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Change in Messier 13 of color index ordinates with distance from the center. Change in Messier 13 of color index ordinates, with arbi- trary zero with distance from the center. Figure III, 4 shows results derived by Hogg 18 from an investi- gation of the integrated light of Messier The changes in color from the center to the edge are seen to be negligible.

It appears, therefore, as far as the present data go, that the colors of the giant stars in globular clusters are not related to the radial distance from the center of the cluster. Types of Stars in Galactic Clusters. In the early work on stellar spectra it was found that the Pleiades are devoid of bright yellow stars, the most luminous members being of Class B. From the neighboring Hyades cluster, on the other hand, the B stars are completely absent; the bluest stars are of Class A, and intermingled with them are a few equally bright K stars.

Fleming examined the spectra of individual stars in a number of loose galactic clusters, mainly southern. She noted not only that each cluster contains more than one spectral class but also that some clusters show a preponderance of blue stars, others of yellow or red stars, 14 thus foreshadowing the present general subdivision into Pleiades and Hyades types.

Spectra in Individual Galactic Clusters. For a number of the largest galactic clusters there are fragmentary data in the Henry Draper Catalogue, the spectra of a few of the brighter stars having been classified in the routine course of the general program. To the extent of their value, these data have been used by various investigators, especially Doig 16 and Raab. Representa- tive papers, giving the colors of individual stars, are those by Hertzsprung 18 and by Scares 19 for N.

All these studies emphasize the diverse spectral structure of galactic clusters. The spectral composition of the individual clusters has been discussed by Doig, 23 ten Bruggencate, 24 and Raab 25 and is in the process of exhaustive analysis by Trumpler 26 at the Lick Observatory. Some of these systems are described in Subsection 5, and Trumpler's results are discussed in Sub- section c.

The Harvard material is taken up first. Swedish Acad. Harvard Studies. Fleming 14 tabulated the spectra for seven galactic clusters: the Pleiades, Praesepe, Coma Berenices, I. The stars for which spectra are classified are distributed over a larger area than that actually covered by the clusters, and foreground stars, of course, cannot generally be differentiated and excluded; very faint stars, barely distinguishable on the plates, are included. Although the classification was crude, and for some stars quite uncertain, the existence is clearly shown of a spectral distribu- tion in Praesepe and Coma different from that in the other five clusters.

It is the same distinction that is now recognized in the Pleiades and Hyades types or in Trumpler's types i and 2. The large percentage of A stars in the Pleiades type of cluster is here seen in detail. It is to the frequency of the A stars in galactic clusters that we owe the attempts to determine spectral paral- laxes for these systems on the basis of the material of the Henry 17 Ibid. See footnote 7, Chapter II, on his later work.

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Spectra in the Brighter Clusters. The Pleiades, 29 the Hyades, 30 and Coma 31 fall under the head of "very loose and irregular clusters" division c in the classification of galactic clusters in Chapter II. References to discussions of their spectra and colors will be found in the special bibliographies 88 Doig, J. Figures III, 5 and III, 6 show, for the Pleiades, the spectrum-magnitude and color-magnitude rela- tions, the former compiled from the Henry Draper Catalogue and Harvard Bulletin , and the latter taken from Hertz- sprung's recent discussion.

Relation of spectrum to luminosity in the Pleiades.

Zoom in to Globular Star Cluster NGC 6397

Ordinates, photographic magnitudes, abscissae, spectral classes. X Persei, Messier 34, Messier n, and two southern clusters are summarized below.

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The Double Cluster in Perseus. The table of spectra and magnitudes for the combined Perseus clusters has been M. The data are plotted in Figure III, 7, which shows also a number of the fainter stars that are plotted, but not individually listed, by Trumpler. Two matters of exceptional interest in this cluster are the fact that a large number of the brighter stars show the c-character and that the numerous bright-line stars are by no means the brightest members of the cluster.

Color-magnitude curve for the Pleiades. Color index ordmate is plotted against apparent photographic magnitude. Messier The magnitude-spectrum relation for Messier 34 is illustrated in Figure III, 8, which is taken from Trumpler's discussion of the classification of open clusters. Messier n. In supplementing my earlier work on the colors in Messier n, I found from plates made with the 83 P.

