Philip's Astronomy Encyclopedia

465 Pages·2002·31.521 MB·English

by  Eugene H. Ehrlich

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ASTRONOMY
ENCYCLOPEDIA
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ASTRONOMY
ENCYCLOPEDIA
F L J. R
OREWORD BY EIF OBINSON
Editor Emeritus, Sky & Telescope magazine
S M W T
TAR APS CREATED BY IL IRION
GENERAL EDITOR SIR PATRICK MOORE
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HOW TO USE THE ENCYCLOPEDIA
PHILIP’SASTRONOMYENCYCLOPEDIA Alphabetical order
First published in Great Britain in 1987 by Mitchell Beazley ‘Mc’ is treated as if it were spelled ‘Mac’, and certain shortened forms as if spelled
under the title The Astronomy Encyclopedia (General Editor out in full (e.g. ‘St’ is treated as ‘Saint’). Entries that have more than one word in the
Patrick Moore)
heading are alphabetized as if there were no space between the words. Entries that
This fully revised and expanded edition first published in 2002 by share the same main heading are in the order of people, places and things. Entries
Philip’s, an imprint of Octopus Publishing Group beginning with numerals are treated as if the numerals were spelled out (e.g. 3C
2–4 Heron Quays follows three-body problem and precedes 3C 273). An exception is made for HI
London E14 4JP region and HII region, which appear together immediately after Hirayama family.
Biographies are alphabetized by surname, with first names following the comma.
Copyright © 2002 Philip’s
(Forenames are placed in parentheses if the one by which a person is commonly
ISBN 0–540–07863–8 known is not the first.) Certain lunar and planetary features appear under the main
element of names (e.g. Imbrium, Marerather than Mare Imbrium).
All rights reserved. Apart from any fair dealing for the purpose
of private study, research, criticism or review, as permitted under
Cross-references
the Copyright Designs and Patents Act, 1988, no part of this
publication may be reproduced, stored in a retrieval system, or SMALL CAPITALS in an article indicate a separate entry that defines and explains the
transmitted in any form or by any means, electronic, electrical, word or subject capitalized. ‘See also’ at the end of an article directs the reader to
chemical, mechanical, optical, photocopying, recording, or entries that contain additional relevant information.
otherwise, without prior written permission. All enquiries should
be addressed to the Publisher. Measurements
Measurements are given in metric (usually SI) units, with an imperial conversion (to
A catalogue record for this book is available from the British Library
an appropriate accuracy) following in parentheses where appropriate. In historical
Printed in Spain contexts this convention is reversed so that, for example, the diameter of an early tele-
scope is given first in inches. Densities, given in grams per cubic centimetre, are not
Details of other Philip’s titles and services can be found on our converted, and neither are kilograms or tonnes. Large astronomical distances are usu-
website at www.philips-maps.co.uk
ally given in light-years, but parsecs are sometimes used in a cosmological context.
Managing Editor Caroline Rayner Particularly in tables, large numbers may be given in exponential form. Thus 103is a
Technical Project Editor John Woodruff thousand, 2 (cid:1)106is two million, and so on. ‘Billion’ always means a thousand million,
Commissioning Editor Frances Adlington or 109. As is customary in astronomy, dates are expressed in the order year, month,
Consultant Editor Neil Bone day. Details of units of measurement, conversion factors and the principal abbrevia-
Executive Art Editor Mike Brown
tions used in the book will be found in the tables on this page.
Designer Alison Todd
Picture Researcher Cathy Lowne
Production Controller Sally Banner Stellar data
In almost all cases, data for stars are taken from the HIPPARCOSCATALOGUE. The very
few exceptions are for instances where the catalogue contains an error of which the
THE GREEK ALPHABET
editors have been aware. In tables of constellations and elsewhere, the combined mag-
αΑalpha η Ηeta ν Νnu τ Τ tau nitude is given for double stars, and the average magnitude for variable stars.
