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Sirius Matters: Fiction & Facts

November 14, 2000 ::

Sirius A B
Chandra X-ray image of Sirius located 8.7 light years from Earth.
Chandra's observation of the white dwarf Sirius B reminds us that for time out of mind, Sirius, the brightest star in the sky, has played a prominent, and sometimes peculiar role in mythology and astronomy. In ancient Egypt, Sirius' hieroglyph, the dog, appeared on monuments and temple walls dating back beyond 3,000 BC. The rising of Sirius just
The Pyramids
The Egyptian Pyramids
before the Sun on the summer solstice signaled the beginning of summer, and coincidentally, the coming flooding of the Nile, an event critical for supplying the water necessary for agriculture. The temple of Isis-Hathor at Denderah and numerous other temples in the Nile valley were oriented to the rising of Sirius.

In ancient Greece and Rome, Sirius was thought to be a bad influence, responsible for drought, disease and madness in dogs. We still speak of the Dog Days of summer in reference to the ancient association of Sirius with the withering heat of July and August.


White Dwarf Illustration
White dwarf stars are dense, burnt-out remnants of stars like the Sun. They are formed when the star contracts after using up its nuclear fuel. When matter from a nearby star falls onto a white dwarf star, X-rays are produced. More on White Dwarfs
The brightness of Sirius led the celebrated philosopher and cosmologist Immanuel Kant to speculate that it was the center of our Milky Way Galaxy, an intriguing but totally wrong idea. Sirius appears so bright because it is very near to Earth, at a distance of 8.7 light years, and because it is intrinsically more luminous than an average star. Sirius was in the news -- at least among astronomers -- in the 1830s and 1840s when the astronomer and mathematician Frederick Bessel concluded that the apparent motion of Sirius was not a smooth arc, but an irregular curved path caused by an invisible companion. The theoretical orbit of this companion was computed, but a search for the companion to Sirius was fruitless.

Then in 1862 Alvan Clark, a renowned telescope maker, wanted to test out his latest creation, an 18.5 inch refracting telescope at the Dearborn Observatory of Northwestern University. He decided to check it out on Sirius, and quickly discovered the dark companion to the Dog Star. Subsequent studies of the companion, which was called Sirius B, or "The Pup," led to the conclusion that it was hotter than the Sun with a diameter less than 1% of that of the sun.


Subrahmanyan Chandrasekhar
Subrahmanyan Chandrasekhar
How such a dense, hot star could exist was a mystery. Finally, in the 1920's R. H. Fowler of Cambridge University, using the new theory of quantum mechanics, proposed that white dwarf stars are supported by degenerate electron pressure, a peculiar property of very dense material. A few years later Subrahmanyan Chandrasekhar, the Chandra Observatory's namesake, worked out the theory in detail.

Chandra used relativity theory and quantum mechanics to show that degenerate electron pressure can do only so much. If the mass of the white dwarf becomes greater than about 1.4 times the mass of the Sun -- called the Chandrasekhar limit -- it will collapse to become a neutron star or black hole, or blow itself apart in a supernova. These so-called "Type I" supernovas are produced when a nearby companion star dumps too much material onto a white dwarf and pushes it over the Chandrasekhar limit.

This discovery created a big stir, since it showed that the theories of quantum mechanics and relativity could help explain cosmic mysteries. Arthur Eddington, the leading astrophysicist of the day, thought that Chandra was wrong about degenerate electron pressure and the implication that massive stars were doomed to collapse. "I think there should be a law of Nature to prevent a star from behaving in this absurd way!" he protested. The controversy stewed for several years, but in the end, everyone who studied Chandra's work -- everyone but Eddington, that is -- agreed that Eddington was wrong, and Chandra was right.


Part Two: In recent times Sirius B is the subject of another debate -- this time involving aliens!

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