Encyclopedia of astronomy and astrophysics Vol. 1, A-Gel.

5306 Pages·2001·143.161 MB·English

by  Paul Murdin

O¨pik(Opik),ErnstJulius(1893–1985) ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
O¨pik (Opik), Ernst Julius (1893–1985)
Born in Estonia, O¨pik studied at Moscow University,
and helped establish Turkestan University in Tashkent,
becomingtheAstronomer(director)atTartuObservatory
in Estonia. He fled the RedArmy by horse cart during
theSecondWorldWarandwenttoArmaghObservatory
(NorthernIreland)in1948. Hiswide-ranginginterestsare
reflected in his discoveries and theories. These include
the discovery of degenerate stars, e.g. white dwarfs, in
his calculation of the density of o2 Eridani (1915). He
calculatedthedistanceofM31as450000parsecsfromthe
Sun(1922). Hecomputedbyhandevolutionarymodels
of main-sequence stars into giants (1938) over a decade
earlier than the computer computations of HOYLE and
SCHWARZSCHILD.Hepredictedthedensityofcratersonthe
surface of Mars, which was confirmed 15 years later by
planetary probes. He put forward an unproven theory
of the IceAges based on a calculation of changes in the
convectionintheinternalstructureoftheSunratherthan
MILANKOVITCHcycles.
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Ångstro¨m,AndersJonas(1814–74) ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
Ångstro¨m, Anders Jonas (1814–74)
Physicist,borninLo¨dgo¨,Sweden. Hewaskeeperofthe
observatory and professor at Uppsala where he studied
heat,magnetismandopticsandexaminedthespectraof
the Sun and auroras. His name is commemorated with
the angstrom unit, 10−10 m, which is used for measuring
wavelengths of light and x-rays, and the separation of
atomsinmoleculesandcrystals.
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61Cygni ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
61 Cygni
Thestar61Cygniisimportantbecauseofitslargeproper
motion, first measured at the Palermo Observatory by
GiuseppePiazzi(1746–1826). Thestarbecamepopularly
knownas‘Piazzi’sflyingstar’.Itsmeasuredannualproper
motionof5.23(cid:1)(cid:1) isstilltheseventhlargestknown, andis
thelargestforanaked-eyestar(apparentmagnitude4.8).
Thisstarwasalsothefirsttohaveitsannualparallax
reliably determined, by Friedrich Bessel (1784–1846).
Bessel’smeasurementof0.314(cid:1)(cid:1),madeatKo¨nigsbergwith
Fraunhofer’s6.25inchheliometerin1838,comparesquite
wellwiththemodernvalueof0.286(cid:1)(cid:1). Determinationsof
stellarparallaxmadeinthesameyearbyotherobservers
werefarlessaccurate. Itistheeleventhcloseststar,ata
distanceof11.4light-years.
61 Cygni is also a well-known binary system, with
a period of 653.3 years. Its components are currently
separated by 30.3(cid:1)(cid:1) at position angle 150◦. The primary
star 61 Cyg A is an orange dwarf, spectral type K5V,
ofapparentmagnitude5.20andabsolutemagnitude7.5.
Its companion 61 Cyg B is also an orange dwarf, of
spectral type K7V, with an apparent magnitude of 6.05
and absolute magnitude 8.3. The system is reputed
to be the most extensively observed double star, some
thousandsofvisualobservationsbeingsupplementedby
morethan34000photographicplates. Preciseastrometric
measurements of these plates have indicated that the
systemhasatleastoneinvisiblecomponent, believedto
beaplanetofsimilarmasstoJupiter,andperhapsasmany
asthree,withorbitalperiodsofbetween5and12years.
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AbastumaniAstrophysicalObservatory ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
Abastumani Astrophysical
Observatory
The Abastumani Astrophysical Observatory (AbAO)—
longitude,42.83;latitude,41.81degrees—wasfoundedin
1932. ItislocatedinAbastumani,inthesouth-westpart
oftheRepublicofGeorgia,250kmwestofthecapitalcity
Tbilisi,onthetopofMountKanobiliat1700m.