β Βbeta θ Θtheta ξ Ξxi υ Υupsilon
γ Γ gamma ι Ι iota ο Οomicron φ Φphi Star Maps pages 447–55
δ ∆ delta κ Κkappa π Πpi χ Χchi
ε Ε epsilon λ Λlambda ρ Ρ rho ψΨpsi Acknowledgementspage 456
ζ Ζ zeta µ Μmu σ Σ sigma ωΩomega
FRONTMATTERIMAGES
Endpapers: Andromeda GalaxyThe largest member of the Local Group, this galaxy is the farthest
MULTIPLES AND SUBMULTIPLES
object that can be seen with the naked eye.
USED WITH SI UNITS
Half-title: Crab NebulaThis nebula is a remnant of a supernova that exploded in the constellation
Multiple Prefix Symbol Submultiple Prefix Symbol of Taurus in 1054.
103 kilo- k 103 milli- m Opposite title: M83Blue young stars and red HII emission nebulae clearly mark out regions of star
106 mega- M 106 micro- m formation in this face-on spiral galaxy in Hydra.
109 giga- G 109 nano- n Opposite Foreword: NGC 4945This classic disk galaxy is at a distance of 13 million l.y. Its stars
1012 tera- T 1012 pico- p are mainly confined to a flat, thin, circular region surrounding the nucleus.
1015 peta- P 1015 femto- f Opposite page 1: EarthThis photograph was obtained by the Apollo 17 crew en route to the
1018 exa- E 1018 atto- a Moon in 1972 December.
SYMBOLS FOR UNITS, CONSTANTS AND QUANTITIES CONVERSION FACTORS
a semimajor axis L luminosity t time Distances
Å angstrom unit Ln Lagrangian points T temperature (absolute), epoch 1 nm = 10 Å
AU astronomical unit (n= 1 to 5) (time of perihelion passage) 1 inch = 25.4 mm
c speedof light l.y. light-year Teff effective temperature 1 mm = 0.03937 inch
d distance m metre, minute v velocity 1ft = 0.3048 m
e eccentricity m apparent magnitude, mass W watt 1 m = 39.37 inches = 3.2808 ft
E energy mbol bolometric magnitude y year 1 mile = 1.6093 km
eV electron-volt mpg photographic magnitude z redshift 1 km = 0.6214 mile
f following mpv photovisual magnitude α constant of aberration, 1 km/s = 2237 mile/h
F focal length, force mv visual magnitude right ascension 1 pc = 3.0857 ×1013 km = 3.2616 l.y. = 206,265 AU
g acceleration due to gravity M absolute magnitude, δ declination 1 l.y. = 9.4607 × 1012km = 0.3066 pc = 63,240 AU
G gauss mass (stellar) λ wavelength Temperatures (to the nearest degree)
G gravitational constant N newton µ proper motion °C to °F : (cid:1)1.8,(cid:4)32
h hour p preceding ν frequency °C to K : (cid:4)273
h Planck constant P orbital period π parallax °F to °C : (cid:5)32, (cid:6)1.8
Ho Hubble constant pc parsec ω longitude of perihelion °F to K : (cid:6)1.8,(cid:4)255
Hz hertz q perihelion distance Ω observed/critical density K to °C : (cid:5)273
i inclination qo deceleration parameter ratio, longitude of ascending K to °F : (cid:1)1.8, (cid:5)460
IC Index Catalogue Q aphelion distance node
Note: To convert temperature differences, rather than points on the
Jy jansky r radius, distance ° degree
temperature scale, ignore the additive or subtractive figure and just
k Boltzmann constant R Roche limit (cid:2) arcminute
multiply or divide.
