The Universe: From Flat Earth to Quasar by Isaac Asimov

Author:Isaac Asimov [Asimov, Isaac]
Language: eng
Format: epub
ISBN: 9780380421923
Amazon: B000AN3J82
Published: 2022-11-12T09:34:08+00:00

Stellar Explosions

16?

Sun, or less. In 1895, the German-American astronomer John Martin Schaeberle (1853-1924) detected Procyon B

and that turned out to be only of the eleventh magnitude.

Even allowing for the fact that Procyon B was a bit farther from us than Sirius B, it could quickly be shown that Procyon B was even dimmer than Sirius B Such stars might not be completely dark, but they were certainly dwarf stars.

It was taken rather for granted at the turn of the century that stars like Sirius B and Procyon B were dying stars that were dim primarily because the stellar fires were flickering out. One might suppose that such stars might fit neatly among the red dwarfs at the tail of the main sequence.

However, even as the H-R diagram was being worked out, it became quite apparent that Sirius B, for instance, would not fit in just that place. To be at the tail end of the main sequence, a star would have to be very cool, and, therefore, a deep red in color. Sirius B, however, was not red. It shone with a clear white light. If it was a dwarf, it was a “white dwarf.”

In 1914, the American astronomer Walter Sydney Adams (1876-1956) succeeded in taking the spectrum of Sirius B and found it to be a spectral class A star, exactly as Sirius A itself was. This meant that Sirius B had to have a surface temperature as high as that of Sirius A (10,000° C.) and higher than the mere 6000° C. surface temperature of the Sun.

But if Sirius B was hotter than the Sun, it should have a surface brighter than the Sun, square mile for square mile.

The fact that Sirius B was so much less luminous than the Sun could only mean that Sirius B possessed very few square miles of surface area. It was a star that was white-hot but very small, just the kind of star you would expect to find in the lower left region of the H-R diagram, the region mentioned at the end of the previous chapter.

In fact, it would have to be quite small. To account for its dimness, Sirius B would have to have a diameter of not more than 17,000 miles and be no larger in size than the planet Uranus. It would be a white dwarf indeed.

And yet Sirius B would still have the mass of the Sun.

That was determined from the gravitational effect of Sirius B on Sirius A, and it could not be argued away. For a star, then, to be as small as Uranus and as massive as the Sun brought up serious questions of density—questions that would have raised insuperable difficulties in the nine

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