AbAO’s primary mission is to enable astronomers
of the former Soviet Union to carry out high-quality
observations. The average number of clear nights is
130 per year with 25% of seeings smaller than one
arcsec. Atpresent, about100staffmembersworkatthe
Observatoryinsixdepartmentsandfourlaboratories.The
Observatory’smainfacilitiesare125cmRitchey–Chre´tien
and70cmmeniscustelescopes.
The major research areas are accretion disks and
pulsars astrophysics, solar system cosmogony, objec-
tiveprismspectroscopy,low-amplitudeshort-periodvari-
ables, AGNs variability, solar physics, solar–terrestrial
phenomenaandatmosphericphysics.
Forfurtherinformationsee
http://gamma.bu.edu/webt/abastumani.
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Abbe,Ernst(1840–1905) ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
Abbe, Ernst (1840–1905)
BorninEisenach,GrandDuchyofSaxe-Weimar-Eisenach
(nowGermany),Abbebecamedirectoroftheobservatory
atJenaandresearchdirectoroftheCARLZEISSopticalworks
in Jena. He discovered the Abbe sine condition, which
describesalensthatwillformanimage,withoutdefects
of coma and spherical aberration. His mathematical
treatmentfoundedthepresent-dayscienceofoptics.
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Abbott,Francis(1799–1883) ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
Abbott, Francis (1799–1883)
Watchmaker,borninDerby,England,convict,transported
toTasmaniain1845. Hemadeastronomicalobservations
attheRossbankObservatoryaftertheendofhissentence
until its closure in 1854 and in his private laboratory in
Hobart.
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AbellClusters ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
Abell Clusters distribution), the composition of the galaxy population
(i.e. the fractions of galaxies of different morphological
Abell clusters are the most conspicuous groupings of types),thedistributionsoftheluminositiesofthegalaxies,
galaxies identified by George Abell on the plates of the the detailed dynamics of the various galaxy classes,
firstphotographicsurveymadewiththeSCHMIDTTELESCOPE dynamicalsubstructureandsegregationandthefraction
atMountPalomarinthe1950s. Sometimes,thetermAbell of the total mass consisting of baryonic (i.e. ‘ordinary’
clustersisusedasasynonymofnearby,opticallyselected nucleonic) matter. An important recent development is
galaxyclusters. thesearchfor,andstudyof,galaxyclustersatverylarge
George Abell constructed a catalogue containing distances(i.e.athighredshifts),whicharethe‘forebears’
2712 of the richest such groupings in the northern sky, ofthelocalrichclustersintheAbellcatalogue. Forthose
which was later extended to the southern sky. It is youngerclustersathighredshifts,theAbellclustersserve
no exaggeration to say that the total sample of 4076 asalocal,present-day,referencepopulation.
clustercandidatesoverthewholeskyhasrevolutionized
the study of the large-scale structure in the universe. Abellclustersasasubsetofthetotalcluster
The Abell catalogue has formed the basis for the first population
quantitativestudiesofthedensestcomponentofthelarge- WhensearchingforclustercandidatesonthePalomarSky
scalestructureinthelocaluniverse. Inrecentyears, the Surveyplates,Abellhadnoinformationaboutdistances
definitionofsamplesofcandidateclustersfromwide-field (or redshifts) of the galaxies. Therefore he used the
survey plates has been repeated with automatic plate- distributionofthegalaxiesinapparentmagnitudetoselect
scanning machines. This showed objectively that the those peaks in the projected galaxy distribution that are
subjectivefactorinAbell’svisualselectionisquitesmall, mostlikelytocorrespondtoaspatiallycompactstructure.