K degrees kelvin s second (cid:3) arcsecond
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CONTRIBUTORS
Philip’s would like to thank the following contributors for their valuable Dr Dale P. Cruikshank, University of Hawaii, USA
assistance in updating and supplying new material for this edition: Professor J. L. Culhane, Mullard Space Science Laboratory, UK
Dr J. K. Davies, University of Birmingham, UK
Alexander T. Basilevsky, Vernadsky Institute of Geochemistry and Analytical M. E. Davies, The Rand Corporation, California, USA
Chemistry, Moscow, Russia Professor R. Davis, Jr, University of Pennsylvania, USA
Richard Baum, UK D. W. Dewhirst, Institute of Astronomy, Cambridge, UK
Peter R. Bond, FRAS, FBIS, Space Science Advisor for the Royal Astronomical Professor Audouin Dollfus, Observatoire de Paris, France
Society, UK Commander L. M. Dougherty, UK
Neil Bone, Director of the BAA Meteor Section and University of Sussex, UK Dr J. P. Emerson, Queen Mary College, London, UK
Dr Allan Chapman, Wadham College, University of Oxford, UK Professor M. W. Feast, South African Astronomical Observatory, South Africa
Storm Dunlop, FRAS, FRMetS, UK Dr G. Fielder, University of Lancaster, USA
Tim Furniss, UK Norman Fisher, UK
Peter B. J. Gill, FRAS, UK K. W. Gatland, UK
Dr Ian S. Glass, South African Astronomical Observatory, South Africa A. C. Gilmore, Mt John Observatory, University of Canterbury, New Zealand
Dr Monica M. Grady, The Natural History Museum, London, UK Professor Owen Gingerich, Harvard-Smithsonian Center for Astrophysics, USA
Dr Andrew J. Hollis, BAA, UK Dr Mart de Groot, Armagh Observatory, N. Ireland
James B. Kaler, Department of Astronomy, University of Illinois, USA Professor R. H. Garstang, University of Colorado, USA
William C. Keel, Department of Physics and Astronomy, University of L. Helander, Sweden
Alabama, USA Michael J. Hendrie, Director of the Comet Section of the BAA, UK
Professor Chris Kitchin, FRAS, University of Hertfordshire, UK Dr A. A. Hoag, Lowell Observatory, USA
Professor Kenneth R. Lang, Tufts University, USA Dr M. A. Hoskin, Churchill College, Cambridge, UK
Dr Richard McKim, Director of the BAA Mars Section, UK Commander H. D. Howse, UK
Mathew A. Marulla, USA Professor Sir F. Hoyle, UK
Steve Massey, ASA, Australia Dr D. W. Hughes, University of Sheffield, UK
Sir Patrick Moore, CBE, FRAS, UK Dr G. E. Hunt, UK
Dr François Ochsenbein, Astronomer at Observatoire Astronomique de Dr R. Hutchison, British Museum (Natural History), London, UK
Strasbourg, France Dr R. J. Jameson, University of Leicester, UK
Dr Christopher J. Owen, PPARC Advanced Fellow, Mullard Space Science R. M. Jenkins, Space Communications Division, Bristol, UK
Laboratory, University College London, UK Dr P. van de Kamp, Universiteit van Amsterdam, The Netherlands
Chas Parker, BA, UK Professor W. J. Kaufmann, III, San Diego State University, USA
Neil M. Parker, FRAS, Managing Director, Greenwich Observatory Ltd, UK Dr M. R. Kidger, Universidad de La Laguna, Tenerife, Canary Islands
Martin Ratcliffe, FRAS, President of the International Planetarium Society Dr A. R. King, University of Leicester, UK
(2001–2002), USA Dr Y. Kozai, Tokyo Astronomical Observatory, University of Tokyo, Japan
Ian Ridpath, FRAS, Editor Norton’s Star Atlas, UK R. J. Livesey, Director of the Aurora Section of the BAA, UK
Leif J. Robinson, Editor Emeritus Sky & Telescopemagazine, USA Sir Bernard Lovell, Nuffield Radio Astronomy Laboratories, Jodrell Bank, UK
Dr David A. Rothery, Department of Earth Sciences, The Open University, UK Professor Dr S. McKenna-Lawlor, St Patrick’s College, Co. Kildare, Ireland
Robin Scagell, FRAS, Vice President of the Society for Popular Astronomy, UK Dr Ron Maddison, University of Keele, UK
Jean Schneider, Observatoire de Paris, France David Malin, Anglo-Australian Observatory, Australia
Dr Keith Shortridge, Anglo-Australian Observatory, Australia J. C. D. Marsh, Hatfield Polytechnic Observatory, UK
