Lectures on Stellar Statistics | Page 6

Sum ... 2222
Spectrum corresponding to a given colour.
Colour Sp. Number W, W+ A0 281 YW- A0 356 YW A5 482 YW+, YW- F3 211 WY G4 264 WY+, Y- K1 289 Y, Y+ K4 254 RY-, RY K5 85 -------------------------------- Sum ... 2222
The signs + and - indicate intermediate shades of colour.
The preceding table drawn up by Dr. MALMQUIST from the colour observations of M��LLER and KEMPF in Potsdam, shows the connection between the colours of the stars and their spectra.
The Potsdam observations contain all stars north of the celestial equator having an apparent magnitude brighter than 7m.5.
We find from these tables that there is a well-pronounced regressionin the correlation between the spectra and the colours of the stars. Taking together all white stars we find the corresponding mean spectral type to be A0, but to A0 corresponds, upon an average, the colour yellow-white. The yellow stars belong in the mean to the K-type, but the K-stars have upon an average a shade of white in the yellow colour. The coefficient of correlation (r) is not easy to compute in this case, because one of the attributes, the colour, is not strictly graduated (i.e. it is not expressed in numbers defining the colour).[5] Using the coefficient of contingency of PEARSON, it is, however, possible to find a fairly reliable value of the coefficient of correlation, and MALMQUIST has in this way found r = +0.85, a rather high value.
In order to facilitate the discussion of the relation between colour and spectrum it is convenient to deal here with the question of the spectra of the stars.
10. Spectra of the stars. In order to introduce the discussion I first give a list of the wave-lengths of the FRAUENHOFER lines in the spectrum, and the corresponding chemical elements.
FRAUENHOFER line Element [lambda] A 759.4 B 686.8 C([alpha]) H (hydrogen) 656.3 D1 Na (sodium) 589.6 D3 He 587.6 E Fe (iron) 527.0 F([beta]) H 486.2 ([gamma]) H 434.1 G Ca (calcium) 430.8 h([delta]) H 410.2 H([epsilon]) Ca(H) 396.9 K Ca 393.4
The first column gives the FRAUENHOFER denomination of each line. Moreover the hydrogen lines [alpha], [beta], [gamma], [delta], [epsilon] are denoted. The second column gives the name of the corresponding element, to which each line is to be attributed. The third column gives the wave-length expressed in millionths of a millimeter as unit ([mu][mu]).
On plate III, where the classification of the stellar spectra according to the Harvard system is reproduced, will be found also the wave-lengths of the principal H and He lines.
By the visual spectrum is usually understood the part of the radiation between the FRAUENHOFER lines A to H ([lambda] = 760 to 400 [mu][mu]), whereas the photographic spectrum generally lies between F and K ([lambda] = 500 to 400 [mu][mu]).
In the earliest days of spectroscopy the spectra of the stars were classified according to their visual spectra. This classification was introduced by SECCHI and was later more precisely defined by VOGEL. The three classes I, II, III of VOGEL correspond approximately to the colour classification into white, yellow, and red stars. Photography has now almost entirely taken the place of visual observations of spectra, so that SECCHI's and VOGEL's definitions of the stellar spectra are no longer applicable. The terminology now used was introduced by PICKERING and Miss CANNON and embraces a great many types, of which we here describe the principal forms as they are defined in Part. II of Vol. XXVIII of the Annals of the Harvard Observatory. It may be remarked that PICKERING first arranged the types in alphabetical order A, B, C, &c., supposing that order to correspond to the temperature of the stars. Later this was found to be partly wrong, and in particular it was found that the B-stars may be hotter than those of type A. The following is the temperature-order of the spectra according to the opinion of the Harvard astronomers.
Type O (WOLF-RAYET stars). The spectra of these stars consist mainly of bright lines. They are characterized by the bright bands at wave-lengths 463 [mu][mu] and 469 [mu][mu], and the line at 501 [mu][mu] characteristic of gaseous nebulae is sometimes present.
This type embraces mainly stars of relatively small apparent brightness. The brightest is [gamma] Velorum with m = 2.22. We shall find that the absolute magnitude of these stars nearly coincides with that of the stars of type B.
The type is grouped into five subdivisions represented by the letters Oa, Ob, Oc, Od and Oe. These subdivisions are conditioned by the varying intensities of the bright bands named above. The due sequence of these sub-types is for the present an open question.
Among interesting stars of this type is [zeta] Puppis (Od), in the spectrum of which PICKERING discovered a previously unknown series of helium lines. They were at first attributed (by RYDBERG) to