The Astrophysics Spectator

Issue 1.10, December 15, 2004

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December 15, 2004

I begin this week's issue with a page on the Kuiper Belt, the disk of planetoids that lies just outside of Neptune's orbit. The best known Kuiper Belt object is Pluto and its binary companion Charon. The Kuiper Belt is of great interest because it contains clues of the Solar System's evolution.

I next introduce the path on gravitational physics. Gravity is the primary force at work on astronomical scales. Problems in gravitational physics range from the simple Keplerian orbits of normal binary stars and of the planets around the Sun to the complex orbits of stars in galaxies and other self-gravitating systems. All aspects of gravitational astrophysics will eventually be covered on this path, including the orbital behavior of objects governed by general relativity.

Because the Keplerian orbit is fundamental to astrophysics, I begin the “Gravitational Physics” path with two pages that discuss this orbit. The first page gives a general overview of the Keplerian orbit. The second page discusses the physics of the Keplerian orbit. This page is the most technical that we have added to this site; it is the first in a series of pages intended to help the reader develop a deep understanding of astrophysics. In the index for a path, such technical pages will be indicated by smaller print and a fourth-level indentation.

Today's commentary is about fame and the astrophysicist.

Jim Brainerd

Holiday Notice

For the next two weeks I will be taking the Christmas holiday off, so the next issue of The Astrophysics Spectator, issue 2.1, will appear on January 5 I wish you all a merry Christmas.


Fame in Astrophysics. The desire of fame drives many astrophysicists in their research. But fame in astrophysics is now an illusion, and today's astrophysicist works in anonymity. (continue)

More commentaries.

New Background

Kuiper Belt Objects. The Kuiper Belt is a broad asteroid belt outside of Neptune's orbit. The objects in this belt are remnants of the ancient solar accretion disk that gave rise to the planets. Kuiper Belt objects fall into several different classes based on their orbits. The objects in one group follow orbits that are resonant with Neptune's orbit; among these are Pluto and Charon. We see in the Kuiper Belt evidence of Neptune's drift to larger distances from the Sun. Objects in the Kuiper Belt are thought to be related to other classes of object, such as the Centaur asteroids, the short-period comets, and the asteroids in the Oort cloud, which gives birth to the long-period comets. The page on the Kuiper Belt is added to the “Solar Planetary Systems” path, which was formerly called the “Solar System Planets” path. (continue)

Gravitational Physics. The gravitational physics path begins with a short and general discussion of gravity and its role in astrophysics. (continue)

Keplerian Orbits. The Keplerian orbit is the fundamental orbit that describes the orbits of binary stars and of the planets around the Sun. The characteristics of the Keplerian orbit was empirically derived by Johannes Kepler from observations of the planets. These characteristics are presented, and the shortcomings of the Keplerian orbit as a description of physically realized orbits are discussed. (continue)

Physics of Keplerian Orbits. Why does the Keplerian orbit behave as it does? This question is answered by presenting the basic equations that describe the orbit. A small amount of mathematics is presented in this page as a means of understanding several important physical principles. (continue)


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