Webb Society Deep-Sky Observers Handbook: Open and Globular Clusters

Webb Society Deep-Sky Observer's Handbook Volume 3 Open and Globular Clusters .

Compiled by the Webb Society Editor: Kenneth Glyn lones, F.R.A.S. Written and Illustrated by Edmund S. Barker, F.R.A.S. With a foreword by Professor Helen Sawyer Hogg (David Dunlap Observatory, University of Toronto)

Enslow Publishers

Hillside, New lersey 072 05

1980

To Professor Helen Sawyer Hogg, Pre-eminent observer of Star Clusters, With gratitude and affection

Copyright

©

1980 by the Webb Society

All rights reserved. No part of this book may be reproduced by any means without the written permission of the publisher.

Library of Congress Cataioging in Publication Data Webb Society.

Webb Society deep-sky observer's handbook. First published in 1975- under title: The Webb Society observers handbook. Includes bibliographies. CONTENTS: v. 1. Double Stars.- v. 2. Planetary and gaseous nebulae.- v. 3. Open and Globular Clusters. 1. AstronomY-Observers' manuals. I. lones, Kenneth Glyn. II. Title. QB64.w35 1979

523

78-31260

In the U.S.A.: ISBN 0-89490-034-X (vol.3)

Manufactured in the United States of America 10 9 8 7 6

CON T ENTS l lu s trations Li s t O f I Fo r eword eface to the Handboo k Gen e r a1 Pr Vo l ume 3: Open and Glo bular Clusters p re fac,e to Open and Globul ar Clus ters In tro duct ion to

vi vii vi i i ix 1

PART ONE: Open Clusters Hi s t or i cal Revi ew 1. O_Ass oci ati o ns 2. G alac tic Open Clusters 3 . Catal o gues o f Open Cluster s 4 . Obs ervati on o f Open Clus ters

3 5 7 17 19

PART TWO: Globular Clus t er s Historical Revi ew 5. Gal actic Globular Clusters 6. Globul ar Clusters in External Gal axi e s 7. Catal o gues of Globular C lu s t ers 8. Observati on o f Globular Clu s ters

25 27 33 35 37

PART THREE A Catalogue o f Open Clusters Li st of Additional Obj ec t s

43 1 45

PART FOUR A Catalogue of Gl obul ar Clusters List of Additi onal Obj ects

1 51 1 89

APPENDI CES Clas s 7 Open Clus ters Furth er Observ at i ons o f Open Cluste rs The Di stance s of Clus ters Pho togr aphic Sourc e s for Open and Globular Cluster s 5 . Bi bl i ography

1. 2. 3. 4.

vii

191 1 97 201 203 205

FOREWORD

LIST OF ILLUSTRATIONS O-B2 A s s o c i ations within 1000 pc o f the Sun Di stri bution on the Gal acti c Pl ane oL 8. 1 Open Clusters Lumino s i ty Function of Open. Clusters and Fi eld Stars Colour-Magnitu de Di agram for the Ple i ades Colour-Magni tude Di agram for M67 Positions of 2 Flare stars in the Ple i ades Galacti c Di stributi on o f 1 29 Glo bular Clusters Colour-Magnitude Di agram for M 15 Di s tributi on o f Globul ar Clusters in NGC 42 1 6 star Di s tributi on i n the Globular Cluster Palomar 4 Bri . Fi eld drawings o f 1 7 6 Open Clu s ters wi ll be found between page s 48 and 1 35, and a further s ample o f 6· Open Clus ters o n page s 1 98 and 1 99 . Fi eld Drawings o f 5 3 Globular Clu s t. found be tween page s 1 56 and 1 8 3 .

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9 12 13 14 24 29 30 33 42 42

of the sky regard star c lusters as the mo st Som e watch ers 'ful ob j ec ts in the heaven s . Several mi l l eni a ago the open beaut1 e Pl ei ade s , s o c aught the imagination o f s ome early clU S t er , th they regulated their year by its heliac al ri sing. peo p l es th at thei r beauty when he wrote that the Pl eiad es Te nnys on captured tarS "Gl i tter l i ke a swarm o f firefl i e s tangl ed in a si lver s

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catchi ng young and profligate blue-white stars a;re braid". Eyeo f many open c lusters , s ome o f whi ch have red stars a fe atu re

to o . Only afte r the development of tel e scopes did globul ar clus ters light . c ome to be noti c ed with their hundreds o f pinpoints o fghtest. stars in Their import ance was recognized as ear1y as 1 7 1 4 when Halley shre wdly sugge sted that the luminous spo t s , inc luding the globular cluster l ater numbered 1 3 by Me s si er , "cannot fai l t o occupy Spac e s immens ely great" . Though globular c lusters are the olde st obj ects in the sky , they are by no means stati c , as Auwers ,

Luther and Pogson l e arned in 1 860 when they s aw Me s s i er 8 0 in Scorpius change its appearance with a s eventh magnitude nova in its core . When and where wi l l the next such nova b e s een in a globular cluster? Frequently change can be no ticed in the c lusters numbered 2 , 3 and 5 by Me s si er when one of their bri ghtest stars bright ens an d fad es as a peri odi c vari ab l e . It i s smal l wonder that s ky watchers wi th telescope s , or even just binoculars , are eager to obs erve s t ar c lusters . Thi s volume is a conven i ent pathway and guide to such obs ervat i ons . For a century great encouragement has been given to sky ob s ervers , first by the Rev . T . W . Webb and l ater by the Soci ety founded in hi s honour . My sincere congratul ati ons go to the Webb Soc i e ty and its editor Kenneth Glyn Jones for producing thi s Handbook on Star Clu s te rs as Vo lu me 3 of the Webb Soci ety Deep-Sky Observer' s Handboo k . H e l en Sawyer Hogg

viii

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General Preface er the Rev. T.W. Webb (1807-1885), an eminent amateur Named aft and author of the classic Celestial Objects for Common a stronomer the Webb Society exists to encourage the study of double stars �lescop�k, objects. It has members in almost every country where amateur deep s y and flourishes. It has a number of sections, each under a director a�tro�� experience in the particular field, the main ones being double w1th lae and clusters, minor planets, sttpernova watch and astrobu stars, n . publications include a Quarterly Journal containing articles photOgr hY ial features, book reviews and section reports that cover the and s , s activities. Membership is open to anyone whose interests are socie are available from tible. Application forms and answers to queries 1 Cramhurst Lane, "Chestnuts", Society, Webb tary, Secre ton, .S. Whis England. rey, Sur Witley,

