Table Of ContentTHE MACMILLAN DICTIONARY OF ASTRONOMY
Contributors
Rosamund L. Briggs, BSc
Peter H. Cadogan, BSc, PhD
Heather Couper, BSc, FRAS
Charles A. Cross, MA, FRAS
John Daintith, BSc, PhD
Tony Dean, BSc, PhD, FRAS
Peter Duffett-Smith, BSc, PhD, FRAS
Professor Andrew Fabian, BSc, PhD, FRAS
Peter Gill, FRAS
Nigel Henbest, BSc, MSc, FRAS
David W. Hughes, BSc, PhD, FRAS
Garry E. Hunt, DSc, PhD, FIMA, FRAS, FMet.S, MBCS
Valerie Illingworth, BSc, MPhil
Professor Philip L. Marsden, BSc, PhD, FIP, FRAS
Alan Pickup, FRAS
James Pinfold, BSc, PhD
Professor Ken Pounds, BSc, PhD, FRS, CBE
Gareth Rees, BSc, PhD, FRAS
Allan J. Willis, BSc, PhD, FRAS
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ASTRONOMY
SECOND EDITION
VALERIE ILLINGWORTH
M
MACMILLAN PRESS
LONDON
© Laurence Urdang Associates Ltd 1979
Material for new edition
© Market House Books Ltd 1985
All rights reserved. No part of
this publication may be reproduced
or transmitted, in any form or by
any means, without permission
First published 1979; second edition 1985 by
THE MACMILLAN PRESS LTD
London and Basingstoke
Associated companies throughout the world
British Library Cataloguing in Publication Data
Macmillan dictionary of astronomy. - 2nd ed
1. Astronomy - Dictionaries
520' .3'21 QB14
ISBN 978-0-333-39243-0 ISBN 978-1-349-17803-2 (eBook)
DOI 10.1007/978-1-349-17803-2
Prepared for automatic typesetting by
Market House Books Ltd Aylesbury
Preface
The tremendous advances in astronomy in the past five years, combined with
the enthusiastic reception of the original edition, have encouraged the
publishers to call for this second edition of the Dictionary ofA stronomy. New
and improved ground-based observational techniques and instruments and
several highly successful astronomical satellites and planetary probes have
produced a flood of information together with new and modified ideas about
celestial objects. These advances in astronomy are described in the new
dictionary: entries in the first edition have been updated, often extensively,
and over 250 entries have been added, bringing the total to more than 2300.
The dictionary contains several long entries in which a word of primary
importance is defined and discussed and in which closely associated words,
printed in italics, are also defined. In these and in the shorter entries, an
asterisk (*) before a word or group of words indicates additional entries that
the reader could consult for further information. Synonyms and abbrevia
tions of entries are placed in brackets immediately following the headword.
Tables at the back of the dictionary contain general information while tables
and illustrations in the text relate to specific entries.
I would like to express my thanks and appreciation to all those involved in
the preparation of the second edition, especially the contributors who
produced the new and updated entries and those at astronomical centres in
the UK, Europe, and the USA who have readily supplied information on
request.
Valerie Illingworth
November 1984
absorption spectrum
A
AA T. Abbrev. for Anglo-Australian and also for those falling at oblique
telescope. angles on the lens or mirror surface.
The four principal aberrations are
Abell catalogue. The standard cata *chromatic aberration (lenses only),
logue of rich *clusters of galaxies visi *spherical aberration, *coma, and
ble from the northern hemisphere. It *astigmatism. *Curvature of field and
was published by George Abell in *distortion are other aberrations.
1958. Chromatic aberration occurs when
more than one wavelength is present
aberration. I. (aberration of starlight). in the incident light beam. For light
The apparent displacement in the po of a single wavelength, only the latter
sition of a star due to the finite speed five aberrations occur. These image
of light and to the motion of the ob defects may be reduced - but not
server, which results primarily from completely eliminated - in an optical
the earth's orbital motion around the system by a suitable choice of optical
sun. It was discovered in 1729 by the materials, surface shape, and relative
English astronomer James Bradley. positions of optical elements and
Light appears to approach the observ stops. See also achromatic lens; cor
er from a point that is displaced recting plate.
slightly in the direction of the earth's 3. A defect in the image produced
motion. The angular displacement, a, by an electronic system using magnet
is given by the ratio vic, where v is ic or electronic lenses.
the earth's orbital velocity and c is
the speed of light. Using the earth's absolute magnitude. See magnitude.
mean orbital speed gives the constant
of aberration, equal to 20.4955 arc absolute temperature scale. See ther
seconds. Over the course of a year, modynamic temperature scale.
the star appears to move in a small
ellipse around its mean position; the absolute zero. The zero value of the
ellipse becomes a circle for a star at *thermodynamic temperature scale,
the pole of the *ecliptic and a i.e. 0 K (-273.l5°C). Absolute zero is
straight line for one on the ecliptic. the lowest temperature theoretically
The maximum displacement, i.e. the possible. At absolute zero molecular
semimajor axis of the ellipse, is 20.5 motion almost ceases.
arc seconds.
