The Astrophysics Spectator

Basic Astronomy

Distance. The size of our universe is difficult to grasp. Each change in scale as we change our focus from the Solar System to the Galaxy, and from the Galaxy to the observable universe, entails an increase of many factors of ten. Each change in scale has demands the development of new methods of measuring distance. With the inventions of radar and space flight, the measurement of distance within the solar system has improved to the point that the positions of planets are known to within tens of meters. The distanced to many stars within about 100 parsecs of Earth are now known to better than 10% from parallax measurements. Determining the distances to the galaxies remains a big problem; standard candles, such as Cepheid variables and type 1a supernovae, and cosmological redshifts are the only handles on the distance to galaxies available in astronomy. (continue)

Astrophysics

Solar Planetary System. The planets, asteroids, and comets of the Solar System are examined along this path. The eight primary planets are discussed in terms of their group properties, so Mercury, Venus, Earth, and Mars are together as the terrestrial planets, Jupiter and Saturn are together as the giant gaseous planets, and Uranus and Neptune are together as the giant ice planets. Pluto is treated as one of the numerous Kuiper Belt objects. (continue)

Stars. The Stars survey path describes the most fundamental object in astronomy, the star. The topics currently covered on this path is a general description of the structure of the star and several pages of discussion about nuclear fusion, the source of energy for a star. (continue)

Galaxies. Galaxies are gravitationally-bound collections of stars. The galaxies vary in their properties, ranging from the beautiful spiral galaxies to the mundane elliptical galaxies. The physics of galaxies is difficult because the motion of their stars is set by the gravitational field produced by those stars; the study of galaxies is the study of collective phenomena. (continue)

Astrophysical Disks. Not all objects in astronomy are spheres. When the centrifugal force in a system balances the gravitational force, the system usually has a disk configuration. One sees disks everywhere in the universe, from the rings of Saturn to the planes of spiral galaxies, but despite this broad range of scales, all disks are governed by the same set of principles. (continue)

Gamma-ray Bursts. Gamma-ray bursts are a phenomena associated with core-collapse supernovae. They are among the most distant observable events, and they are among the most energetic. This makes them interesting from the standpoint of the evolution of the galaxy and from the standpoint of the physics involved in producing this highly-relativistic phenomena. (continue)

Cosmology. Cosmology is the study of the universe as a whole. We know that the universe is expanding, so that the galaxies are moving apart. We also know that at one time the universe was extremely dense. The expansion of the universe is described very simply as motion counteracted by gravitational force and accelerated by pressure in the early universe and perhaps a vacuum repulsion in the current epoch. (continue)

Branches of Astronomy

X-ray Astronomy. X-ray astronomy came into existence with birth of rocketry. X-rays are unable to penetrate a significant distance into Earth's atmosphere, forcing astronomers who want to study the x-rays emitted from astronomical objects to travel above the atmosphere. Today we have three observatories in Earth orbit dedicated to the study of x-ray sources outside of our Solar System. With these observatories we are able to study an x-ray sky crowded with stars, galaxies, and black-hole candidates. Because the physics underlying the creation of x-rays is somewhat different from the physics behind the creation of visible light, x-rays provide us with information about a source that cannot be provided by observations of visible light from the source. X-rays tell us about the physics within the hot regions of our universe. (continue)

Physics

Gravitational Physics. Gravity governs the evolution of all things in astrophysics. The theory for the basic characteristics of planetary and binary star motion is very simple. When the systems become more complex, as is the case for a galaxy, the theory itself is complex. All of these systems are described by Newtonian gravity, but under the most extreme conditions, one must describe gravity with the more precise theory of general relativity. (continue)

Electromagnetic Radiation. Most of our knowledge about astronomical sources comes from studying the light that they produce. This light spans the electromagnetic spectrum from the radio band to the gamma-ray band. By studying the spectrum of an astronomical source we can determine the structure and composition of the source's photosphere. This path on the electromagnetic radiation produced by astronomical sources starts with a discussion of thermal electromagnetic radiation. (continue)

Special Relativity. Special relativity was developed to make the equations of electromagnetism consistent. Under special relativity, measurements of length and time depend on the motion of the observer. Among the more important effects is the slowing of time for an observer who is accelerating. Special relativity and the time of propagation of light combine to modify the appearance of objects moving at close to the speed of light. These effects are seen in many astronomical sources, such as gamma-ray bursts, extragalactic jets, and jets from compact binary star systems. (continue)

General Relativity. General relativity is the modern theory of gravity that supplanted Newtonian gravity after the development of special relativity. Unlike in Newtonian gravity, in which changes to the gravitational field propagate instantaneously, changes to the gravitational field in general relativity propagate through space at the speed of light. General relativity is based on the equivalence principle, which asserts that gravitational acceleration is identical to acceleration by a rocket in special relativity. This principle inspired Einstein to place the effects of gravity into the definitions of space and time. We see the effects of general relativity in a handful of places in astrophysics. Within our own Solar System we see the gravitational Doppler shift, bending of light, and the drift in Mercury's perihelion, all consequences of general relativity. Farther from home, we see general relativity in the black hole candidates of compact stellar binaries and at the cores of galaxies, in the loss of energy by compact binaries through the generation of gravitational waves, and in the curvature of space time within our expanding universe. These topics are explored on the “General Relativity” path. (continue)