Mm mm mJLi no i iw Mi i iwJLhJlm Jmii mJLi in nJLw mm am

' ■ I ■ ■ * ■ 1 * ■ 1 * ■ ■ * ■ ■ ■ 00h48m42s 45s 48s 51s 54s

Fig. 19.5 Pulses from the pulsar PSR B0329 as observed with the Nancay radio telescope in France (see color supplement). The interval between the pulses is exactly 0.714 s (courtesy of I. Cognard & G. Theureau)

19 Stars: Cosmic Fusion Reactors Table 19.3 Comparison of properties of the Sun and white dwarfs

Quantity

Sirius B

The Sun

Mass

Radius

Luminosity

1.00

1.00

1.00

5,700

1.41

Surface temperature (K) Average density (g/cm3) Central density (g/cm3) Central temperature (K)

27,000

Gravitational redshift (km/s)

What exactly are white dwarfs, supernovae, and their remnants, to which Hoyle referred? At the early part of the twentieth-century observations started to suggest that there are fantastically dense stars, with sizes like the Earth and masses like the Sun. An example is the companion to Sirius, called Sirius B (see Table 19.3). The density of such stars is about a million times greater than the density of ordinary rock! Arthur Eddington remembered how the scientific community reacted: "When the message from Sirius was decoded, it read: I am made of matter which is 3000 times denser than any matter that you know of; a ton of my matter is such a small piece that you can put it in a matchbox. What can you answer to this message? Most of us answered in 1914: Shut up. Don't be silly."

It was not until 1926 that it was realized that Sirius's message was not nonsense. American Ralph H. Fowler applied the newly found Pauli's exclusion principle to an electron gas in white dwarf stars. In the high density prevailing in white dwarfs, the electrons have no room to circle around the atomic nuclei, but they form a gas of their own. A white dwarf star is like a huge atom covered by a cloud of innumerable electrons. Pauli's principle applies to electrons in this cloud just the same as it applies to them in ordinary atoms. Electrons cannot settle in a state which is identical to the state of any other electron in the cloud. When the star cools, all electrons cannot slow down since there are not enough states corresponding to slow motion. Some electrons are bound to have high speeds, and the resulting pressure prevents further shrinkage in the star, even if the temperature were to approach absolute zero.

Referring back to the HR diagram given earlier (Fig. 19.3), we see the white dwarf stars in the lower left part of the diagram, hot and of low energy output compared to the Sun.

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