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Webb's Celestial Objects for Common Telescopes, first published in 1859 must have been among the most popular books of its kind ever writ en. Running through six editions by 1917, .it still is in print although the text is of more historical than practical interest to the amateur of today. Not only has knowledge of the universe been transformed totally by modern developments, but the present generation of amateur as tronomers has telescopes and other equipment that even the professional of Webb's day would have envied.

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The aim of the new Webb Society Deep-Sky Observer's Handbook is to provide a series of observer's manuals that do justice to the equipment that is available today and to cover fields that have not been adequately covered by other organisations of amateurs. We have endeavoured to make these gu ides the best of their kind: they are written by experts, some of them professional astronomers, who have had considerable practical exper­ ience with the problems and pleasures of the amateur astronomer. The manuals can be used profitably by the beginner, who will find much to stimulate his enthusiasm and imagination. However, they are designed primarily for the more experienced amateur who seeks greater scope for the the exercise of his skills. Each handbook is complete with respect to its subject. The reader is given an ade quate historical and theoretical basis for a modern under­ ing ?f the physical role of the objects covered in the wider context e un verse. He is provided with a thorough exposition of observing � m th O s h s mlcrometers and simple spectroscopes. Each volume contains a . de a �ed and comprehensive catalogue of objects for the amateur to locate and observe with an eye made more perceptive by the knowledge he has O gained

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ope that these volumes will enable the reader to extend his ab i t e l �, to exploit his telescope to its limit, and to tackle the cha lle n succes glng difficulties of new fields of observation with confidence of s. xi

Prefac e Volume 3: Open and Globular Clusters of c eles ti al obj e c t s covered in thi s volume can both Th e t wo kOnds as s t ar-clus ters . However , in astronomical terms bed cri d be l o o � elYer : all respec t s . Open clusters are young in the st almo �n th e y d: ff mo s t o f them are c omparatively near to us in and e scal evolut � on a:Y the globular clu s t er s on the o ther hand have hi s tori e s our G al Y · to the very beginnings o f galaxy-formation , and some of th at go aC k di s tant that they may be vi rtually free o f the gravi tati onal th e m ar e so e Gal axy . fi eld of th the amat eur observer they pre s ent no t only very di fferent vi sual bu t also require very di fferent methods o f observation. The as pe c to s eparate from the bac kground � ste rs may o ften be di ffi culte asy o pen to r e solve: in drawing at the generally are they but _ d iel ' t r however , great skill and pati ence are required to produc e an e s cope usually conspi cuous in are s er t s clu Globular . on entati ; s epre aC rat e for a large number o f them , the resolution into but ance , ar e nsti tuen t star s may t ax the observer ' s vi sual' acui ty to the utmo st . The imaginati on may well be s ti rred by the s i ght of one of the ' great ' globular clusters - such as omega Centauri or M 1 3 - in all the glory of i t s myri ad s t ars: the open clusters have a more ae s thetic appeal , and not even the fines t as tronomi c al pho tograph c an do justi c e to the pri smatic beauty o f the double-cluster in Perseus ( h and chi Persei ) or the ' J ewel Box' in Crux as s een in even the m o s t mode st o f amateur telescopes . Thi s volume provides a more than ade quate hi s tori c al and theoretical background for the reader to appre c i at e the importanc e o f both clas s e s o f star-clus ters i n their di fferent astronomic al aspects . Expert guidan ce is given in the selection, location and clas si fi c ation of objec ts in each fi eld , and as in e arlier volumes , useful hints are provided on observing t echnique and recording. In addi tion a selected cat alogu e in each clas s , c omplete wi th �umerous fi eld drawings , has been compiled from observations mad e by members of the Webb Soci e ty over the year s . Observers will find thi s an invaluable ai d to rec ogniti on for many o f t h e more ' di fficult ' o b j e c t s , and a useful mp ari s on for their own o b servations . Altoge the r , some 205 open us � er s ( 1 7 6 o f them wi th drawings ) and 6 3 globular clu s t ers (53 wi th raW1ng s ) are included . Almo st the EdmUOd Bar er whole of the text has been wri tten in thi s instance by � , who as Director of the Nebulae and Clu s t er s Section, h as C -o r d1nated the many deep- sky observing schem e s undertaken world­ Wi d Y memb ers o f the Webb Soci e ty . In addi tion, Ed Barker ' S skill as d augh t sman i s mani fes t throughout the whole o f thi s volume: we are i n e ed for tunate that such talent s are given so uns tintingly . gr a t W th all our productions , the Edi tor wi she s to acknowledge wi th u e the con tri buti on of our Publi cati ons Offi c er , Eddi e Moore ,

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who has kept a watchful and expert eye on all s t age s o f preparati o n o f thi s volume . We extend our thanks als o to Ri dley M. Enslow J r . , Presi dent o f Enslow Publi shers , for hi s enthusi asti c support in our publi shing activi ti e s , and especially for hi s gui danc e and experti se in the reproduc tion of the all-important fi eld drawings in thi s and previ ous volume s . Finally , we wi sh to expr e s s to Profe s sor Helen Sawyer Hogg our deep grati tude for doing us the si gnal honour o f acc epting the dedi c at ion o f thi s volum e , and for so generously contri buting a foreward . Her eminence in as tronomy , especi ally in the fi eld of globular cluster obs ervation and analyti c al res earch , i s internat ionali acknowledge d , as i s her abi ding intere s t in and encouragement of many; the activi t i e s o f amateur as tronomers .

xiv

GENERAL

INTRODUCTION.

