Topics in Astrophysics

Introductory

The Structure of Our Universe.  This topic path gives an overview of the principal objects studied in astronomy.  Each of these objects???planets, stars, galaxies, and the universe???defines a scale for our universe.  The size of our universe is difficult to grasp. Each change in scale???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 demanded the development of new methods of measuring distance.   With the invention 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 distances 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

Planets.  The planets, asteroids, and comets of the Solar System and of other planetary systems, as well as the Jupiter-like objects moving freely within the Galaxy, 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.  This topic path describes the most fundamental object in astronomy, the star. This path covers the structure and evolution of a star.  This discussion covers the nuclear fusion that takes place within a star.  Included among these pages are simulators that allow the reader to experiment with the thermonuclear fusion of hydrogen, helium, and other light elements.   Nuclear fusion is the process that separates stars from planets.   Stars begin life composed principally of hydrogen and helium. As they age, they convert these elements to carbon, oxygen, and other elements.   Eventually a star ends its life as a degenerate dwarf, neutron star, or black hole.  (continue)

Degenerate Objects.  The topic of degenerate objects covers two classes of object: the electron degenerate objects???comprising the giant gaseous planets, the brown dwarfs, and the white dwarfs???and the neutron stars.  Degenerate objects are the endpoints of evolution for most bodies in the Galaxy.  The white dwarfs and neutron stars are the endpoint of stellar evolution for all but the most massive stars.  Isolated neutron stars are seen as radio pulsars.  Neutron stars are found in x-ray binary systems.  White dwarfs are found in binary systems that are called cataclysmic variables.  (continue)

Supernovae.  The two most dramatic explosive events we see in our universe are the explosion of a massive star and the explosion of a heavy white dwarf.  The first event is a core-collapse supernova, occurring when the core of a massive star collapses under its own gravity, becoming a neutron star.  The second event is a thermonuclear supernova, which is called a type Ia supernova by observers; it occurs when a white dwarf is pushed over the Chandrasekhar limit, causing a gravitational collapse that initiates a thermonuclear runaway.  These explosions are seen from the most distant parts of the universe.  On rare occasion, they are seen within our own galaxy.  (continue)

Milky Way Galaxy.   Our own galaxy has been strikingly difficult to understand.  Living in the middle of a thin sheet of stars, gas, and dust, we are like a traveler in a forest, able to see those things that are nearby or overhead, but unable to see far into the forest itself.   But the development of radio, infrared, and x-ray astronomy has opened to observation much of the Galaxy that is hidden at the visible wavelengths.   The picture we now have of the Milky Way is of a barred spiral galaxy that contains a massive black hole at its center.   (continue)

Galaxies.  Galaxies are gravitationally-bound collections of stars and gas.   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 the stars within a galaxy 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)

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 the motion of an object away from a central gravitating object.   The motion we see of a galaxy moving away from us is identical to the motion of a spacecraft moving directly away from the Sun.  (continue)

Observational Astronomy.  This incomplete path discusses the instruments used in observational astronomy and the classification schemes for stars.  To this point, the discussion of instrumentation is limited to x-ray and gamma-ray observatories.  This discussion includes descriptions of the various objects seen at x-ray energies, and what is learned by observing the x-rays from these objects.  The discussion of stellar classification describes the magnitude, color, and stellar type systems used by astronomers observing stars with ground-based telescopes.  (continue)

Physics

Electromagnetic Radiation.  Most of our knowledge about astronomical sources comes from studying the light 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 topic path on the electromagnetic radiation produced by astronomical sources starts with a discussion of thermal electromagnetic radiation.  (continue)

Newtonian Gravity.  Gravity governs the evolution of all things in astrophysics.  The theory for the basic characteristics of planetary and binary star motion are 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)

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 by the Sun, 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)