Part 2 (1/2)

_Type B_ (Orion type, Helium stars). All lines are here dark. Besides the hydrogen series we here find the He-lines (396, 403, 412, 414, 447, 471, 493 ).

To this type belong all the bright stars (, ?, d, e, ?, ? and others) in Orion with the exception of Betelgeuze. Further, Spica and many other bright stars.

On plate III e Orionis is taken as representative of this type.

_Type A_ (Sirius type) is characterized by the great intensity of the hydrogen lines (compare plate III). The helium lines have vanished.

Other lines visible but faintly.

The greater part of the stars visible to the naked eye are found here.

There are 1251 stars brighter than the 6th magnitude which belong to this type. Sirius, Vega, Castor, Altair, Deneb and others are all A-stars.

_Type F_ (Calcium type). The hydrogen lines still rather prominent but not so broad as in the preceding type. The two calcium lines H and K (396.9, 393.4 ) strongly p.r.o.nounced.

Among the stars of this type are found a great many bright stars (compare the third chapter), such as Polaris, Canopus, Procyon.

_Type G_ (Sun type). Numerous metallic lines together with relatively faint hydrogen lines.

To this cla.s.s belong the sun, Capella, a Centauri and other bright stars.

_Type K._ The hydrogen lines still fainter. The K-line attains its maximum intensity (is not especially p.r.o.nounced in the figure of plate III).

This is, next to the A-type, the most numerous type (1142 stars) among the bright stars.

We find here ? Andromedae, Aquilae, Arcturus, a Ca.s.siopeiae, Pollux and Aldebaran, which last forms a transition to the next type.

_Type M._ The spectrum is banded and belongs to SECCHI's third type. The flutings are due to t.i.tanium oxide.

Only 190 of the stars visible to the naked eye belong to this type.

Generally they are rather faint, but we here find Betelgeuze, a Herculis, Pegasi, a Scorpii (Antares) and most variables of long period, which form a special sub-type _Md_, characterized by bright hydrogen lines together with the flutings.

Type M has two other sub-types Ma and Mb.

_Type N_ (SECCHI's fourth type). Banded spectra. The flutings are due to compounds of carbon.

Here are found only faint stars. The total number is 241. All are red.

27 stars having this spectrum are variables of long period of the same type as Md.

The spectral types may be summed up in the following way:--

White stars:--SECCHI's type I:--Harvard B and A, Yellow ” :-- ” ” II:-- ” F, G and K, Red ” :-- ” ” III:-- ” M, ” ” :-- ” ” IV:-- ” N.

The Harvard astronomers do not confine themselves to the types mentioned above, but fill up the intervals between the types with sub-types which are designated by the name of the type followed by a numeral 0, 1, 2, ..., 9. Thus the sub-types between A and F have the designations A0, A1, A2, ..., A9, F0, &c. Exceptions are made as already indicated, for the extreme types O and M.

11. _Spectral index._ It may be gathered from the above description that the definition of the types implies many vague moments. Especially in regard to the G-type are very different definitions indeed accepted, even at Harvard.[6] It is also a defect that the definitions do not directly give _quant.i.tative_ characteristics of the spectra. None the less it is possible to subst.i.tute for the spectral cla.s.ses a continuous scale expressing the spectral character of a star. Such a scale is indeed implicit in the Harvard cla.s.sification of the spectra.

Let us use the term _spectral index_ (_s_) to define a number expressing the spectral character of a star. Then we may conveniently define this conception in the following way. Let A0 correspond to the spectral index _s_ = 0.0, F0 to _s_ = +1.0, G0 to _s_ = +2.0, K0 to _s_ = +3.0 M0 to _s_ = +4.0 and B0 to _s_ = -1.0. Further, let A1, A2, A3, &c., have the spectral indices +0.1, +0.2, +0.3, &c., and in like manner with the other intermediate sub-cla.s.ses. Then it is evident that to all spectral cla.s.ses between B0 and M there corresponds a certain spectral index _s_.