NOTES. 



ASTROXOMV. 



By A. C. D. CROMMlil-lN, B.A., D.Sc, F.K.A.S. 



I HAVK decided to use my column this month to give an 

 explanation of the Harvard svstem of classification of star- 

 spectra, which is now in general use. .-\5 spectra are often 

 quoted simply by their letters in this classification, and as the 

 full description in Harvard Atnials, \'o\. X.XVIII, is probably 

 not readily accessible to many of our readers, it seems likely 

 that it will be convenient to give a summary of it here. 



The m.ain classes of spectra are denoted by the letters O, 

 B. A. F, G. K, .\f, X, which originally followed each other in 

 alphabetical order (in the Draper Catalogue), but have now 

 been put into the order which is conjectured to be the 

 chronological order of their life-history. The fact, recently 

 discovered by Professor Campbell, that average radial velocities 

 increase as we pass from the beginning to the end of this set 

 of letters, is a distinct confirmation of the theory that it is a 

 true chronological sequence. Besides the above eight letters, P 

 and O are used, the former for gaseous nebulae, the latter for 

 peculiar spectra having bright lines. Small letters following 

 the capital ones, as Oa, Mb. and so on, indicate sub-divisions of 

 main classes. Where two capitals are used with a numeral 

 between, as BIA, B3A, B5A, the meaning is that the type is 

 intermediate between B and A, and one-tenth, three-tenths, 

 five-tenths respectively of the interval from the former to the 

 latter type. 



We now proceed to the description of the types. The letter 

 O in general corresponds to Secchi's Fifth Type. 



Oa. — There are no very bright stars of this type. The 

 spectra consist of bright bands on a faint continuous 

 background. Two of the bright bands are hydrogen 

 ones, 4101-8 = Ho, 4340-7 = H7. A fainter one at 

 4471-8 appears to be helium. There is a broad con- 

 spicuous band at 4633. 



Ob. — There are no very bright stars of this type. Spectrum 

 similar in general character to Oa, but positions and 

 intensities of the bands differ. Several hydrogen lines 

 bright, including Ht, Ho, H7, H/i; no helium lines seen. 

 .\n intensely bright band has centre at 4688. 



Oc. — Xo very bright stars of this type. The bands are much 

 narrower than in Ob. Ho, H7, H/S, 4471-8 (helium) and 

 4688 arc again bright. 



Od. — Typical star i Puppis. All lines are dark except 4633 and 

 4688. which are bright. The dark lines are narrow. 

 Hydrogen lines dark. Only four other dark lines. 



Oe. — Typical star, 29 Canis Maj. ; like Od, but far more dark 

 lines. 



Oe5B. — Typical stars, t Can. Maj., land S' Orionis. No bright 

 bands. Resemble Oe in hydrogen spectrum, B in helium 

 spectrum. K (calcium) sharply defined. 



B. — This is sometimes known as the " Helium " or " Orion " 

 type. Typical stars, e. (, Orionis. Helium lines more 

 conspicuous than hydrogen ones. 



BIA. — Type, f* Centauri. More faint lines than in B. Helium 

 lines intense, including D,. 



B2A. — Type, y Orionis. The helium lines attain their 

 maximum intensity in this class. 



B3A.— Types, a Pavonis, t' Orionis. Diminution in strength 

 of " Orion " lines. 



B5A. — Type, 1 Tauri (Alcyone). Helium still present, but less 

 conspicuous ; hydrogen, calcium, and so on, more con- 

 spicuous. 



B8A. — Type. 7 Gruis. Hydrogen lines still increasing in 

 intensity, and are hazy ; twelve " Orion " lines still seen. 

 The solar lines seen in Class A now begin to appear. 



B9A. — Type. X Centauri. Spectrum like A class, with addition 

 of helium. 



A (Sirian stars). — Types, Sirius and Vega. Secchi's First 

 Type. Hydrogen lines attain their maximum strength in 

 this and in the next class. Calcium fairly conspicuous, 

 and there are ninety-three solar lines. 



\Z\'. — Type, 1 Centauri. Calcium and solar lines are stronger 

 than in last. 