atleastforthericherandmorenearbyclusters. Taking the magnitude of the 10th brightest galaxy as an
The reality of the cluster candidates in Abell’s approximate‘standardcandle’,aredshiftwasestimated;
catalogue has been the subject of some debate, until this yields the angle subtended by a fixed linear size
spectroscopicobservationsoflargenumbersofgalaxiesin of 1.5h−1 Mpc at the distance of the cluster (where h is
the directions of theAbell clusters showed convincingly thevalueofthepresentHubbleparameter, expressedin
thatonlyasmallfractionoftherichclustersaretheresult units of 100 km s−1 Mpc−1). In a circular aperture with
ofchancesuperpositions. Thatis,averylargefractionof radiusequaltothatangle,thenumberofgalaxieswitha
therichclustercandidatesinthecataloguemadebyAbell magnitudenotmorethantwomagnitudesfainterthanthe
(or,includingthesouthernclusters,byAbell,Corwinand third-brightestgalaxywascounted. Finally,thenumberof
Olowin)representcompact,localizedpeaksinthespatial unrelatedgalaxiesintheaperture(anddowntothesame
distributionofgalaxies,mostlywithredshiftslessthan0.2, magnitudelimit)wasestimatedfromthegalaxydensity
andheldtogetherbygravity. inbackgroundfieldswithoutobviousclustercandidates.
Alreadyinthe1930s,FritzZWICKYhadconcludedthat Thecorrectednumberofgalaxies(therichnesscount,
theluminousmatter(i.e.thegalaxies)inclustersrepresents i.e. the estimated number of members in the aperture
onlyabout10%ofthetotalclustermass,mostofwhichcan above the magnitude limit) was found to have an
therefore be detected only through its gravitation. This uncertainty of about 17. Therefore, only clusters with
has led to estimates of the total mass (both visible and a corrected galaxy count of at least 50 were considered
dark matter) by various means. The most common of by Abell to have been sampled in an unbiased fashion
those are the velocities of the galaxies in the cluster, the outtoredshiftsof0.1–0.2. InAbell’soriginal(‘northern’)
amount and temperature of the hot (x-ray-emitting) gas catalogue,1682ofthe2712clustercandidateshaveacount
and the distortion of the images of galaxies at distances of at least 50. The lower limit in richness count must
wellbeyondthatoftheclusterbyGRAVITATIONALLENSING. be applied if one uses the Abell catalogue for statistical
For a long time, several of the better-known Abell purposes.Clearly,manylessrichclustersexistbutatlarger
clusters, like those in the COMA BERENICES and PERSEUS distances–redshiftstheircontrastwithrespecttothefieldis
constellations, have shaped our vision of the class of toolowtoallowarobustdefinitionofastatisticallyreliable
rich, populous, clusters. In this schematic view, rich sample.
clustersaresmooth,roundandvirializedstructures. This In recent years, an extensive redshift survey (the
idealizedpicturecoexistedwiththeknowledgethatthere ESO Nearby Abell Cluster Survey) has been made of
aresignificantvariationsinthevariouspropertiesofthe close to 6000 galaxies in about 100 cluster candidates
Abell clusters. This has led to many studies of those (mostlyfromthesouthernpartoftheAbell,Corwinand
properties,andofcorrelationsbetweenthem,aswellasto Olowin catalogue) with a richness count of at least 50
severalattemptstodescribetheformationandevolution andestimatedredshiftslessthan0.1(seeGALAXYREDSHIFT
of rich clusters. It is now realized that clusters are still SURVEYS). The contamination in these redshift surveys by
formingandevolvingatthepresentepoch. galaxiesthatdonotbelongtothemainclusterisfarfrom
Among the cluster properties that can be studied, negligible, i.e. about 25%. However, the majority of the
andforwhichtheoreticalpredictionshavebeenmadeare redshift surveys contains a spatially compact cluster to
the 3D shape (or rather, the axial ratios of the galaxy whichatleast50%ofthegalaxieswithmeasuredredshifts
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AbellClusters ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
belong. Onlyabout10%ofthecandidateclustersappear or any combination of those. The projected distribution
to be a superposition of two almost equally rich (but of the galaxies has many forms and ranges between the
relatively poor) systems at different redshifts along the followingextremes. Theremaybeacentralconcentration
samelineofsight. of bright galaxies, generally of early type, i.e. ellipticals,
For the spatially compact systems, the velocity and frequently one of them is a cD galaxy, i.e. a
dispersionshowsaglobalcorrelationwithrichnesscount giantellipticalsurroundedbyanextendedenvelope(see
(clusters with higher richness counts on average have ELLIPTICALGALAXIES).Attheotherextremethereareclusters
larger velocity dispersions), but the correlation is very thatdonothaveaclearcentralconcentration.