The aberration due to the earth's absorption lines, bands. See absorption
orbital motion is sometimes termed spectrum.
annual aberration to distinguish it
from the very much smaller diurnal absorption nebula. See dark nebula.
aberration that results from the earth's
rotation on its axis. Compare annual absorption spectrum. A *spectrum that
parallax. is produced when *electromagnetic ra
2. A defect in the image formed by diation has been absorbed by matter.
a lens or curved mirror, seen as a Typically, absorption spectra are pro
blurring and possible false coloration duced when radiation from an incan
in the image. Aberrations occur for descent source, i.e. radiation with a
all light rays lying off the optical axis *continuous spectrum, passes through
abundance 2
cooler matter. Radiation is absorbed in a gravitational field. It is equal to
(i.e. its intensity is diminished) at se GM/ R2, where G is the gravitational
lective wavelengths so that a pattern constant and M and R are the mass
of very narrow dips or of wider and radius of the planet. The acceler
troughs - i.e. absorption lines or ation is thus independent of the mass
bands - are superimposed on the of the accelerated object, i.e. it is the
continuous spectrum. same for all bodies (neglecting air re
The wavelengths at which absorp sistance) falling at the same point on
tion occurs correspond to the energies the surface of the planet, satellite, etc.
required to cause transitions of the On earth the standard value of the
absorbing atoms or molecules from acceleration of gravity is 9.806 65 me
lower *energy levels to higher levels. tres per second per second. The value
In the hydrogen atom, for example, varies from place to place on the
absorption of a photon with the re earth's surface because of different
quired energy results in a 'jump' of distances to the earth's centre and
the electron from its normal orbit to greater acceleration towards the equa
one of higher energy (see hydrogen tor. In addition, it is affected by local
spectrum). deposits of light or heavy materials.
The absorption lines (or bands) of
a star are produced when elements accretion (aggregation). The increase
(or compounds) in the outermost lay in mass of a body by the addition of
ers of the star absorb radiation from smaller bodies that collide and stick
a continuous distribution of wave to it. The relative velocity of any two
lengths generated at a lower level in colliding bodies must be low enough
the star. Basically the same elements for them to coalesce on impact rather
occur in stars. Since each element has than fly apart. Once a large enough
a characteristic pattern of absorption body forms, its gravitational attrac
lines for any particular temperature tion accelerates the accretion process.
(and pressure) range, there are several Accretion is now assumed to have
different types of stellar spectra de had an important role in the forma
pending on the surface temperature of tion of the planets from swarms of
the star. See spectral types. See also dust grains. See solar system, origin.
emission spectrum.
accretion disc. See black hole; mass
abundance. See cosmic abundance. transfer.
Acamar. See Eridanus. Achemar (a Eri). A conspicuous blu
ish-white star that is the brightest in
acceleration. 1. Oinear acceleration). the constellation Eridanus. Illy: 0.5;
Symbol: a. The rate of increase of ve M_.: -2.2; spectral type: B5 V; dis
locity with time, measured in metres tance: 35 pc.
per second per second (m s-2), etc.
2. (angular acceleration). Symbol: a. Achilles. The first member of the
The rate of increase of *angular ve *Trojan group of *minor planets to
locity with time, usually measured in be discovered, in 1906. It lies east of
radians per second per second. Jupiter and precedes the latter in its
orbit of the sun. See Table 3,
acceleration of gravity. Symbol: g. The backmatter.
acceleration to the centre of a planet
(or other massive body such as a nat achondrite. Any of a class of *stony
ural satellite) of an object falling free meteorites that lack the characteristic
ly without air resistance, i.e. *chondrules of the *chondrites. They
acceleration due to downward motion are usually more coarsely crystallized
3 active regions
than the chondritic stones and are 1t 1s the radio-emitting lobes created
more similar chemically and minera by *beams emanating from the pow
logically to some terrestrial rocks. erhouse that are observed. There is
They contain very little iron and nick mounting evidence that quasars and
el. Seyferts are associated with spiral ga
lactic structure while radio galaxies,
achromatic lens (achromat). A two-ele BL Lac objects, and possibly N gal
ment lens - a doublet - that greatly axies might represent activity in ellip
reduces *chromatic aberration in an tical galaxies.
