Contents

• Introduction
• Space Exploration and Solar System
• Stars and Nebulae
• Milky Way
• Galaxies
• more Astrophysics
• Observations/Instruments
• other Stuff
• Statistics Simulations
• Numerical Maths
• Radio and Electronics
• my Home Page
• Other Material for Teaching Astronomy

Most of the applets are done with the JDK 1.05 only (and thus work also on Netscape3), but some are JDK 1.2.

Space Exploration and Solar System

• The PassFinder applet gives prediction of satellite passes over a ground station. For a period of days, following either the current time or any user-specified time, it computes the Equator Crossings, the start and end times of a pass, the azimuts at start and end, the maximum elevation, the minimum range, as well as the expected maximum signal strength of the satellite's transmitter with specified EIRP. This applet is an adaptation of the Java program PassFinder by P.J.Fernandez
• The Orrery applet permits to view the Sun, the Moon, any planet, or any moon of any planet and a small portion of sky around it, for any time and date, and as seen from various places on Earth. Apart from a rendering of that heavenly body, all data on its position in the sky, rising and setting times, distances from Earth and Sun, and other information can be obtained. The progress of lunar and solar eclipses can be displayed, as well as the transits across the face of the Sun of the inner planets Mercury and Venus. Also, occultations of the planets by the Moon can be inspected. This applet was written by Sébastien Poirier in summer 1999 during an internship.
• Simulation of the Flight of a Launch Vehicle allows the computation of the behaviour of a multistage rocket from the launch site into orbit. The user may change the parameters of the rocket, such as the type and mass of the fuel, the thrust of the engines, the satellite weight, the orientation of the thrust vector during the powered flight etc. One can study the characteristics of the ascent trajectory and the achieved orbit. This is an applet designed and programmed by Guillaume Gouge, Sébastien Majerowicz, Sébastien Poirier, and Vincent Prud'homme in winter 1999 as a second-year project of the ENSPS school of physics.

• Two Body Simulation shows the movement of a body in the attractive field of another body, e.g. a planet around the Sun. This is the simple Kepler problem, but done for a general attractive force. Also, one of several different numerical methods can be used (Old version).
• Two Body Simulation (NEW) shows the movement of a body in the attractive field of another body, e.g. a planet around the Sun. This is the simple Kepler problem, but done for a general attractive force. Also, one of several different numerical methods can be used. This new version allows to make time plots of the distance to the central body, the speed, and the kinetic and potential energy as well as their sum
• Three Body Simulation subjects a small satellite to the combined gravitational pull from both Earth and Moon as they circle each other. Kepler's laws no longer hold true for the satellite, because of the perturbations by the Moon. Changing the mass ratio of Moon/Earth permits also to simulate a planet moving around a binary star.
• Three Stars' Ballet For three stars of equal mass, there are known a couple of stable configurations, where the three masses will orbit each other always in the same way. Euler's and Lagrange's solution have been known for a long time, while the Figure-of-8 configuration had been found only a few years ago. The applet permits to display the orbits, as well as to study the stability of the configuration, by allowing to perturb the initial positions and velocities as well as the masses from their ideal values.
• Orbits under MOND show how bodies move under the MOdified Newtonian Dynamics theory: Here, Newton's Law of Gravity applies only if its acceleration is larger than a certain value a0; for lower values, one takes a slightly higher value. This approach can explain successfully the 'flat' rotation curves found in spiral galaxies without postulation of the enigmatic Dark Matter. The applets allow to explore the effects on stellar orbits under various values of the constant a0, whose usual value is 10^-10 m/s².
• Precession of Orbits under MOND is similar to the two-body version of the above applets, but it also provides numerical results for the precession rate of the orbit. It also includes the possibility of adding a tidal field, such as by gravitational attraction by a third mass outside the system.
• Orbital Perturbations This simulates how the orbit of a planet (red curves) is modified by the gravitational attraction of another planet (blue curves), in comparison with the unperturbed orbit (green).
• Saturn's Ring is a simulation of how the orbits of dust particles around a planet are perturbed by the presence of a moon.
• Stability of Saturn's Ring: In 1865, in his Essay which won him the Adams Prize, J.C.Maxwell showed - among other things - that a ring of moons in circular orbit around Saturn could be stable, if the number of satellites does not exceed a number which depends on the mass ratio of the ring and the planet. At this site, I collect several applets which demonstrate various aspects of Maxwell's study, including a full n-body simulation of the problem.
• Lagrangian Points are equilibrium points for a satellite in the gravitational field of two large masses which orbit each other. This applet gives a graphical representation of the effective potential for the satellite, as seen by an observer who rotates with the orbiting primary masses. It allows to inspect this potential, to locate the Lagrangian points, and investigate their neighbourhood.