A3F. — Type, r-' Eridani. Calcium more intense; solar lines 

 more numerous and more intense. 



ASF. — Type, "■ Pictoris. Hydrogen begins to weaken, solar 

 lines continue to strengthen. All lines somewhat hazy. 



F. — Type, a Carinae. This class is intermediate between 

 Secchi's first and second types. Hydrogen only half as 

 intense as in A. Ultra-violet spectrum strong. Three 

 hundred and twenty-six solar lines present. 



F2G. — Type, ir Sagittarii. The G band in the spectrum grows 

 stronger. 



F5G. — Type, Procyon. Hydrogen, though only half as intense 

 as in Sirius, is still two or three times as intense as in 

 Sun, while solar lines are fainter and fewer than in Sun. 



F8G. — Type, a Fornacis. Resembles following class, except 

 that hydrogen is twice as intense. 



G (solar stars). — Secchi's second type. Type Capella. Hy- 

 drogen one-fifth as intense as in Sirius. The calcium 

 H,K lines, and the band G are the most conspicuous 

 features. 



G5K. — Types, a Reticuli and /S Corvi. Hydrogen still grows 

 weaker, and absorption at violet end begins. 



K. — Intermediate betw-een Secchi's second and third types. 

 Types a Phoenicis and f Scorpii. Hydrogen one-twelfth 

 as intense as in Sirius. The K line of calcium attains its 

 maximum intensity in this and the next class. There are 

 brightish bands whose limits are 4470 to 4525, 4614 to 

 4648 in this and preceding class. Arcturus belongs to 

 this class; its spectrum resembles that of a sunspot, so 

 that it is probably much more spotted than the Sun. 



K2M. — Type, " Librae. Like following class in intensities of 

 solar lines and absorption at violet end. But the G band 

 is still continuous as in Class K. 



K5M. — Type. Aldcbaran. Like Secchi's Third Type. 

 Hydrogen lines one-twentieth as intense as in Sirius, and are 

 inconspicuous among solar lines. The spectrum beyond K 

 too faint to photograph. H,K are conspicuous. The G 

 band is broken up into lines with bright intervals. There 

 are the same brightish bands as in Class K and some 

 others. 



Ma. — Types, j3etelgeux and 7 Hydri. This is Secchi's Third 

 Type. Spectrum banded. There are now well marked 

 bands from 4762 to 4954, and 4954 to 5168, which were 

 faintly seen in K5M. "The edges 4762, 4954, 5168 are 

 brighter than the adjacent continuous spectrum, and the 

 change in intensity from these edges towards the end of 

 greater wavelength is abrupt." There are bright bands, 

 4470 to 4525, 4556 to 4586, 4657 to 4668. Hydrogen 

 lines similar to Aldebaran. The H,K bands are barely 

 seen. The hnes that formed the band G in Classes G to 

 K are now well separated. 



Antares is of Class Ma except that hydrogen lines H)3, 

 H7, H5 are stronger, and H-n, Hfare present as in Class 

 A. Betelgeux has also stronger hydrogen lines than the 

 normal Ma star. 



Mb. — Typical stars, 7 Crucis, o Librae. The abrupt change 

 of light from the edges of the bands towards the end of 

 the spectrum of greater wave-length is more marked than 

 in .Ma. Numerous bands brighter than adjacent portions 

 of the spectrum are present. Hydrogen even fainter than 

 Ma, and is only one-thirtieth as intense as in Sirius. 

 Line 4227 reaches its maximum intensity in this class. 



Md. — This class resembles Mb, except that HS, H7, H^ 

 (hydrogen lines) are bright. The lines of the band G are 

 even less conspicuous than in Class Mb. o Ceti (Mira) is 

 of this class. 



N. — This corresponds to Secchi's Fourth Type. It consists 

 of small red stars with banded spectra, the bands being 

 sharply defined at the red end and fading away gradually 

 at the blue end (the opposite to the Third Type). The 

 bands are due to carbon and cyanogen; those of the 

 Third Type have been identified by Fowler with titanium 

 oxide. 