1

of information can be to which the maximum amounta direct The derrr eme any consequence of is study under ct obje ined i r s and method used, and nowhere is this more so than obtama the tef�e�d of observational astronomy . Often , however , the method n the ian be in advance of the means , and the time comes when c beiesee�n tovisual observation have virtually exhausted the abili:i � of a given telescope . It is at this point that financial pote�iO:S ari se; greater light-�athering q�alities demand a larger ue this in turn requlres a conslderable cash outlay. aqpers ture,ereand -lnches nces between telescopes 0f , for examp I e , 8 and 1 6· The diff view of of point the performance from both ble, sizea aP:rturecosare t. If we take galaxies as a class of object for visual of has access to a 1 6-inch would willingly o�: ervation, nouseoneofwho 8-inch. Bearing this in mind , it is an the to urn that deep-sky objects do exist in realizing ile worthwh fore ere �: a high percentage of information can be obtained with the which , and most of the trumentation ins inexpensive ely relativ of use present volume is concerned with such objects . In the field of visual observation of star clusters , the observer is confronted with objects that , in a large number of cases, exhibit a considerable amount of detail . Even quite small telescopes are oapable of fully resolving numbers of open clusters , while many globular clusters will allow at least peripheral resolution to be obtained, and in a number of cases total resolution is the order. With such morphology being relatively easy to determine , s tar olusters may legitimately be said to outrank other deep-sky objects in the context of resolution of constituent parts . Unlike many nebulae and , open and globular clusters are (with inevitable exceptions)galaxies not objects that present extreme problems in locating. Indeed, many examples abound where visual observation will reveal an object that can well-nigh dominate its ticular field , whereas a photograph of the region with i�esc a ope increases field s tar distribution to such an extentlarge that � t e cluster merge s to a large degree with the field. ver the whole sky a large number of clusters is shown on the At l � Coeli ts , and a selection of objects not plotted in Coeli is o�� of thechar aims of the present volume. A number of such clusters a h e served , and appear in the catalogue sections while a selec t�oen ob We hav enndof others will be found in the list of additional objects. ea�oured to achieve an adequate balance between the more we l l_ � wn ob les ser_�o Ject s that many observers are aquainted with , and wn but often equally interesting clusters. Ob server s lnv . ticular t p ariably find themselves gravitating towards a P� th e � e of object upon which they spend the greater part of opeinrColub:�rvlng time. This is illustrated in this volume, where the OQncen trateerds are primarily the province of a few observers who have to a large degree upon these objects. Interest and V

2

General Introdu ction. determination can produce qualitative comprehensive resul in observational astronomy , and hopefuland ly the results set out tsi n the present catalogues will act as a catalys t for those not Ye t aware of the variety of objects within a single , as well as showing how much can be achieved with moderateclass telescopes . Acknowledgements . David Allen contributed to the Appendix on cluster distanc es as well as contributing many observations to the es . David Pike and John Alexander of the Royal Greenwicatalogu ch Observa tory kindly supplied data on globular clusters and stellar associatio while further data on open clusters was supplied by Guy Hurst. ns Much of the research for this volume was undertaken at the libr ary of the Royal Astronomical Society and at Universi ty College 10nd on, The writer expresse s his thanks to the 1everhulme Research rds Committee for a research grant to aid in the compilation ofAwa this Handbook.

I

PART ONE : OPEN CLUSTERS. SISTeR!

3

CAL REVIEW. associated in cluster-like forms was noted o.e s !�: t �:omers !bateasrli : the Hyades and Pleiades (both true es by t he always been recognised as distinct have , e 1 p s ) for examthe grouping of the conste11a t·10n Taurus. clusteirsms larger wi thin however , defied resolution by Pleiades asterters sma ller than the and the, few Clus 1y unai that were observed before , ight eyes d de optical . on of under the vague classed all were telescope the the invetnhn f ' ebulae' (little clouds) or 'nebulosae' (cloudy d escri f O t � lem� listed seven of these, but only four, the double ( stars ) ) Ppers eus (h and� Persei) , Praesepe in Can�er, M7 in in clust �� and the 'cloudy convolution s' of Coma Beren1ces are true Scorp clusters , the other three being small asterisms. mer , Al Sufi, who , in about AD 954 , was the Tbe Persian astrono Nebula , also li�ted of the Andromeda existence the t or rep to . first asterism whi c h the 11ttle the 'nebulae' of ptolemy and added includes 4 and 5 Vulpeculae, and which has become well-known as 'Briochi's' cluster. He also included the star cluster . around kinds omicron Velorum (le 239 1 ). Other false clusters of var10US were added in the star catalogues of Ulugh Begh and Tycho Brahe, but with the advent of the telescope in the hands of Galileo , we first arrive at the recognition that 'the nebula called Praesepe is not one star only but a mass of more than forty small stars'. Galileo thought that all nebulae could be resolved into component stars, and this impression was to persist for several centuries. As telescopes improved in light-grasp and resolution , many new and diverse nebulous objects were discovered , including the open clusters Ma (Flamsteed in 1 680) , M1 1 (Kirch , 168 1 ) , NGC 2244 and M41 (Flamsteed , 1690 and 1702), M50 (Cassini, 1 711 ) . In the years 1745-4 6 the French astronomer de Chesaux discovered M6 , lC 4665 , Bce 6633 , M1 6 , M25, M35, and made a more-or-less successful attempt to differentiate between nebulae which were composed of stars and se 'rightly-styled nebulae which when seen in th. largest teltho esc opes, never appear as anything but white clouds. Two of the well-known open clusters in Auriga, M36 and M38 , �ere diScovered and demons trated to consist only of stars by Le n til 1 7 49 , (M37 was found by Messier in 1 764). In 1 751-52 t� e Abbein1ac aille , his expedition to the Cape of Good Hope, :��Piled a list of in 42 nebulous objects of the southern skies , h i some 16 new , genuine open clusters. 1acaille also atu: tCmptendcluaded fo d , di more rigorous classification of the 'nebulae' he had id g them into Class I - nebulae without stars; Class 11 ebulVouin s star by - n clusters and Class III - stars accompanied glo�:�Ulosity. Thi ss in scheme did in fact give a rough separation of clUst ers , open clusters nebulae, but the d1st1nar c tion was not entirely sharpandnordiffuse consistent enough to be

4

5 Historical Review.

1 . 0-ASSOCIATIONS.

meaningful at the time.