• Comet Chaser is an elementary demonstration of how one determines the orbital elements of a comet from a number of observed positions. Here we consider the very simplified case if the comet's positions were given as the heliocentric longitudes in the orbital plane of the comet. This permits to reduce the complexity of the analysis to finding the four parameters which best match the positions observed at a number of times.

• EarthShooter simulates the collision of an asteroid with the Earth, in a simple two-body treatment. It shows the effects of "gravitational focussing", and that for small approach speeds the Earth presents a larger effective cross section than its geometric cross section.
• NEO Simulator allows to study the effect of exploding an asteroid or trying to deflect it in order to avoid collision of the asteroid with Earth. This takes place in the gravitational sphere of influence of the Earth, and addresses what could be done in the very last moments.
• Comet Halley's Orbit in the Solar System is an application of the wonderful Applet OrbitViewer by O.Ajiki and R.Baalke to the comet Halley. It allows to view the positions of the planets at any time and to view their orbits from any angle.
• The Michelin Guide to the Solar System is an extension of the above applet, which permits to find the launch windows for interplanetary missions: If you want to plan your next holidays on Mars, or travel to Saturn for a close-up view of the rings, you need to know when there are possible times to start on such a trip, and how costly it will be in terms of rocket fuel. This is the aim of this applet developped by Isabelle Edmond, Pauline Ginestet, Vivien Raymond, and Matthieu Rieser in spring 2007 as their second-year project of the ENSPS school of physics.
• The Michelin Guide to the Solar System: Slingshots is a further extension of the above Michelin Guide. It also allows to compute the gravity assist maneuvers, by which the space ship passes closely by a planet and thus its trajectory is strongly altered, which permits to arrive at a farther planet with no extra fuel. This applet was developped by Sarah Lawday, Jérémy Lebreton, Axel Richard, and Florence Gris in winter and spring 2008 as their second-year project of the ENSPS school of physics.
• Orbits and Travels in the Solar System uses Keplerian orbits to show how a body travels through the Solar System, such as a planet, asteroid, or comet. We can inspect the body's distance from Earth, and see how it could be detectable by its brightness or velocity. We can also launch a probe from Earth to intercept the body.
• Trip to Jupiter: Schedule departure and arrival is an extended version. It uses Keplerian orbits to compute the time windows available for travelling from the Earth to another body orbiting the Sun, such as a planet, asteroid, or comet. For these mission windows, it displays the various properties of the mission, such as departure and arrival speeds, orbital eccentricity and period. It thus allows to optimize an interplanetary space mission.
• Near Earth Objects are judged to be potential hazards to the Earth. This applet allows to show how such an object on collision course could be detected from Earth before the collision, and to estimate the effects of the impact on the Earth surface. We can also try to find the orbital trajectory for an intercepting mission.
• Hit and Deflect a Near Earth Object is an extension of the above applet: it also allows to find the launch windows for an intercept mission, and to estimate how much it would take to try to deflect the body sufficiently to avoid the collision with Earth.

• Decision map displays on a map of NEO diametre and the speed at its impact on Earth various data: the energy of the impact (to judge how severe the consequences of such an event would be). Then the time before collison at which the body could be detected, given a limiting visual magnitude. And the energy that would be required to cause a given deflection of the trajectory, depending on the time of launch and the speed of an interceptor. All calculations are done in a rather rough approximation, that the NEO approaches on a straight line at constant speed. This neglect of the effects of gravity by the Earth and by the Sun limits such a study to fast objects, but one can still get a rather good impression about the principal issues.

• SpaceMirrors studies whether it is possible to heat the waters of an atoll by the sunlight reflected from mirrors in Earth orbit, for example to generate electricity. The simulation permits to compute the solar flux reflected to the atoll as a function of time, for any number of mirrors placed in orbit at any height; it computes the average irradiation, compared to the direct sunlight, and with a simple model for the heat losses estimates the evolution of the water temperature. This applet is a further developed version of the designed and programmed by Baptiste Rouillier, Amandine Fallet, Stéphane Hau, and Mounir El-Mendili in 2005/06 as their second-year project of the ENSPS school of physics.
• The TV satellite position computer 1 (in English)
• The TV satellite position computer 2 (in French)

