Astrophysics Simulations

• J.M. Anthony Danby of North Carolina State University
  • binaries : visual1 , visual2, eclipse, spectro , tidal, roche, accrdisk
  • nbmenu : twogalaxies, astroids, nbodies, planets, playback, elements
  • galaxies :rotation, oortcons, arms21cm
• Richard Kouzes of West Virginia University
  • stellar
  • evolve
• Charles Whitney of Harvard University
  • atmos
  • pulse

• STELLAR (Stellar Models), written by Richard Kouzes, is a simulation of the structure of a static star in hydrodynamic equilibrium. This provides a model of a zero age main sequence star, and helps the user understand the physical processes that exist in stars, including how density, temperature and luminosity depend on mass. Stars are self gravitating masses of hot gas supported by thermodynamic processes fueled by nuclear fusion at their core. The model integrates the four differential equations governing the physics of the star to reach an equilibrium condition which depends only on the star's mass and composition.

• EVOLVE (Stellar Evolution), written by Richard Kouzes, builds on the physics of a static star, and considers (1) how a gas cloud collapses to become a main sequence star, and (2) how a star evolves from the main sequence to its final demise. The model is based on the same physics as the STELLAR program. Starting from a diffuse cloud of gas, a protostar forms as the cloud collapses and reaches a sufficient density for fusion to begin. Once a star reaches equilibrium, it remains for most of its life on the main sequence, evolving off after it has consumed its fuel. The final stages of the star's life are marked by rapid and dramatic evolution.

• BINARIES, written by Anthony Danby, is the driver program for all Binaries Programs:
  • VISUAL1 (Visual Binaries - Proper Motion), enables you to visualize the proper motion in the sky of the members of a visual binary system. You can enter the elements of the system and the mass ratio, as well as the speed at which the center of mass moves across the screen. The program also includes an animated three-dimensional demonstration of the elements.

  • VISUAL2 (Visual Binaries - True Orbit), enables you to select an apparent orbit for the secondary star with arbitrary eccentricity, with the primary at any interior point. The elements of the orbit are displayed. You can see the orbit animated in three dimensions, or can make up a set of "observations" based on the apparent orbit.

  • ECLIPSE (Eclipsing Binaries), shows simultaneously either the light curve and the orbital motion of the light curve and an animation of the eclipses. You can select the elements of the orbit and radii and magnitudes of the stars. A form of limb-darkening is also included as an option.

  • SPECTRO (Spectroscopic Binaries), allows you to select the orbital elements of a spectroscopic binary, and then shows simultaneously the velocity curve, the orbital motion and a moving spectral line.

  • TIDAL (Tidal Distortion of a Binary), models the motion of a spherical secondary star around a primary that is tidally distorted by the secondary. You can select orbital elements, masses of the stars, a parameter describing the tidal lag, and the initial rate of rotation of the primary. The equations are integrated over a time interval that you specify. Then you can see the changes of the orbital elements, and the rotation of the primary, with time. You can follow the motion in detail around each revolution, or in a form where the equations have been averaged around each revolution.

  • ROCHE (The Photo-Gravitational Restricted Problem of Three Bodies), follows the two-dimensional motion of a particle that is subject to the gravitational attraction of two bodies in mutual circular orbits, and also, optionally, radiation pressure from these bodies. It is intended, in part, as background for the interpretation of the formation of accretion disks. Curves of zero velocity (that limit regions of possible motion) can be seen. The orbits can also be followed using Poincaré maps.

  • ACCRDISK (Formation of an Accretion Disk), follows some of the dynamical steps in this process. The dynamics is valid up to the initial formation of a hot spot, and qualitative afterward.

• NBMENU, written by Anthony Danby, is the driver program for all Programs on the motion of N interacting bodies:
  • TWOGALAX (The Model of Wright and the Toomres) is concerned with the interaction of two galaxies. Each consists of a central gravitationally attracting point, surrounded by rings of stars (which are attracted, but do not attract). Elements of the orbits of one galaxy relative to the other are selected. as is the initial distribution and population of the rings. The motion can be viewed as projected into the plane of the orbit of the galaxies, or simultaneously in that plane and perpendicular to it. The positions can be stored in a file for later viewing.

  • ASTROIDS (N-Body Application to the Asteroids) uses the same basic model, but a planet and a star take the place of the galaxies and the asteroids replace the stars. Emphasis is on asteroids all having the same period, with interest on periods having commensurability with the period of the planet. The orbital motion of the system can be followed. The positions can be stored in a file for later viewing. An asteroid can be selected, and the variation of its orbital elements can then be followed.

  • N-BODIES (The Motion of N Attracting Bodies) allows you to choose the number of bodies (up to 20) and the total energy of the system. Initial conditions are chosen at random, consistent with this energy, and the resulting motion can be observed. During the motion various quantities, such as the kinetic energy, are displayed. The positions can be stored in a file for later viewing.

  • PLANETS (Make Your Own Solar System), is similar to the preceding program, but with the bodies interpreted as a star with planets. Initially conditions are specified throughout the choice of the initial elements of the planets. The positions can be stored in a file for later viewing.

  • PLAYBACKenables a file stored by one of the proceeding programs to be viewed.

  • ELEMENTS (Orbital Elements of a Planet), shows a three-dimensional animation that can be viewed from any angle.

• GALAXIES, written by Anthony Danby, is the driver program for Galactic Kinematics Programs:
  • ROTATION (The Rotation Curve of a Galaxy) allows you to "design" a galaxy, consisting of a central mass and up to five spheroids (that can be visible or invisible). It then displays the galaxy and can show the animated rotation or the rotation curve.

  • OORTCONS (Galactic Kinematics and Oort's Constants) allows you to design your galaxy, choose the location of the "Sun" and a local region around it, and to observe the kinematics in this region. It also shows graphs of radial velocity and proper motion with comparison with the linear approximation, and computes the Oort constants.

  • ARMS21CM (The Spiral Structure of a Galaxy) allows you to design your galaxy, construct a set of spiral arms and select the position of the "Sun." Then, for different galactic longitudes, you can see observed profiles of 21 cm lines.

• ATMOS (Stellar Atmospheres), written by Charles Whitney, permits the user to select a constellation, see it mapped on the computer screen, point to a star, and see it plotted on a brightness-color diagram. The users task is to build a model atmosphere that imitates the photometric properties of observed stars. This is done by specifying numerical values for the basic stellar parameters: radius, mass, and luminosity. The program then builds the model and displays it on the brightness-color diagram, and it also plots the spectrum and the detailed thermodynamic structure of the atmosphere. With this program the user may investigate the relation between stellar parameters and the thermal properties of the gas in the atmosphere. Two atmospheres may be superposed on the graphs for easier comparison.

• PULSE (Stellar pulsations), written by Charles Whitney, illustrates stellar pulsation by simulating the thermo-mechanical behavior of a "star" modeled by a self-gravitating gas divided by spherical elastic shells. The elastic shells resemble a set of coupled oscillators. The program solves for the modes of small-amplitude motion, and it uses Fourier synthesis to construct motions for arbitrary starting conditions. The screen displays the thermodynamic structure and surface properties, such as temperature, pressure and velocity. Animation displays the nature of the pulsation. By showing the motions, temperatures and energy flux, the program demonstrates the heat engine acting inside the pulsating star. The motions of the shells and the spatial Fourier decomposition into eigenmodes are displayed simultaneously, and this will help you visualize the meaning of the Fourier components.

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