broad (at least a factor of 2 in both quantities). The In some clusters, the galaxy distribution is quite
uncertainty in the visually estimated richness counts smooth, and in general those clusters contain relatively
mightbethoughttoberesponsibleforthis,butthewidth few spiral galaxies. When the fraction of spiral galaxies
oftherelationdoesnotdecreaseifoneusesrichnesscounts islarge,thegalaxydistributionisingenerallessregular.
basedonmachinescanninginsteadoftheoriginalones. Therelativefractionsofearly-andlate-typegalaxiesare
Forasampleofabout150Abellclusterswithredshifts correlatedwithrichnesscount,andthisisamanifestation
less than 0.15, cluster masses were calculated from the of the morphology–density relation. The latter shows
relativevelocitiesandpositionsofthegalaxies,assuming a clear correlation between the relative fractions of
thatthevirialtheoremholdsinthecentralregionsofthe ellipticals, lenticulars (S0s) and spiral galaxies, and the
clusters. The cluster masses correlate fairly well with (local) projected galaxy density (and therefore radial
the velocity dispersions, but the mass distributions in distance). TheS0smaycontributeupto50%inthecenter,
the various intervals of richness counts appear to have
with ellipticals not far behind and spiral galaxies about
considerable overlap. Therefore, application of a limit
10%. Intheouterparts,ellipticalsarealmostabsentwhile
in richness count to a sample ofAbell clusters (which is
spiralgalaxiesmayrepresentupto60%. Notethatthese
necessary for practical reasons) induces quite a diffuse
areglobalvalues: individualclustersshowaconsiderable
limitinmass.
spreadaroundthese.
The clusters, or rather cluster candidates, inAbell’s
EventhoughinasizeablefractionoftheAbellclusters
cataloguewithrichnesscountsofatleast50arethereforea
the galaxy distribution is not very regular or circularly
subsetofallclustersinthemassrangefromabout4×1013
symmetric, one can always derive the azimuthally
to 2 × 1015M(cid:3). However, for clusters with a velocity averagedprojectednumberdensityprofile(cid:4)(R),inwhich
dispersion of at least 800 km s−1, essentially all richness R is the projected distance from the cluster center, i.e.
countsarelargerthan50. Inotherwords,allclusterswith
theshortestdistancebetweenthelineofsightthrougha
avelocitydispersionofatleast800kms−1 arecontained
galaxy and the cluster center. Several expressions have
inthesamplewithalimitingcountof50,andtheestimate beenproposedforthemathematicaldescriptionof(cid:4)(R),
oftheirspacedensityisunbiased. Clusterswithapparent
velocitydispersionsgreaterthanabout1200kms−1 turn avlalluofewofh(cid:4)ic(hRh),aiv.ee.t(cid:4)h(rRee=pa0r)a,macehtearrsa.cTtehroissteicalreentghtehcRen(ttrhael
outeithertobesuperpositionsortohavelotsofdynamical c
distance at which (cid:4)(R) has decreased by a given factor,
substructure.
say2)andameasureofthedecreaseof(cid:4)(R)intheouter
With the advent of all-sky x-ray surveys like those
parts(generallythelogarithmicslopeαof(cid:4)(R)).
fromtheEINSTEIN(HEAO-2)andROSATmissions,ithasbecome Recently, (cid:4)(R) has been derived for galaxies of
possible to construct complete samples of clusters for
different morphological types in about 70 rich Abell
whichthex-rayfluxfromthehotgasinthepotentialwellof
clusters. In individual clusters, the number of galaxies
theclusterislargerthanathresholdvalue. Thisproduces
of a particular type is generally not sufficient to allow
clustercataloguesthatarefundamentallydifferentfrom,
an accurate estimate of the three parameters of (cid:4)(R).
andthuscomplementaryto,theAbellcatalogue,although
By properly combining data for many clusters one can
there is quite some overlap. The mass of the x-ray gas
compare the representations of (cid:4)(R) for ellipticals, S0s,
is generally at least as large as the mass of the cluster
spiralsandgalaxieswithemissionlines(mostlyvery‘late’
galaxies, but the combined mass of these two baryonic
spirals, such as Sc and Sd, with ionized gas in their
components is typically only 10–15% of the total mass.
