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Issue 3.02

The Astrophysics Spectator

January 14, 2006

Two new pages are added to the introductory topic The Structure of Our Universe this fortnight on The Astrophysics Spectator. The first page, Planets, describes the basic characteristics of the planets both in and outside of our Solar System. The Planets page contains links to pages under the Planets topic path. The second page, Stars, describes the basics characteristics of stars. This page contains links to pages on the Stars topic path.

We have a good natural sense of what constitutes a planet, and what constitutes a star: planets are cool bodies inside of our own Solar System, while stars are distant hot objects similar to the Sun. These distinctions, however, blur as more low-mass stars and high-mass planets are found. At some point, a dividing line between the two types of object must be drawn.

In fact, a whole class of objects sits between the stars and the planets: the brown dwarf. A brown dwarf most closely resembles the giant gaseous planets like Jupiter, although a brown dwarf is much larger than Jupiter. Thermonuclear fusion is the physics that distinguishes the planets, brown dwarfs, and stars. In a planet, no thermonuclear fusion can take place. The pressure and temperature at the core of a planet is too low to initiate thermonuclear fusion. At the core of the more-massive brown dwarfs, the pressure and temperature are high enough to drive the thermonuclear fusion of deuterium, a rare heavy isotope of hydrogen. Hydrogen and other light elements are unable to fuse in brown dwarfs. This limited source of fusion makes a brown dwarf a very cool object compared to a star. Stars are distinguished by their ability to initiate the thermonuclear fusion of hydrogen. Hydrogen is the most abundant element in a star, and the amount of energy released when hydrogen fuses together to form helium is more than the amount released in any other fusion process. This makes a star very bright and very long-lived. While a brown dwarf will burn deuterium for less than a billion years, a low-mass star will burn hydrogen for hundreds of billions of years.

Next Issue: The next issue is scheduled for February 1.

Jim Brainerd

The Structure of the Universe

Planets. The planets are the smallest bodies studied in astrophysics. The ancient astronomers new of only of five planets in addition to Earth, but these were enough to eventually develop a theory of gravity and of planetary motion. Today we know of nine planets within our own Solar System—ignoring the issues of what constitutes a planet—and more than a hundred planets around other stars. There is even speculation that planet-sized bodies move freely within our Galaxy, far from any star system. This page discusses what is a planet, and what distinguishes them from stars. This page also discusses the brown dwarf, the planet-like body that generates energy by burning deuterium. (continue)

Stars. Stars are objects that some time during their life burn hydrogen into helium. The smallest stars, which are cool and red, are very long-lived, living for hundreds of billions of years before exhausting their supply of hydrogen. The largest stars are hot, blue, and short-lived, burning their hydrogen in as little as 2 million years. The nearby stars are predominately the small stars, but the brightest stars in our sky are predominately large stars. (continue)

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