optical system. The components, one Observations of the motions of
converging and the other diverging in stars and gas in galaxies such as M87
action, are of different types of glass (see Virgo A) and NGC 4151 (see
(i.e. they have different *refractive in Seyfert galaxy), in addition to theoret
dices); the combination focuses two ical arguments (see quasar), strongly
selected colours, say red and blue, at suggest that the energy output is de
a common image plane with a small rived from the gravitational pot-ential
spread in focal length for other of a supermassive *black hole: the en
colours. The difference in optical ergy would arise from an accretion
power (reciprocal of focal length) for disc of gases spiralling into the black
the two colours in one element must hole. This gas could come from the
cancel that in the other element. By a interstellar medium of a spiral galaxy
suitable choice of glasses and surface (especially when perturbed by gravita
curvatures, the doublet can be *apla tional effects in *interacting galaxies),
natic as well as achromatic, so that from the tidal disruption of stars near
three major aberrations are mini the black hole, or from flows of inter
mized. galactic gas on to the central galaxy
Residual colour effects in an achro of a *cluster of galaxies, as the gas
mat can be further reduced by using cools (see Perseus cluster).
a compound lens of three or more el
ements - an apochromatic lens; each active optics. The techniques and
element has an appropriate shape and technology by which any distortion,
dispersive power so that three or arising from atmospheric effects, of a
more colours can be focused in the *wavefront entering a telescope can
same image plane. be sensed and information fed back
to a deformable primary mirror whose
acronical rising (or setting). The rising profile is continuously adjusted to
(or setting) of a star at or just after compensate. This should in theory
sunset. When the rising is acronical lead to diffraction-limited perform
the setting is cosmic and vice versa. ance at all times (see Airy disc).
Acrux. See Alpha Crucis. active prominence. See prominences.
active galaxy. A galaxy that is emit active-region filament. See prom-
ting unusually large amounts of ener inences.
gy from a very compact central
source. (A separate category, active regions. Regions of intense lo
*starburst galaxy, is now employed calized magnetic field on the sun that
for a galaxy where a high infrared lu extend from the *photosphere,
minosity arises from intense star for through the *chromosphere, to the
mati'on.) This central powerhouse may *corona. They may encompass a vari
be observed directly, as in *Seyfert ety of phenomena, such as *sunspots,
galaxies, *N galaxies, '~<BL Lac ob *faculae/*plages, *filaments (or
jects, or *quasars; in *radio galaxies *prominences), and *flares, and are
active sun 4
characterized by enhanced emission of seconds per century. This agrees al
radiation at x-ray, ultraviolet, and ra most exactly with the discrepancy be
dio wavelengths. tween the experimentally determined
value for the advance and that pre
active sun. The term applied to the dicted by classical Newtonian me
sun around the maximum of the chanics. It was therefore an important
*sunspot cycle, when the profusion of confirmation of general relativity. Re
*active regions ensures a high level of cent measurements of the advance of
*solar activity. Compare quiet sun. the perihelia of Venus and Earth have
also been very close to Einstein's pre
Adhara (e CMa). A very luminous re dicted values for those planets.
mote blue-white giant that is the sec
ond brightest star in the constellation aeon. A period of one thousand mil
Canis Major. It is a visual double star lion years: 109 years.
with an 8th-magnitude companion at
a fixed separation of 8". IDy: 1.5; Mv: aerial. See antenna.
-5.0; spectral type: B2 II; distance:
200 pc.
aerolite. See stony meteorite.
adiabatic process. A process that takes
aeronomy. The physics and chemistry
place in a system with no heat trans
of the upper atmosphere of the earth,
fer to or from the system. In general,
i.e. its temperature, density, motions,
a temperature change usually occurs
composition, chemical processes, reac
in an adiabatic process.
tions to solar and cosmic radiation,
etc. The term has been extended to
adiabatic theory. See galaxies, forma
include the physics and chemistry of
tion and evolution.
the atmospheres of the other planets.
Adonis. A member of the *Apollo
aerospace. The earth's atmosphere
group of *minor planets. Discovered
and the space beyond.
in 1936 when it passed 0.015 AU
from the earth, it was not observed
again until it was recovered in 1977. Ae stars. Hot stars of *spectral type
Its *perihelion distance of 0.44 AU is A that in addition to the normal ab
the fourth smallest among the minor sorption spectrum have bright emis
planets. See Table 3, backmatter. sion lines of hydrogen. These lines are
thought to arise in an expanding at
advance of the perihelion. The gradual mospheric shell of matter lost from
movement of the *perihelion of a the star. Like some *Be stars they are
planet's elliptical orbit in the same di probably similar to *T Tauri stars ex
rection as that of the planet's orbital cept that they have larger masses.
motion. This advance results from the
slow rotation of the major axis of the afocal system. An optical system in
planet's orbit due to gravitational dis which both the object and the second
turbances by other planets and to the ary image are infinitely distant. It is
curvature of *spacetime around the the usual adjustment in a simple re
sun. The small contribution from the fracting telescope and is produced
curvature of spacetime was predicted when the objective and eyepiece
by Einstein's general theory of *rela lenses are separated by the sum of
tivity. their focal lengths: the lenses are then
The value of this relativistic contri confocal.
bution towards the advance of the
perihelion of Mercury is about 43 arc AG catalogue. See AGK.
