together with the Right Ascension (RA), which
is measured in hours and minutes, it is used to pinpoint an object's position
on the celestial sphere.
Degree of Condensation
(abbrev DC) - a measure of the light distribution within the coma
(q.v.), measured on a scale from 0 (diffuse) to 9 (stellar or disk-like).
Dust Tail
- often curved and yellow in colour, dust tails are visible by the reflection
of sunlight off solid particles. The tails become curved when distant dust
particles thrown off the nucleus lag behind the coma. Tails only form when
a comet comes close to the Sun, and they always face in a direction opposite
to the Sun.
Fountains
- diffuse features observed on the sunward side of the coma (q.v.)
Gas Tail
- also called the ion tail, it is usually straight, blue and filamentary.
Gas tails form when ultra-violet light ionizes the cometary gases, causing
them to fluoresce. Like the dust tail (q.v.), a gas tail only forms
when the comet comes close to the Sun and always faces away from it. The
tail of a comet may stretch over 93 million miles (150 million km) through
space.
Hood
- a parabolic-shaped feature enveloping the nucleus, only seen in bright
comets near perihelion (q.v.)
Jet
- a fine and delicate feature seen emanating from a cometary nucleus (q.v.),
always curving and often spiral. They are caused by the outgassing of material
from the nucleus, the spiralling effect being caused by the rotation of
the nucleus itself, much like a Catherine Wheel.
LPR Filter
- abbreviation for Light Pollution Reduction Filter. A special type
of filter, attached to a telescope eyepiece, which suppresses the effects
of sky-glow (light pollution) from streetlights by only allowing selected wavelengths to
pass through the eyepiece. The so-called nebula filters transmit
only in the wavelengths at which nebulae shine most brightly (mostly Oxygen
III and Hydrogen Alpha), effectively darkening the sky and increasing
the contrast of the nebulae against the sky.
Messier
Number (M44, M70 etc.)
- designations from the Messier Catalogue of nebulous objects compiled
by French astronomer Charles Messier and published in 1784. He listed a total of
109 objects, comprising gaseous nebulae, planetary nebulae,
galaxies, open clusters and globular clusters. Messier compiled the catalogue
in order to prevent confusion with newly discovered comets, for which he
was a keen searcher.
Magnitude
-
a measure of the relative brightness of stars and other celestial objects.
The brighter the object, the lower the assigned magnitude value. Because
of the logarithmic nature of the magnitude scale, stars of magnitude 2
are 2.5 times fainter than those of magnitude 1, and magnitude 3 stars
are (2.5 x 2.5) = 6.3 times fainter than magnitude 1, and so on. Objects
brighter than magnitude 1 have a zero or even a negative value. The values
given in the current context are apparent magnitudes (sometimes referred to as visual
magnitude,
symbol v) i.e. the magnitudes
as they appear in visible light from the Earth's surface. Sample magnitude
values are given below:
Sun |
- 26.7 |
Full Moon |
- 12.7 |
Venus (at brightest) |
- 4.4 |
Sirius (brightest star) |
- 1.4 |
Naked Eye limit (approx) |
~ + 6.0 to + 6.5 |
Neptune (at brightest) |
+ 7.7 |
Pluto (at brightest) |
+ 13.6 |
Faintest objects imaged by the Hubble Space Telescope |
~ + 30* |
* Reached in an exposure of 11.3 days in 2004
Cometary magnitudes are difficult to give precisely because they are extended objects with a bright nucleus, a diffuse coma and often with a tail. In practice, the magnitude given for a comet refers specifically to the coma (q.v.) as it would be if its total light was compressed into a star-like point. The vast majority of comets do not become bright enough to be visible with the naked eye. The brightness of a comet is dependant on many factors; the size of the nucleus, the activity of the nucleus, the distance of the comet from the Sun and the relative geometry of the Sun, Earth and comet.
The twenty-one brightest stars in the night sky are listed in the table above (click for full-size image, 70 KB). The letter 'v' in brackets indicates that the star has a variable brightness (Sources: "The Guinness Book of Astronomy Facts & Feats" by Patrick Moore (Guinness, Enfield, 1983) and "Norton's Star Atlas" edited by Ian Ridpath (Longman, London, 1989)
NGC
- abbreviation for the New General Catalogue of Nebulae and Clusters
of Stars, prepared by Danish astronomer J. L. E. Dreyer in 1888. Each
item is assigned a number in the catalogue. Together with the two subsequent
Index
Catalogues (IC) there are over 13,000 objects listed.
Nucleus
- an icy rubble-pile, typically a few kilometres or more across, forming
the permanent part of a comet. It comprises mainly water ice and dirt.
Whenever the comet comes close to the Sun the nucleus becomes active, outgassing
and sublimating material as the comet rotates. A typical cometary nucleus
will survive around 200 or so close approaches to the Sun before disintegrating.
Oort Cloud
- a theoretical cloud of comets orbiting the solar system at a distance
of about a light year (63, 240 AU) from the Sun. First proposed by the
Dutch astronomer Jan Oort, it is considered to be the origin of most of
the comets which enter the inner solar system. It is thought that the gravitational
pull of nearby stars causes the comets in the cloud to be pulled either
into the solar system or out towards the distant star. Other comets may
originate from a belt of comets beyond Neptune known as the Kuiper Belt.
Opposition -
an alignment of celestial bodies such that the Sun, the Earth and another
celestial body form a straight line, with the Earth in the centre of the
alignment. The celestial body in question will then be at a point opposite
the Sun in the sky, crossing the observer's meridian at around local midnight.
Perihelion
- the point in the orbit of a planet or comet at which it is closest to
the Sun. Its most distant point from the Sun is called the aphelion.
At its aphelion, Comet Hale-Bopp reaches a distance of 360 AU (q.v.)
from the Sun - nine times further out than Pluto.
Position Angle
(abbrev pa) - the angle at which a feature is orientated relative
to the direction of celestial North. It is measured from 0 to 360 degrees
from North through East, South and West. For example, a comet whose tail
is at pa = 270° has its tail facing
to the West.
Pseudo-Nucleus
- a term given for the appearance of the nuclear region of a comet through
a telescope. Because of the small size of a cometary nucleus (q.v.),
it cannot be seen through even the most powerful telescopes. The bright
stellar-like concentration at the centre of the coma is therefore termed
the "false", or pseudo-nucleus.
Shells -
a series of arcs seen ahead of the coma of a bright comet when it comes
close to the Sun. Shells are caused by the outgassing of material from
a cometary nucleus. As the nucleus rotates, the material forms a series
of spirals around it, much in the manner of a Catherine Wheel. The telescopic
view is usually of a series of concentric arcs ahead of the coma.
Spectroscopy
- the study of the spectra from astronomical sources of radiation. The
lines and bands in stellar spectra are characteristic of the atoms, molecules
and ions producing them. They are measured across the entire range of the
electromagnetic spectrum, and are the main source of information on the
composition and nature of celestial bodies.
Copyright Martin J Powell 2001
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