Neither Charles Messier, nor his imitator in the compilat ion Of catalogues of nebulae, J.E. Bode, made any attempt to classif y th e nebulae they listed, but the former gathered in his fruitful net a dozen new open clusters, although one of the finest of these M6 7 in Cancer, was first discovered by J.G. Koehler at Dresden 177 i

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It was, of course, Wm. Herschel who made the great leap for ward both in the discovery of new nebulae and in a more rigorous clasS' ification of them. His eight categories, from I ( bright nebula e to VIII coarsely scattered clusters begin to show divisions whi ch are meaningful, and although he did not include a separate class for globular clusters, he used the term as a description of many objects we now know to be of that class. The open or galactic clusters, as might be expected, are found among those objects he classified in the categories VI very compressed and rich Clusters VII compressed clusters of small and large stars and VIII coarsely scattered clusters .

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However, even Herschel's catalogues, and their successor, the NGC, did not always succeed in giving a clear separation between open and globular clusters. Even now, we know several objects such as the well-known M71 in Sagitta - so indeterminate in form that the distinction is difficult to apply, at least purely on morphological grounds. _

It was the recognition by WaIter Baade in 1944 of the existenc e of two separate populations of stars in our Galaxy the young Population I stars of the disc and the older Population 11 stars of the halo which served to distinguish more clearly the differences between the open and globular clusters.

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b inappropri ate to commence a survey o f the vari ous It WOUldrt a��ni ng to open clusters without first consi dering tho s e pe faotor s gr oupS known as O-as sociati ons and their relation to open ar l l s te m se two type s o f star group s are s�m� 1 ar �n many ways , olus ter s . 1 h ases are found to be c l o s ely linke d . and in s om e " stincti on betwe en 0-as so c1at1ons and open .T o de f":- ne a tru e di if we must pinpoint a di fference it would easy: olust ers � s not diameter , which , for the as soci ations , i s linear f o b e that hav� 1 00 pc. . The lower limi t o f linear s i ze f o order the f o tYP�·c lY em to be about 30 p c , and we c an compare e s would i ons for a- as so ci at lowe r limit for open clu sters , wh"�ch h overs aroun d a th the . w�. th�S diameters o f as sociations are , howeve r , qui t.e fi gur e o �so 1 pc . The ne ; mem b ersh1p S'" " � e ior any · " sl " t r1nS1C " and h enc e 1n di ffi cul t to determi many problems , and thus the membersh1p aspect posing st ell ar group can be s e en to parallel that of open clusters . f a_a sso ciatio ns en for the smaller as sociations r . is di ffi cult , but at least in such cas e s the boundari e s are fairly well -defin ed, whi l e with increasing si ze they become considerably more vague . As i s the c ase with open clusters , O-as so c i ati ons are unstable groups , and under the rel entle s s shearing e ffects of gal acti c rotati on they wi ll eventually di sperse into the surrounding star fields . Nany as sociati ons show an extreme overall ellipticity whi l e others di splay n o tendency towards e longati on i n the gal acti c plane, and thi s furni she s us with a c lue regarding the age s of as sociati ons . Tho se stars in an as sociation which are clo ser to the gal acti c centre move at a greater speed than tho se which are more di stant, and as a consequenoe o f thi s an as soci ati on wil l gradually spread out around the centre o f the Galaxy . Theory shows that , by thi s acti on, an as so ciation wil1 undergo a doubling o f its l ength on a time scal e of about 30 :le 1 0 6 yr . For as sociati ons younger than thi s , their resp ect ive expan sion velociti e s wi l l mask the elongation pro c e s s . A� the upp er end o f the age scal e , where we are c'o nc erned wi th lm e s OVe r twice that o f the above , the pro c e s s o f di s s o luti on wi ll e. wel l adv anced . exampl , di splays a 6 q�te pronoun ced The Sco-Cen as sociati on , for o f an e age of 70 x 1 0 elongati on whi ch i s indi c ative contr ast , the as soci ation around zeta Per , covering an area 1n the sky , i s a much younger group , with an age o f only about 1 0 Yr . T S CO-Cen as sociation contains the well-known dark clouds near rh o and al so contains a number o f sub-groups . Other associati ons also b and G r o � the latt , e n N 7 6 are q ? 1 0 re si. d e . Connecti ons between clusters and associati ons Ult e common , and i n the present catalogue c lu sters l inked with

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O-Associati ons .

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O-assoc i ati ons wi l l b e referr ed t o i n the data secti ons prec e din�c the visual o bserva ti ons . Finall y we must consid er the questi on o f ini t i al i denti f . o f an O-assoc i at i o n . This is made by determin ing a regi on i c atl On whe r e the densi ty o f stars o f types O-B2 shows a larger -thanmean fi e l d densi ty than i s acc ep tabl e as a chance arrang ement . Where the c ase , fUrthe r work i .8 necess ary i n order to obtain datathi s . m embership . An O-asso c i at i on wi l l give a fairly wel l-defi n on c o lour-m agni tude di agram as l ong as i ts back-t o-fron t rat i ed t o o great ( fo r ful l detai ls o f colour -magni tude di agrams o i s n 0 t see Chapte r 2 ) . Distan ces t o ass o c i at ions c an be estima ted from th e proper moti ons o f the const i tuent stars , the distan ce o f Sco-C en assoc i at i on being about 1 7 0 pc . In Fi gure 1 below the is pro j ec ted on t o the galact ic plane , O-B2 associ ati ons wi thin sh own, a distan ce o f 1 000 pc o f the Sun .