interstellar medium). In other words, by sacrificing the
When the total mass of a cluster can be estimated both
detailed properties of individual clusters, one obtains a
from the kinematics of the galaxies and from the x-ray
temperatureandbrightness,thetwoestimatesingeneral pictureofanaveragerichAbellcluster.
agreereasonablywell. Thereappearstobeaclearcorrelationbetweengalaxy
type and (cid:4)(R): the characteristic length R increases
c
PropertiesofthegalaxypopulationinAbell markedlyfromearlytolategalaxytype(fromabout0.1to
clusters 0.5Mpc).Thisshowsthatellipticalsareindeedmuchmore
Inthepast, severalschemeshavebeenproposedforthe centrally concentrated than spirals, while the emission-
classification of Abell clusters. All of them summarize linegalaxiesformthemostextendedpopulation. These
in one way or another the distribution of the cluster differences must be accompanied by differences in the
galaxies in position, magnitude or morphological type, kinematics of galaxies of the various types, because all
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AbellClusters ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
galaxyclassesmoveinthesameclusterpotential,which The shapes of Abell clusters have been derived
ismostlydeterminedbythedarkmatter. from the projected distributions of galaxies. Using the
Such kinematical differences are indeed observed: galaxy positions irrespective of galaxy type, one can
the ellipticals and S0s show the smallest dispersion of calculatetheapparentellipticityofacluster. Ingeneral,
the line-of-sight component of their velocities, and this the richer clusters are less elongated than the less rich
dispersion varies little with projected distance from the ones. The apparent ellipticities for a cluster sample
center. Spirals, and in particular emission-line galaxies, of about 100 northern Abell clusters suggest that the
have a larger velocity dispersion (by as much as 20– elongated clusters are prolate (cigar like) rather than
30%)whichdecreasesmarkedlytowardslargerprojected oblate. Comparison of these data with the results of
distances. Actually, the kinematics of the emission-line numericalN-bodycalculationscanconstrainthetheories
galaxies indicates that they have not yet traversed the ofstructureformation.
densecentralcores,whichisprobablythereasonwhythey Thefulldistributionofthemassesofavolume-limited
havenotyetlosttheirline-emittinggasinencounterswith sampleofAbellclusters(i.e.itsshapeandnormalization)
othergalaxies. can also give information for cosmological structure
Combining the projected galaxy distributions with formationtheory. AsthesampleofAbellclusterswitha
the kinematics one may estimate the distribution of the limiting richness count of 50 has a rather badly defined
total (visible plus dark) mass via the Jeans equation of completeness at smaller masses, one must restrict the
stellar dynamics. By comparing the distribution of the comparisonbetweenobservationsandpredictionstothe
dark matter with that of the luminosity of the galaxies, most massive clusters for which the Abell catalog is
complete.
one can in principle study the variation of the so-called
Itisfarfromtrivialtoderiveindependentinformation
mass-to-light ratio with distance from the cluster center.
fortheseveralparametersintheformationtheoriesthat
Thismaygivecluesaboutdetailsoftheformationprocess,
influence the properties of the most massive structures.
suchastheeffectsofgalaxyencounters,theroleofthedark
Yet, there seems to be general agreement that the latter
matterhaloesofthegalaxies,etc.
aremorenaturallyunderstoodinauniverseinwhichthe
matter density is considerably smaller than the critical
Abellclustersascosmologicalprobes
density.
Several observational properties of Abell clusters have
beenusedtoconstrainthetheoriesofformationoflarge-
Bibliography
scalestructureintheuniverseandtheparametersinthose
Book:
theories (see also UNIVERSE: SIMULATIONS OF STRUCTUREAND
GALAXYFORMATION). These properties include the spatial Giurin G and Mezzetti M (ed) 1999 Observational
distribution of Abell clusters, their shapes and their Cosmology:TheDevelopmentofGalaxySystems(Astron.
masses. Indifferentways,theseallcarryinformationon SocietyofthePacificConf.Ser.176)
thewayinwhichthelargestwell-developedstructuresin
Journalarticles:
theuniversehaveformedthroughthegrowthoftheinitial
fluctuationsinthematterdensity.