Dr Andrew T. Sinclair, former Royal Greenwich Observatory, UK Dr J. Mason, UK
Pam Spence, MSc, FRAS, UK Professor A. J. Meadows, University of Leicester, UK
Dr Duncan Steel, Joule Physics Laboratory, University of Salford, UK Howard Miles, Director of the Artificial Satellite Section of the BAA, UK
Nik Szymanek, University of Hertfordshire, UK L. V. Morrison, Royal Greenwich Observatory (Sussex, UK)
Richard L. S. Taylor, British Interplanetary Society, UK T. J. C. A. Moseley, N. Ireland
Wil Tirion, The Netherlands Dr P. G. Murdin, Royal Greenwich Observatory (Sussex, UK)
Dr Helen J. Walker, CCLRC Rutherford Appleton Laboratory, UK C. A. Murray, Royal Greenwich Observatory (Sussex, UK)
Professor Fred Watson, Astronomer-in-Charge, Anglo-Australian Observatory, I. K. Nicolson, MSc, Hatfield Polytechnic, UK
Australia J. E. Oberg, USA
Dr James R. Webb, Florida International University and the SARA Observatory, Dr Wayne Orchiston, Victoria College, Australia
USA Dr M. V. Penston, Royal Greenwich Observatory (Sussex, UK)
Dr Stuart Weidenschilling, Senior Scientist, Planetary Science Institute, USA J. L. Perdrix, Australia
Professor Peter Wlasuk, Florida International University, USA Dr J. D. H. Pilkington, Royal Greenwich Observatory (Sussex, UK)
John Woodruff, FRAS, UK Dr D. J. Raine, University of Leicester, UK
Dr R. Reinhard, European Space Agency, The Netherlands
Contributors to the 1987 edition also include: H. B. Ridley, UK
Dr D. J. Adams, University of Leicester, UK C. A. Ronan, East Asian History of Science Trust, Cambridge, UK
Dr David A. Allen, Anglo-Australian Observatory, Australia Professor S. K. Runcorn, University of Newcastle upon Tyne, UK
Dr A. D. Andrews, Armagh Observatory, N. Ireland Dr S. Saito, Kwasan & Hida Observatories, University of Kyoto, Japan
R. W. Arbour, Pennell Observatory, UK Dr R. W. Smith, The Johns Hopkins University, USA
R. W. Argyle, Royal Greenwich Observatory (Canary Islands) Dr F. R. Stephenson, University of Durham, UK
H. J. P. Arnold, Space Frontiers Ltd, UK E. H. Strach, UK
Professor W. I. Axford, Max-Planck-Institut für Aeronomie, Germany Professor Clyde W. Tombaugh, New Mexico State University, USA
Professor V. Barocas, Past President of the BAA, UK R. F. Turner, UK
Dr F. M. Bateson, Astronomical Research Ltd, New Zealand Dr J. V. Wall, Royal Greenwich Observatory (Sussex, UK)
Dr Reta Beebe, New Mexico State University, USA E. N. Walker, Royal Greenwich Observatory (Sussex, UK)
Dr S. J. Bell Burnell, Royal Observatory, Edinburgh, UK Professor B. Warner, University of Cape Town, South Africa
D. P. Bian, Beijing Planetarium, China Professor P. A. Wayman, Dunsink Observatory, Dublin, Ireland
Dr D. L. Block, University of Witwatersrand, South Africa Dr G. Welin, Uppsala University, Sweden
G. L. Blow, Carter Observatory, New Zealand A. E. Wells, UK
Professor A. Boksenberg, Royal Greenwich Observatory (Sussex, UK) E. A. Whitaker, University of Arizona, USA
Dr E. Bowell, Lowell Observatory, USA Dr A. P. Willmore, University of Birmingham, UK
Dr E. Budding, Carter Observatory, New Zealand Dr Lionel Wilson, University of Lancaster, UK
Dr P. J. Cattermole, Sheffield University, UK Professor A. W. Wolfendale, University of Durham, UK
Von Del Chamberlain, Past President of the International Planetarium Society Dr Sidney C. Wolff, Kitt Peak National Observatory, USA
Dr David H. Clark, Science & Engineering Research Council, UK K. Wood, Queen Mary College, London, UK
Dr M. Cohen, University of California, USA Les Woolliscroft, University of Sheffield, UK
P. G. E. Corvan, Armagh Observatory, N. Ireland Dr A. E. Wright, Australian National Radio Astronomy Observatory, Australia
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FOREWORD
The progress of astronomy – or, more precisely, astrophysics – One of the greatest successes of astrophysics in the last century
over the past century, and particularly the past generation, is was the identification of how chemical elements are born.