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OMon OB1 z = +27

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DUCTION.

purpose , namely t? of s t ar cluste rs have a twofold of their evolutl on , stud i e s ull anding underst an ble ossi p y as f s a n overal l struc ture the i to this a and to relate o bt t and fu ture , Galaxy . Both observa ti onal and theore t·lca 1 bo th pasut . n of the ft�d e.vol lO. chapter we shall .... re d'In this c ontext , and in this qul e . wor k'ln th e f'le Id 0 f . de r the various aspects o f thlS d at a lS r nsl co bri e fl Y ste rs . u cl n e p o . . . ; is divi de d into five secti ons: 1 . Classlflc atlon Th e cha te r the Galaxy ; 3 . Cluster morphology; 4. Ste � lar to . 2. Rel ati o d 5 Col our-Magni tude di agrams . These sectlons glve an ns ati � po pul ons moti stellar as a func tion o f . f0 rmatlon. on c l �ster li fetimes In , the re 1 a t·lon 0 f c Ius t ers derlvatlons age , tation ro c actl an d gal . .. clus t er s t ars lndlvldual , Galaxy the f o re tu struc . t o th� sP iral c luster reddening. The l atter has a bearlng upon ll na d i Y an ' ! ll wi be ch whi , on i derivat distance of on t questi the imP or tan 3. Appendix in lusters c ar globul and open both for d ere consi d

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Ori OB1 z = - 1 30

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INTRO

GALACTIC OPEN CLUSTERS.

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Lac OB 1 z = -1 65

Cep OB3 z = +37

Cep OB2 = +72

z

Sco - Cen 500 pc

Classi fic ati on . The classi fication of open c lusters wi th whi ch the. maj ori ty of the amateurs wi ll be fami l i ar is that o f Shapley , who devlsed following scheme: ( c ) very l o ose and i rregular ; ( d) loose and poor; ( e) intermedi ate rich ; ( f ) fairly ri ch; ( g) consi � erably . of baslc rich and concentrate d . This gives a good outllne cluster morphology, but is now superseded by a cl assi fi cati on scheme which ori ginated wi th Trumpler and which . be � i des givi � g a bri ef description of cluster struc ture , also furnlshes detalls on stellar membershi p . De tai ls of this classi fi cati on are shown below. Detached wi th strong c entral c oncentration. Detached wi th li ttle c entral conc entrati on . III Detached wi th no c entral concentration. IV No t wel l detached but apparently l i ke a strong fi eld c oncentrati on . The se Roman numerals are foll owed by Arabi c numerals , whi ch Used to con inform at i on on the luminosi ty func tion of the lg te st starsvey ( for on luminosi ty functi ons see section 3 inally , lett ers details are used to deno te the number of stars . p po or , less than 50 stars . m medium ri ch , 50 - 1 00 stars . r ri ch , over 100 stars . Th�s to tak e the cluster NGC 687 1 as an example , the Trumpler � cl is IV 3 p. This parti cular scheme is used in the o Randbo ogu e o f op en c lusters to be found in Part Three of this cIUs t k . In s� c ti on. 3 we shall c onsi der the morphology of open fUr th er , and in parti cul ar i t s rel ation to field star Ut i on and the questi on of cluster recogni t i on . I

11

�r7 � J •

Fi gure 1 . Posi ti ons o f O-B2 assoc­ i at i ons wi thin 1 000 pc o f the Sun . above ( + ) and Z = distances in pc below ( -) the galac tic pl ane .

ca�=�lflcation

d1.tr��s

7

8

Galactic Open Clusters.

Galactic Open Clusters.

Relation to the Galaxy. Open clusters are to be found inhabiting two population group in the Galaxy; young clusters , along with 0 and E stars , occ ur isn the extreme Population I regions , older clusters in intermediat Population I. Stellar populations will be discussed in section 4e and we shall therefore, in this section , consider the role of open clusters in defining spiral structure and also the association of clusters and nebulosity. The spatial distribution of open clusters varies as a func tion of galactic latitude, and this can only be ascertained by obtaining distance determinations that are as accurate as possible. Onc e the cluster distances have been derived , then it is possible to arrive at figures showing their distances perpendicular to the galactic plane. These, as we have seen in Figure 1, with regard to stel lar associations , are defined by the letter which denotes these distances in pc. In star atlases , such as Atlas Coeli , the maj ority of open clusters appear within the confines of the Milky Way i80photes . This , however, gives us no indication of their distribu tion in nor does it show how clusters can aid in defining the spiral s tructure of the Galaxy, as the great circle of the Milky Way , seen with the eye, is comprised of different spiral arms. In some cases , e.g. , Cygnus , we are looking along a spiral arm , while in others , e.g. , Auriga, our viewpoint is essentially at right angles, Nearby clusters such as the Hyades and Pleiades appear to the eye as being outside the apparent boundaries of the galactic . plan e , This is purely a result of their small distances from the solar system , and the further we see the more clusters will appear to be located within the Milky Way confines. Nevertheless , the Hyades are at a fair distance from the galactic plane, with -23, wh ile another naked-eye cluster , Mel 111, better known , perhaps , as t h e Coma star cluster , has +85. As we approach the plane of the galaxy the space density of open clusters increases , and this is a suitable point to discuss clusters as delineators of spiral structure. In Figure 2 overleaf the distribution of 81 clusters is shown projected on to the galactic plane. The circle enclosing a dot defining the position of the sun. The delineation of section s of three spiral arms is easily apparent , the section at the upper right being the Perseus arm , the central section , including the�, US sun , the Carina - Cygnus arm and the lower section the Sagittar arm. A further correlation can be made by plotting H 11 regions, H I regions and stellar associations on to such a diagram. These too will be found to be indicators of spiral structure, and occupY similar positions to those of the open clusters.

