They
(Sirius, Vega, etc.) are, in fact, known as "hydrogen stars," and
their temperature is generally computed at between 20,000 and
30,000 degrees C. A few stars, such as Procyon and Canopus, seem
to indicate a stage between them and the yellow or solar type.
But we may avoid finer shades of opinion and disputed classes,
and be content with these clear stages. We begin with stars in
which only hydrogen and helium, the lightest Of elements, can be
traced; and the hydrogen is in an unfamiliar form, implying
terrific temperature. In the next stage we find the lines of
oxygen, nitrogen, magnesium, and silicon. Metals such as iron and
copper come later, at first in a primitive and unusual form.
Lastly we get the compounds of titanium and carbon, and the
densely shaded spectra which tell of the thickly gathering
vapours. The intense cold of space is slowly prevailing in the
great struggle.
What came before the star? It is now beyond reasonable doubt that
the nebula--taking the word, for the moment, in the general sense
of a loose, chaotic mass of material--was the first stage.
Professor Keeler calculated that there are at least 120,000
nebulae within range of our telescopes, and the number is likely
to be increased.
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