not easily pigeon-holed. Hydrogen, helium, and traces of others originated in the Big Bang;
On the one hand, profound truths have tumbled abundantly heavier elements through iron derive from the cores of stars; and
from the sky. Here are four diverse examples: still heavier elements are blasted into space by the explosions of
1. Our universe began some 14 billion years ago in a single very massive stars.
cataclysmic event called the Big Bang. The discovery of pulsars in 1967 confirmed that neutron stars
2. Galaxies reside mainly in huge weblike ensembles. exist. Born in supernova explosions, these bodies are only about 10
3. Our neighbouring planets and their satellites come in a kilometres across and spin around as rapidly as 100 times a
bewildering variety. second. Whenever a pulsar’s radiation beam, ‘focused’ by some of
4. Earth itself is threatened (at least within politicians’ horizons) by the strongest magnetic fields known, sweeps over the Earth, we see
impacts from mean-spirited asteroids or comets. the pulse. In addition to being almost perfect clocks, pulsars have
On the other hand, ordinary citizens may well feel that allowed studies as diverse as the interstellar medium and relativistic
astronomers are a confused lot and that they are farther away than effects. Finally, unlike any other astronomical object, pulsars have
ever from understanding how the universe is put together and how yielded three Nobel Prizes!
it works. For example, ‘yesterday’ we were told the universe is Tantalizing though inconclusive evidence for extraterrestrial life
expanding as a consequence of the Big Bang; ‘today’ we are told it accumulates impressively: possible fossil evidence in the famous
is accelerating due to some mysterious and possibly unrelated Martian meteorite ALH 84001, the prospect of clement oceans
force. It doesn’t help that the media dine exclusively on ‘gee-whiz’ under the icy crust of Jupiter’s satellite Europa, and the organic-
results, many of them contradictory and too often reported compound rich atmosphere of Saturn’s moon Titan. And then
without historical context. I can’t help but savour the pre-1960s there is the burgeoning catalogue of planets around other stars and
era, before quasars and pulsars were discovered, when we naïvely the detection of terrestrial life forms in ever more hostile
envisioned a simple, orderly universe understandable in everyday environments. All this suggests that we may not be alone. On a
terms. higher plane, despite many efforts to find extraterrestrial intelligence
Of course, all the new revelations cry out for insightful since Frank Drake’s famous Ozma experiment in 1960, we haven’t
interpretation. And that’s why I’m delighted to introduce this picked up E.T.’s phone call yet. But the search has barely begun.
brand-new edition. So much has been discovered since it first The flowering of astrophysics stems from the development of
appeared in 1987 . . . so much more needs to be explained! ever larger, ever more capable telescopes on the ground and in
It’s sobering to catalogue some of the objects and phenomena space. All the electromagnetic spectrum – from the highest-energy
that were unknown, or at least weren’t much on astronomers’ gamma rays to the lowest-energy radio waves – is now available for
minds, only a generation or two ago. robust scrutiny, not just visible light and long-wavelength radio
The one that strikes me most is that 90% (maybe 99%!) of all emission as was the case in as recently as the 1950s.
the matter in the universe is invisible and therefore unknown. Equally impressive has been the development of detectors to
We’re sure it exists and is pervasive throughout intergalactic space capture celestial radiation more efficiently. In the case of the CCD
(which was once thought to be a vacuum) because we can detect (charge-coupled device), trickle-down technology has allowed small
its gravitational influence on the stuff we can see, such as galaxies. amateur telescopes to act as though they were four or five times
But no one has a cogent clue as to what this so-called dark matter larger. Augmented by effective software, CCDs have caused a
might be. revolution among hobbyists, who, after nearly a century-long hiatus,
Masers, first created in laboratories in 1953, were found in can once again contribute to mainstream astrophysical research.