M Hertzsprung, B. The magnitude-spectrum relation for stars in h and x Persei. Trumpler; , Payne. A circle enclosing the symbol indicates a c-star.

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Magnitude- spectrum curve for Messier Or- dinates are apparent visual magnitudes, abscissae are spectral classes. The broken line represents the dwarf branch. After Trumpler. D Spec- trum Magni- tude Note B. Hertzsprung B A N. Nearby stars suspected of membership from their spectra and magnitudes 4. S P , 38, , Twenty- five stars that he plots but does not tabulate are omitted from this table.

Magnitude from the B. The color indices are system- atically too great, as shown by my own spectrum plates unpublished and sub- sequently by the similar work of Lindblad and Trumpler. A correction of 0. There is a possibility of differ- ential light absorption within the cluster. The spectral classes of 59 stars in and around the cluster are given by Trumpler, 39 and his data are shown in condensed form in Table III, IX. Successive columns contain the number from StratonofTs catalogue, 40 Trumpler's spectral class, and my photographic magnitude.

Globular cluster

Becker from his own photographs made at La Paz. Fleming recorded 93 per cent of the stars as Class A. Trumpler's Investigations. The kinds of spectral distribu- tion among the stars of a galactic cluster, first recognized in the early Harvard work, and amplified by all subsequent studies, are defined in some detail in Trumpler's scheme of classification Section 4 in Chapter II , and a number of galactic clusters are now assigned to the classes and their subdivisions.

The essentials of the classification are shown by Figure III, 9, which is reproduced from Trumpler's paper. The Lick studies, published and unpublished, have provided for the classification of 52 galactic clusters by means of their spectra, and the relative numbers in the various classes are as follows: Type Number ib 24 ia 6 2a 20 2f I Others I It will be seen that the Pleiades type preponderates among the systems bright enough to classify. This may, however, be an effect of selection, as Type ib contains far brighter stars than any of the others.

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The bearing of this selection on estimates of distance is considered later Section 58, last footnote. Fortunately, the field is rich. Almost a thousand variable stars are enumerated in the census contained in the present chapter. The first four sections refer to the many variables in globular clusters, the next one to their scanty appearance in galactic clusters, and the following sections deal with observa- tional considerations of the nature of Cepheid variability.

A Summary of Known Variables. Examining some of his earlier photographs of globular clusters, Dr. Common 1 noted the probable variability of some stars in Messier 5. Pro- fessor E.

Which fall into these classes

Pickering 2 in and Mr. David Packer 3 in also made some early observations of the variables in globular clusters, which were independently confirmed by Barnard a few years later. Employing mainly the photographs made at Arequipa with various telescopes, Bailey has found the majority of cluster variables now known and has made by far the most important investigations of light curves and periods.

Aside from Bailey's work, the discovery and 1 M.

api.vinylextras.com/tomorrow-in-the-battle-think-on.php Larink has made an extensive check of Bailey's periods for cluster-type variable stars in Messier 3, finding that, after a 2o-year interval, 82 periods were unchanged and 29 probably had varied. My similar check on 54 of Bailey's variables in Messier 5 results in periods accurate to within a tenth of a second; in this cluster the periods are nearly all constant throughout an interval of 30 years.

The data at present available concerning the variable stars in globular clusters are summarized in Table IV, I.

Clusters examined with care but without discovery of variable stars are also included in the table. Some stars suspected of vari- ability are omitted in the absence of numerous or decisive observations. Three of these, for instance, are in the Hercules cluster, 5 where my measures on a few plates cannot be con- sidered to furnish sufficient evidence of variability. In crowded regions and for close doubles, the photographic development Eberhard effect 6 may produce spurious variability, for it varies from plate to plate under ordinary working conditions.

Aside from this and similar uncertainties, which lead to the inclusion and exclusion of suspected variables, there is an ele- ment of incompleteness in these tabulated results because of the difficulty of thoroughly examining the centers of clusters, and also because of the small number of plates sometimes involved in the surveys. In general, also, it may be said that scarcely a cluster has been examined with the accuracy and thoroughness necessary to detect ordinary eclipsing stars of short range or narrow minimum and to exhaust the possibility of Cepheids of small range.

Eberhard, Phys. Bailey, H. Obs , unpublished. Miss Swope, unpublished.