9

2.

I

0 0

, " ." 0

z,

o

, ,',

'

',' .

.

o

•

0

.

.

. , .

" o

00

0 ,

:

.

,

'

o

•

, "

0

0

z,

z

z

=

=

1

kpc Figure 2 . Distribution on the galactic plane of 81 open clusters , showing the delineation of three spiral arms.

On the eding page we have mentioned how H 11 regions can be fit ted to prec the plotting shown in Figure 2 . It will thus be obvious that incluster many cases associations of clusters and nebulosity occur , and we shall briefly discuss such amalgams below. ObVious associations of nebulosity and clusters are objects :uCh as MS (comprising the cluster NGC 6530 and the bright and ta8r: nebulosity catalogued as NGC 6523) and M16. A further example al tho h here the cluster aspect is not as obvious as in prev4i2, ous two ug cas es . i mp o rtant effeot of dark nebulosi ty is its ability to dim 8ta� r an d c lusters associated with nebulosity will often be qUitaight, h e a V i 18 a e ly reddened by obscuring material. This dark nebulosity when seen in oonjunction with regions of brigh t nebuIPar aa , nast einsoutgh he case with M8 , but often no bright matter is '.eQ � '� J,� d i s uoh n cases where a cluster is affected by interstel lar , Pho tome try can supply details of the dimming. The young

10 Galactic Open Clusters. cluster lC 348 lies within a region of dark material which Cont at least one heavily obscured luminous star , and it is thereforeaine highly likely that the visible part of lC 348 is only a small par t of the total. Bearing in mind the patchy nature of the interstellar medium . is evident that the effects of this upon clusters will vary qUitelt considerably. Absorption figures range from as low as 0.15 mag NGC 6281 to over 1 2 mag for a recently discovered cluster. Furthfaer details of clusters and nebulosity will be found in Chapter 4, Wh�l below , in Table 1 , we show the diameters , in pc , of selected as soc�e iations of clusters and nebulosity.

Galactic Open Clusters. a:n

11

personal designation s. These asterisms d even given tive fields in much the same way as respec r ll in thei , and a few are to s edgenUl :: open clu sters of sparse membership as bona-fide ly plotted erroneous , Atlas som und in Norton' s star b e fo clu s ter s cal parameters of a cluster are useful ; they define Theageeom�hterinumeric al richness of its stars and their density sh 'and thes e alone will be enough to differentiate an open igrts ie ad n�fro majority of globular clusters. Nevertheless , m the greatristics clust� r . of a cluster are also important , and racte cha the p ayS1ecal composition , age and luminosity chemical the by ined def thesc�i � of the cluster stars. To extend knowledge of a·cluster O photographic appearance, therefore , detailed photometry fun Table 1 . Clusters and Associated Nebulosity. b eyond its Diam. Neb·(Ec } Diam. C.(EC} Association. and spectros copy of t s s t ars are needed of astronomy , it is natural that the brighter , In all bran ches 25 18 NGC 1 893/IC 4 1 0 receive the maximum amount of attention. In the will cts obje a g l er r 15 8 2 1 75 objects such as those in the Messier cata­ clusters open of field 50 12 2244 and detailed observations go back for many , ed well-studi are gue lo 7 7 2264 large , bright clusters not in Messier have also been Other years. 1 50 4 2579 subjected to the full range of observational procedures , but we 14 14 651 4 (M20) still find that a large number of clusters are either only partially 40 21 6523/30 (M8) studied or not studied at all, apart from basic classification. In 30 6 this context it is worth recalling that the first photometric 661 1 (M1 6) observations of NGC 2420 did not take place until 1 962. As well as 90 12 IC 1 805 relatively little-studied clusters in the NGC , there are also many 30 14 1 848 more among the discoveries made by Ruprecht , Dolidze and others. 8 8 69 Cr Furthermore, with numbers of recently discovered clusters or 30 35 302 possible clusters cropping up in the southern sky , the situation is unlikely to change for some time, bearing in mind the great amount of work required in order to study a cluster fully. 3. Cluster Morphology. In this section we shall concentrate primarily upon the problems Lum�osity Function of Open Clusters. e � sity function of a cluster is obtained by counting that can be encountered in identifying open clusters . It may seem e n umino er of at first thought that nothing could be simpler than defining a:ny li th star s in each magnitude interval or step, and this � W fer from the luminosity function of a non-cluster field. open cluster; that any not-too-compres sed , globular-like object . eouol Inillthe Ifat case , the luminosity function is seen to increase be nothing but an open cluster, and vice versa. This , however , lthSen with de rete: the case, for it must be realised that morphology alone is no t fu ncti � � slns gamagnitude , while in the case of a true cluster the rise to the brighter cluster stars followed by a determining factor in cluster recognition. Many other factors aruSe tell decreaose �oowthe fainter one s. valid, not the least being the types of stars comprising any cl Figure 3 on page 1 2 shows the mean luminosity be it open or globular. o function of true ' pen l c us .e eld lstarters comp�red with the apparent luminosity function of In the past , difficulties have often been encoun.tered in �he f�1I11 fCilus �he dlfferences are immediately apparent ; the open of cluster classification , it being uncertain whether to ass1gnc tSt# th ters s��wlng e clusters to either the open or globular category. Examples of e 11 nitufield tar s dia mean maximum absolute magnitude of Mv =+5 while splay maximum numbers at fainter apparent mag­ des. H�w individual cases will be given in Chapter 7 , and therefore we sh� i l l a whil � ver, teS e the greater percentage of open clusters a � Sho 1 agglomer from clusters open true of now discuss the sifting w os ity function equivalent to Figure 3a , it is rue to s ay t�amin field star distribution. t not necessarily all will do so. � Lying among the rich star fields of the galactic plane, the:eg to be found groupings of stars which give the impres sion of belned coarse open clusters. Such s tellar concentrations are often logg �ate ur s, out. ta n

.