space only 12 years later. These intense emitters of coherent Increasingly, astronomers are no longer limited to gathering
microwave radiation have enabled astronomers to vastly improve electromagnetic radiation. Beginning late in the last century, they
distance determinations to giant molecular clouds and, especially, started to routinely sample neutrinos, elementary particles that
to the centre of our Galaxy. allow us to peek at such inaccessible things as the earliest times in
A scientific ‘war’ was fought in the 1960s as to whether clusters the life of the universe and the innards of exploding stars. And the
of galaxies themselves clustered. Now even the biggest of these so- gravitational-wave detectors being commissioned at the time of
called superclusters are known to be but bricks in gigantic walls this writing should allow glimpses of the fabric of spacetime itself.
stretching across hundreds of millions of light-years. These walls Astronomy has involved extensive international collaborations
contain most of the universe’s visible matter and are separated for well over a century. The cross-disciplinary nature of
from each other by empty regions called voids. contemporary research makes such collaborations even more
The discovery of quasars in 1963 moved highly condensed compelling in the future. Furthermore, efforts to build the next
matter on to astronomy’s centre stage. To explain their enormous generation of instruments on the ground and especially in space
and rapidly varying energy output, a tiny source was needed, and are so expensive that their funding will demand international
only a black hole having a feeding frenzy could fill the bill. Thus participation.
too was born the whole subdiscipline of relativistic astrophysics, Where do astronomers go from here? ‘Towards the unknown’
which continues to thrive. Quasars are now regarded as having the may seem like a cliché, but it isn’t. With so much of the universe
highest energies in a diverse class called active galaxies. invisible or unsampled, there simply have to be many enormous
Gamma-ray bursts, the most powerful outpourings of energy surprises awaiting!
known in the universe, only came under intense scrutiny by When it comes to the Big Questions, I don’t know whether we
astronomers in the 1990s (they had been detected by secret are children unable to frame our thoughts, or teenagers at sea, or
military satellites since the 1960s). The mechanism that leads to adults awash in obfuscating information. Researchers find the
this prodigious output is still speculative, though a young, very plethora of new discoveries – despite myriad loose ends and
massive star collapsing to form a black hole seems favoured. conundrums – to be very exciting, for it attests to the vibrancy and
A decades-long quest for extrasolar planets and closely related maturation of the science. Yet, as we enter the 21st century,
brown-dwarf (failed) stars came to an abrupt end in 1995 when astronomers are still a very long way from answering the two most
the first secure examples of both entities were found. (By a common and profound questions people ask: what kind of
somewhat arbitrary convention, planets are regarded as having universe do we live in, and is life pervasive?
masses up to several times that of Jupiter; brown dwarfs range
from about 10 to 80 Jupiters.) Improved search strategies and
techniques are now discovering so many of both objects that Leif J. Robinson
ordinary new ones hardly make news. Editor Emeritus, Sky & Telescopemagazine
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absolute temperature
AAOAbbreviation of ANGLO-AUSTRALIANOBSERVATORY a ‘corrected lens’. Examples are the composite OBJECTIVES
in astronomical refractors and composite EYEPIECES. A
AAT Abbreviation of ANGLO-AUSTRALIANTELESCOPE Curvature produces images that are not flat. When A
projected on to a flat surface, such as a photographic film,
AAVSO Abbreviation of AMERICANASSOCIATIONOFVARI- the image may be in focus in the centre or at the edges,
ABLESTAROBSERVERS but not at both at the same time. Astronomers using CCD
cameras on telescopes can use a field flattener to produce
Abbot, Charles Greeley (1872–1961) American a well-focused image across the whole field of view. Often
astronomer who specialized in solar radiation and its this is combined with a focal reducer to provide a wider
effects on the Earth’s climate. He was director of the field of view.
Smithsonian Astrophysical Observatory from 1907. Distortion occurs where the shape of the resulting
Abbot made a very accurate determination of the solar image is changed. Common types of distortion are pin-
constant, compiled the first accurate map of the Sun’s cushion and barrel distortion, which describe the effects
infrared spectrum and studied the heating effect of the seen when an image of a rectangle is formed. Some binoc-
solar corona. He helped to design Mount Wilson Solar ular manufacturers deliberately introduce a small amount
Observatory’s 63-ft (19-m) vertical solar telescope. of pin-cushion distortion as they claim it produces a more
natural experience when the binoculars are panned across
Abell, George Ogden(1927–83) American astronomer a scene. Measuring the distortion in a telescope is
who studied galaxies and clusters of galaxies. He is best extremely important for ASTROMETRYas it affects the pre-
known for his catalogue of 2712 ‘rich’ clusters of galaxies cise position measurements being undertaken. Astromet-
(1958), drawn largely from his work on the PALOMAR ric telescopes once calibrated are maintained in as stable a
OBSERVATORY SKY SURVEY. The Abell clusters, some of condition as possible to avoid changing the distortion.
