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Uranus and Neptune

The two outermost planets, Uranus and Neptune, owe their discovery to the telescope. Uranus was found quite by accident in 1781, during a systematic search of the sky by the great English astronomer William Herschel. It is just barely visible to the naked eye, and in fact had been identified as a faint star on a number of sky maps prepared during the preceding hundred years. Herschel suspected Uranus to be a planet because, through the telescope, it appeared as a disk rather than as a point of light. Observations made over a period of time showed its position to be changing relative to the stars, and its orbit was determined from these data. The discovery of Neptune in 1846 was made as the result of predictions based on it gravitational effect on other planets.

Uranus and Neptune are large bodies, each with a diameter about 3 ½ times that of the earth. In most of their properties Uranus and Neptune resemble Jupiter and Saturn. Their atmospheres are largely methane, which accounts for their greenish color, with some hydrogen present as well. Because these planets are so far from the sun, their surface temperatures are below – 2000 C, and any ammonia present would be frozen out of their atmospheres.

Stellar Evolution

A star shines because it is a large, compact aggregate of matter that contains abundant hydrogen. A body of this sort cannot avoid being luminous because of the energy liberated in the conversion of its hydrogen into helium. We may imagine as the starting point in a star’s history a stage when its matter was an irregular mass of cool, diffuse gas and small, solid particles. Gravitation in such a mass would concentrate it into a smaller space. The gradual contraction would heat the gas, much as the gas in a tire pump is heated by compression. At length the temperature would grow high enough for hydrogen to be converted into helium, and the mass would begin to glow brightly. From this time on the tendency to contract would be counterbalanced by the pressure of radiation from the hot interior, so shrinking would stop and the star would maintain a nearly constant size. The diameter of a star is thus determined by equilibrium between gravitational forces pulling its material inward and forces due to radiation pushing its material outward.

A star does not shine because some occult force has started I shining; it shines because it has a certain mass and a certain composition. If we could somehow build a star by heaping together sufficient matter of the right composition, it would start to shine of its own accord.

A star consumes its hydrogen rapidly if it is large, slowly if it is small. A fairly small star like our sun makes its supply of hydrogen last for a period of the order of 10 billion years; probably the sun is now about halfway through this part of its career. When the hydrogen supply at last begins to run low in a star like the sun, the life of the star is by no means ended but enters its most spectacular phase. Further gravitational contraction makes the interior still hotter and other nuclear reactions become possible - particularly reactions in which atoms of heavier elements are made by a combination of helium atoms. These reactions, once started, give out so much energy that the star expands to become a giant. Energy is now being poured out at a prodigious rate, so the star’s life as a giant is much shorter that the earlier part of its existence.

Eventually the new energy-producing reactions run out of fuel, and again the star shrinks – although probably not without a few last brief flare-ups, which we see from the earth as novae (“new stars”) that shine brilliantly for a week or two and then subside into insignificance. The shrinking ultimately reduces the star to the white dwarf state. As a slowly contracting dwarf the star may remain luminous for billions of years more with its energy now coming from the contraction, from nuclear reactions involving elements heavier than helium, and from proton-proton reactions in a very thin outer atmosphere of hydrogen.

Stars much more massive than the sun have somewhat different histories. Eventually they become unstable and explode violently, emitting enormous amounts of material. Such explosions we observe as supernovae, flare-ups 10,000 or more times as luminous as ordinary novae. Having lost perhaps half its mass, a star of this kind can then subside like its smaller brethren into a dwarf star.

Today astronomers believe that the residual dwarfs of supernovae are different from ordinary white dwarfs because of the large mass of their parent stars. These hypothetical dwarfs are calculated to have densities far in excess of ordinary dwarfs, with masses comparable to that of the sun packed into spheres perhaps 15 km (9 mi) in diameter. The matter of such a star would weigh billions of tons per cubic inch. (If the earth were this dense, it would fit into a large apartment house). Under the pressures that would be present the most stable form of matter is the neutron. Pulsars, which emit brief, intense bursts of radio waves at regular intervals, are believed to be rotating neutron stars with magnetic fields that lead to radio emission in narrow beams; as a pulsar rotates, its beams swing with it to produce the observed fluctuations. A notable pulsar is located at the center of the Crab nebula, which is the remnant of a supernova that was seen in A.D. 1054 and has been expanding and glowing brightly ever since.

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