. 1

•

12

Galactic Open Clusters.

Galactic Open Clusters.

100 90 BO 70 60

N (Mv) ,.... l�

f-

-

,-

a.

f-f-I-

-

I

+2

I

I

T

h 1

Mv

-

+10

50 40 30 20 10

b. V

+B

+12 +16

Figure 3. a, Mean luminosity function of real physical clust er s selected by van den Bergh (1 961). b , Apparent luminosity func­ tion in the case of field stars. stellar Populations. In the historical review we have seen that the concept of stellu populations originated with Baade. Nowadays the scheme has been elaborated , and comprises five population divisions , as can be seen below in Table 2 .

4.

Table 2 . Stellar Population Group Extreme Pop. II Intermediate Pop. 11 Disc Population Intermediate Pop. I Extreme Pop. I

ColoUr-Magnitude Diagrams. evolutionary s tage and lity to cast light on the current of T � aabista unction great the is cluster r � . a colour-magnitude . o a� am T he use of such a d�agram �s not conf�ned to clusters dl agr however ; certain types of field s tars are also subjected to alucohn e�alysis , as well as luminous s tars in nearby galaxies . s to s tar fields within the Galaxy , when we look at I con tra st or a globular cluster we are seeing s tars which are open an e ith:prrox age and with the same initial chemical the ely imat of :p ition. A colour-same of a cluster is obtained by magnitude oom osing the brightne ss of as many of its stars as possible in sur visual (V) wavelengths and then plotting V agains t ��:e (B) and the latter defining the star's colour. If stars in an open j-V a well-defined locus is obtained , known as olu : ter are so plotted certain s tars will form a t he m ain- sequence. In some clusters giant the as branch; in very young known diagram the region of will be lacking while in old clusters it is more in this ters olus evidence. While, as described above, the apparent magnitudes of cluster stars can be plotted on to a colour-magnitude diagram, to obtain data on the ages of clusters their distances are required in order that the apparent magnitudes can be converted into absolute magnitudes (M ), which is the magnitude a s tar or any ot her object would dis play at a distance of 1 0 pc .

�

N(V)

I-

-

13

Populations in the Galaxy. % of Met al� Type of star 0.3 Halo globular clusters 1.0 RR Lyrae and halo s tars 2.0 Main-sequence stars 3. 0 Old open clusters 4.0 Young open clusters O-B stars.

From the above table it can be seen that there is a wide diSpetilr�l', sion in metal abundance as a function of galactic latitude , and will be further discussed in Chapter 5 with regard to globular clusters. As well as the metal-rich open clusters, Population I regions of the Galaxy also contain classical Cepheid stars. TheS eet variables are also to be found in some open clusters (e.g. , UY p and VY Per in the Perseus Double Cluster , USgr in M25) and are useful in deriving distances of the clusters in which they occur. Further details of this will be found in Appendix 3 .

M

v

-6 -4

Figure 4 . Colour-magnitude diagram for the Pleiades. The abscissa is the colour index (B-V), the ordinate is the absolute visual magnitude (Mv).

-2 o

+2 +4 +6

+0. 4 +O.B +1.2 +1.6 (B-V)

14

Galactic Open Clusters.

Galactic Open Clusters.

15

On the preceding page is shown a colour-magnitude diagram fo ges. The star, characterised by a larger angular size Pleiades. The band running from B-V + 1 . 2 to M -3 is the cll.' th, undergo ���ur, defined as a red giant, and many well-known stars s m�in-sequenc?; individually plotted cluster sta�s would basical� tel and r;d i type,ise.g., Antares, Aldebaran and Betelgeuse. th S 11e along th1s line. It will be noted that there is no red giantY are 0 from the above that the lower the turn-off point branch for this cluster, which is indicative of its young age. be seenquenc Itthoan e the older the cluster. It is apparent, there­ old open cluster M67 does, however, have a giant branch, as canThbee -se ain fro� e � Figures 4 and 5 that the pleiades is a young cluster and seen below in Figure 5. fore, �7°�s decidedly older in that it has a well-developed giant Mv that M further examples of well-known clusters, the Perseus b ranoeh·ClTwo and the Hyades, show this difference in a less extreme er ust DOubl the Pers -6 eus clusters the main-sequence is virtually intact, n i fo�; the Hyades has already lost its blue stars to the giant branch ..,hile of cluster ages also demands a good deal of theorderivationwhich -4 Th work observations can be compared. Theoretical with � of stars in a cluster in order Figure 5. Colour-magnitude e ti�a�odels can be fitted to a sample c e ' whil. ions, derivat �AM ge diagram for M67 . a in aid -2 from the ZAMS are also important. In the past, these t�a cks of stars nary tracks for stars often agreed poorly with evolutio ical ret theo in the form of a colour-magnitude diagram, plotted observed dataages derived o for clusters are well below these early t curren and deriv9tion of the age of the open figures; for example, an early cluster NGC 1 88 was about 1 4 - 1 6 x 1 0 yr, while a 9recent figure by +2 Hirshfeld, McClure and Twarog ( 1 97 8 ) gives 5 x 1 0 yr as an age for this cluster. At the other end 09 the scale there is the Hyades, to which age of only 0 . 7 x 1 0 yr has been assigned. For an even younger stellar group we can take the CMa R1 association, in which +4 a number of hot stars are to be found illuminating nebulosity. In this association, which includes the emission-line stars Z CMa and RD 53367, no stars later than B5 are on the main-seq�ence, leading ) yr. to a possible age for the group of only about 3 x 1 0 The derived age for the Pleiades is about 2 0 x 1 0 6 yr, but it is :� on of the youngest clusters, the Perseus Double Cluster and -0 . 4 0 . 0 + 0 . 4 +0 . 8 +1 . 2 +1 . 6 236 2 both being younger, the age of the latter being put at ab ut one (B-V) ntie�h �hat of the Pleiade�. Another yo�ng cluster is M39 Wh retwe the maJorl. ty of stars are st111 on the mal.n-sequence, and � a ew of the In stellar evolution, a proto-star will lie off to the right of just brightest are in the process of evolving towards the red g iant stage. the main-sequence. As the star evolves, it will eventually make contact with the main-sequence, the point where it does so being bu! further consideration regarding cluster ages is that of metal dependant upon its mass. A large mass star will make contact with a d ar ll the main-sequence at the upper left of the diagram, where ste ­ forme��� · aBeing r?latively you�g objects, open clusters were s�age l.n the evolut10n of the Galaxy when the inter­ luminosities are high, while a small mass star will join the main stella a l from which they coalesced was much richer than at sequence at the lower right, and will hence be an object of low t1'n! earlierr emapoter1 �hs , this being due to metal enrichment of the inter­ stella intrinsic luminosity. The locus of points at which stars of diffe r �ed1 um by successive generations of stars, each generation fO rm in masses begin life on the main-sequence is referred to as the Zer o Age Main Sequence (ZAMS). From then on the evolutionary track of a earlierg l.� ra more metal-rich environment due to mass loss from HoweYe S � s by Yarious means, including supernova explosions. star is determined by its mass, the larger mass, high luminosity r t ere is a significant spread in metal abundance in clusters Of a stars evolving more rapidly due to the high rate of their nuclear gi � l a t �, :� age, and hence interstellar material in the disc of the reactions. Although a star will spend the majority of its life ime : re clusters reside, has obviously been unevenly mixed i � n � on the main-sequence, it will eventually move off to the upper g Pa epochs of galactic evolution. Older open clusters such S right of the colour magnitude diagram as its nuclear reactions =