which are 3 billion l.y. distant, are important because they
define the Universe’s large-scale structure. Abell Abetti, Giorgio (1882–1982) Italian solar physicist,
successfully calculated the size and mass of many of these director of ARCETRI ASTROPHYSICAL OBSERVATORY
clusters, finding that at least 90% of the mass necessary to (1921–52). As a young postgraduate he worked at Mount
keep them from flying apart must be invisible. Wilson Observatory, where pioneering solar astronomer
George Ellery HALEbecame his mentor. Abetti designed
aberration (1) (aberration of starlight) Apparent and constructed the Arcetri solar tower, at the time the
displacement of the observed position of a star from its best solar telescope in Europe, and used it to investigate
true position in the sky, caused by a combination of the the structure of the chromosphere and the motion of
Earth’s motion through space and the finite velocity of sunspot penumbras (the Evershed–Abetti effect).
the light arriving from the star. The effect was discovered
by James BRADLEY in 1728 while he was attempting to ablation Process by which the surface layers of an object
measure the PARALLAX of nearby stars. His observations entering the atmosphere (for example a spacecraft or a
revealed that the apparent position of all objects shifted METEOROID) are removed through the rapid intense
back and forth annually by up to 20(cid:1) in a way that was heating caused by frictional contact with the air. The heat
not connected to the expected parallax effect. shields of space vehicles have outer layers that ablate,
The Earth’s movement in space comprises two parts: preventing overheating of the spacecraft’s interior.
its orbital motion around the Sun at an average speed of
29.8km/s (18.5mi/s), which causes annual aberration, absolute magnitude (M) Visual magnitude that a star
and its daily rotation, which is responsible for the smaller would have at a standard distance of 10 PARSECS. If m =
of the two components, diurnal aberration. The former apparent magnitude and r= distance in parsecs:
causes a star’s apparent position to describe an ELLIPSE M=m(cid:3)5(cid:4)5logr
over the course of a year. For any star on the ECLIPTIC, this For a minor planet this term is used to describe the
ellipse is flattened into a straight line, whereas a star at the brightness it would have at a distance of 1AU from the
pole of the ecliptic describes a circle. The angular dis- Sun,1 AU from the Earth and at zero PHASEANGLE(the
placement of the star, (cid:2), is calculated from the formula Sun–Asteroid–Earth angle, which is a physical
tan(cid:2)=v/c, where vis the Earth’s orbital velocity and cis impossibility).
the speed of light.
Diurnal aberration is dependent on the observer’s posi- absolute temperature Temperature measured using
tion on the surface of the Earth. Its effect is maximized at the absolute temperature scale; the units (obsolete) are
the equator, where it produces a displacement of a stellar ºA. This scale is effectively the same as the modern
position of 0(cid:1).32 to the east, but drops to zero for an thermodynamic temperature scale, wherein temperature
observer at the poles.
Bradley’s observations demonstrated both the motion of
the Earth in space and the finite speed of light; they have
influenced arguments in cosmology to the present day.
angle of
aberration
aberration(2) Defect in an image produced by a LENS
or MIRROR. Six primary forms of aberration affect the
quality of image produced by an optical system. One of
these, CHROMATIC ABERRATION, is due to the different
amount of refraction experienced by different
wavelengths of light when passing through the boundary
between two transparent materials; the other five are
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The five Seidel aberrations are SPHERICALABERRATION, path away from the optical axis
COMA, ASTIGMATISM, curvature and distortion. All but produces coma, drawing the
spherical aberration are caused when light passes through star image into a ‘tail’ (3). Offset
the optics at an angle to the optical axis. Optical designers of the star’s position (2) can
strive to reduce or eliminate aberrations and combine lens- 1 2 3 reduce the effectiveness of the
es of different glass types, thickness and shape to produce telescope for astrometry.
1