AbellGO1958Thedistributionofrichclustersofgalaxies
The spatial distribution of Abell clusters has been
Astrophys.J.Suppl.3211–88
analyzedthroughthetwo-pointcorrelationfunctionξ(r),
AbellGO,CorwinHGandOlowinRP1989Acatalogof
i.e.thefractionofclusterpairswithacertainseparation,
richclustersofgalaxiesAstrophys.J.Suppl.701–138
in excess of the expected number of pairs for a random
distribution,whichhasbeenderivedforclustersofvarious Reviews:
richness counts. In general, the correlation function
is found to have a power-law form: ξ(r) = (r/r )−γ; Bahcall N A 1977 Clusters of galaxies Ann. Rev. Astron.
0
the exponent γ (about 2) does not appear to depend Astrophys.15505
on the limiting richness count, but the value of the Bahcall N A 1988 Large-scale structure in the Universe
correlation length r does, and is larger for the richer indicated by galaxy clusters Ann. Rev. Astron.
0
clusters (with a characteristic value of about 20 Mpc). Astrophys.26631
SarazinCL1986X-rayemissionfromclustersofgalaxies
In principle, these data allow one to derive the value of
Rev.Mod.Phys.581
thecosmologicaldensityaswellastheamplitudeofthe
fluctuationspectrum.
Another aspect of the distribution of rich Abell PeterKatgert
clusters is that they are generally located in the vertices
where the sheets and filaments in the general galaxy
distribution come together. Therefore, the distribution
of rich clusters has sometimes been compared with
the distribution of the vertices in so-called Voronoi
tesselations,whicharegeometricpartitioningsofspace.
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Aberration ENCYCLOPEDIAOFASTRONOMYANDASTROPHYSICS
Aberration
(1) The apparent displacement of a star from its mean
position on the celestial sphere due to the velocity of
the Earth in its orbit around the Sun. The phenomenon
was discovered in 1729 by James Bradley (1693–1762)
whowas,infact,tryingtomeasurestellarparallax. The
displacementiscausedbythecombinationofthevelocity
oftheEarthandthevelocityoflightapproachingfromthe
source.IftheEarthwerestationary,lightfromastarwould
arrivefromthetruedirectionofthissource,butthemotion
oftheEarthcausesthelighttoappeartobeapproaching
from a point which is slightly displaced in the direction
oftheEarth’smotion. Inthecourseofayear,astheEarth
travelsroundtheSun,astarwilltraceoutasmallellipsein
theskyaboutitsmeanposition. Themaximumradiusof
thisellipse(inradians)isequaltotheratioofthespeedof
theEarthtothespeedoflight(30kms−1:300000kms−1),
that is about 20.5 seconds of arc. The eccentricity of the
ellipse depends on the celestial latitude of the star (the
figure becomes a circle at the pole of the ecliptic and a
straightlineontheecliptic).
Thedisplacementduetoaberrationismuchgreater
thanthatduetoparallax(theannualparallaxofthenearest
star is 0.76 seconds of arc) and this must be allowed for
beforetheparallaxcanbedeterminedforastar.Asimilar,
thoughsmaller,aberrationeffectoccursduetothespeed
ofrotationoftheEarthonitsaxis.Thisisknownasdiurnal
aberration.
(2) In optical systems, such as lenses and curved
mirrors,aberrationreferstotheinabilityofthesystemto
produce a perfect image. Unlike a plane mirror, which
doesnotcreateaberrations,alensorcurvedmirrorisan
imperfect image producer, becoming ideal only for rays
passingthrough(orreflectingfrom)itscenterparallelto
theopticalaxis(alinethroughthecenter, perpendicular
tothelenssurfaces). Themainaberrationsarechromatic,
spherical,comaandastigmatism.
Seealso: astigmatism,atmosphericrefraction,chromatic
aberration,coma,scintillation,sphericalaberration.
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