::

•

an

16

Galac t i c Open Clus ters .

3 . CATALOGUES O F . OPEN CLUSTERS .

as NGC 2 4 2 0 and NGC 2 506 show a degree o f me tal defi c i ency . Th e s e clusters , along wi th o thers , l i e in the regi on o f the gal ac t i c ant��t 1. c entre , and 2 t o 4 kpc from the gal ac ti c nuc leus . Data on m e t al defici ency for a l arge number of c lus ters now ind i c a t e s that wi th increasing di s t ance from the gal ac ti c c entre , c lus ters show a d ec 9 in m e t al abundanc e , and that for about the last 5 x 1 0 yr the p o �a1t of a c luster in the Gal axy d e t ermine s i t s me tal c onten t , and no t t !, e c lu s t e r age .

� �

Finally we c om e t o the the que s t i on o f the s tabi l i ty of open c lu s t ers , o r , t o put i t ano ther way , over what peri o d o f time is a c lu s t e r liab l e t o retain i t s i denti ty as a uni t? For a c lus t e r to be s t abl e , i t s int ernal gravi tation mus t exc eed the t i dal forc e s of th p arent gal axy , and t o do thi s i t mus t have a den s i ty greater than ae specific figure . We can arrive at thi s fi gure by mul t i plying the maaB o f the Gal axy by 1 0 and divi ding i t by the cube o f the di s t anc e be tween the two rel evant c entroids , that o f the Gal axy and that of th ' c lus t e r . From such c omputati on s i t c an be demons trat e d that all tho se 3 c lus ters whi ch have s t ar dens i ti e s l e s s than one s t ar per 1 0 pc are very uns t abl e , and wi l l therefore be qui ckly di srupted by gal actic ti dal orc e s . In the P l e i ade s , the c entral densi ty i s about 0 . 1 s t� 5 per pc , whi ch makes the s e regi ons fai rly s tabl e , although n o t by a l arge amoun t . The den s i ty o f s tars in the Hyades i s about one qu arter of the P l e i ad e s fi gure , and the degree o § instabili ty i s thus much gre ater . On a time scale of about 2 x 1 0 yr about half o f the origim s tars in a cluster wi l l have di spersed owing t o ti dal forc e s and the perturbati on s o f n e arby s t ars , and thi s add s further we i ght to our knowl edge that o p en c lus ters are young o b j ec t s . From thi s i t can be s e en that many c lus ters whi ch once exi s t e d have now bro ken up , and that their c on s ti tuent s t ars have di spersed wi thin the s t ar fi elds of the Gal axy .

17

clusters th �t the great maj ori ty o f amat eurs l i s t of ope� in the Atlas C o e l i c atal o gue , . l i ar wi th l S that contalned e the who l e sky . Two of the s e , over ers t s lu c 3 2 r f o s 4 l ai ram: det a ves 'tIhio h gl , NGC 6 5 58 and ar e erron eously c at al o gued gl obul ar c lus t ers 2 Fr om the point o f vi ew o f thi s vo0 lume , whi ch has ( wi th a bo we NGC n.s ) a declinati on l imi t of _ 3 0 , Coeli l i s t s 1 8 7 open � X c e tio f ew s gives a l arge and vari ed s ampl e , but by no means Thi e s lu S · th e numbe r o f c lu s t ers avai l ab l e t o the vi sual observer , � e xhau S s clu s t e r s being o f e qual or gre ater int ere s t than r the omi t ted m any ° Coeli chart s . the on ed ott tho s e pl eur re ference , i t i s ins truc t ive W . th coel i as the s t andard amat C o e l i open c lu s ters were t aken . the ch whi from our�e s the at k t l o c ompi l e d by Shap l ey in 1 9 3 0 , ers t s 2 clu f 9 o e 4 T i S i s the catal o gu i er l i s t s such a s that o f arl e on advance great a was t i nd whil e s e d as c omprehensive for the s a l c be t anno c t i ) ers t s clu 2 6 1 ( e el ott p%e sent day obs erveT . Sinc e Shap l ey ' s c at al o gue app e are d , many new clu ster s have been s i ft e d from the rich s t ar fi elds of the Gal axy , 'tIhi l e any e