Stars and Nebulae

• Optical Stellar Spectra of the different spectral types are shown in a coloured representation, as one would see if we looked at them through a spectroscope. The data are taken from the library of stellar spectra by Jacoby, Hunter, and Christian (Astrophys.J.Suppl.Ser., 56, 257 (1984)) based on observations with the No.1 91 cm telescope at Kitt Peak. The spectral resolution is 4.5 Angstroms.
• The formation of an Absorption Line in a stellar atmosphere is computed for a simple case. One can study how the shape of the line changes if one alters the line opacity and the radial velocity as a function of depth in the atmosphere
• The formation of an Blended Absorption Line considers a line which is situated near another, strong line. One can study how the shape of the line changes if one alters the parameters of the blending line and the opacity and radial velocity of the line of interest
• Interstellar Extinction is a small utility that allows the conversion of extinction, optical depth, attenuation for any optical, ultraviolet and infrared wavelength, using the standard interstellar extinction curve.
• Blackbody plots this curve in various ways and displays the integrated energy and photon fluxes.
• Compute the radius of the Strömgren Sphere around a source of ionizing radiation, given the number of ionizing photons.
• Analysis of an emission nebula as a Strömgren Sphere assumes a simple model to estimate the distanace of the object from its observed Hbeta flux, angular diametre, and electron density. Also, we get the number of ionizing photons necessary to make the nebula, and the luminosity of the central star.
• When the central star of a nebulma evolves sufficiently fast, we must consider Time-dependent Ionization in the nebula. This is done here for a small test volume of hydrogen gas, placed at some distance from the central star. The star's output of ionizing photons can be set to any time history, and the nebula's response is computed.
• Plasma Diagnostics interprets the emission line spectrum of ionized gaseous nebulae: from the observed intensities of the lines, temperature, density, and the abundances of the elements in the nebula are derived. Also, given these values, the expected line intensities are synthesized. This is a scaled-down version of my professional-level code HOPPLA.
• Plasma Diagnostics (version3) also permits to show the results obtained by a number of analysis methods based on the strong lines only.
• Plasma Diagnostics of IR lines is similar to the applet above, but deals with the infrared lines: for given values of extinction constant, and the temperature and density of the electrons, the abundances of the elements in the nebula are derived from the observed line intensities.
• The Spectrum of a single Ionic Species computes the intensities of major emission lines, if you specify the observed intensity of one of the lines. Helps you to check whether you have indeed got all the lines of this ion that you should have been able to observe.
• Molecular Clouds computes the populations of the levels of a molecule - e.g. CO - in an interstellar cloud, subject to collisions with hydrogen molecules and radiation from an external radiation field, taking into account the absorption of the cloud in the various spectral lines of the molecule. From the level populations the brightness temperature in these lines is computed. As special interest are the possibility that under certains conditions the CO molecule has population inversions, leading to (weak) MASER lines.
• Stellar Evolution shows the computed evolutionary paths of any star between 0.8 and 120 solar masses in the Hertzsprung-Russell diagram, as well as the isochrones, i.e. the locus of stars of any given age (NEW version, but old Help pages ....)
• The Light curves of eclipsing binary star by Terry Herter at Cornell University.
• The Orbits of a binary star by Terry Herter at Cornell University.
• The Orbits of a binary star my own version ...
• Internal structure of a star computes how density, temperature, luminosity etc inside a star depend on radius or mass coordinate, for isothermal, polytropic, purely radiative, and radiative/convective models. This demonstrates the simple shooting method which integrates the stellar structure equations, starting from the guessed conditions at the centre, and aiming to meet the proper conditions at the surface. Note that only simple opacity tables are used!

our Milky Way galaxy

• Radial velocities in the Milky Way shows which part of the gas moving in the Galactic Plane on its rotation around the Galactic Centre moves with respect to the solar position.
• Spiral Arms in the Milky Way shows how the gas moving in spiral arms in the Galactic Plane will appear in the diagram of galactic longitude and radial velocity, which is the result of mapping the emission of the 21 cm hydrogen line in the Galactic Plane. The user can change the parameters of the arms and hence see the effects in the map.
• A very nice Applet demonstrating the decomposition of Rotation Curves of disk galaxies into the visible and dark matter components. By Chris Mihos and colleagues at Case Western Reserve University

Galaxies

• Galactic Potential shows the circular and escape speeds for any position in a galaxy composed of bulge, disk and dark matter halo. It also shows the positions and values of the maximum restoring force in vertical direction, as well as allowing to show the parameters as a function of the mass inside that radius. Apart from these plots, a false colour map of the galaxy's gravitational potential can be displayed.
• Solar Photospheric Abundances gives the data from Asplund et al. (2009) in the usual logarithmic values as well as the linear mass fractions. If the user changes one of the values, and hits the return key, all values are recomputed.
• Various Initial Mass Functions of the stars can be displayed. Also the locked-up mass fractions and a crude estimate for the metal yield are computed.
• The Integrated Galactic Initial Mass Function (IGIMF) is the distribution function of masses with which the stars in a galaxy are born. Since stars are formed in clusters, one has to take into account that in small clusters the presence of massive stars is less likely than in more massive clusters. This concept, developed by C.Weidner and P.Kroupa, also shows that the shape of the IGIMF depends on the actual star formation rate. The applet computes and displays the resulting IGIMF. It also computes the number of stars more massive than a certain value. For details, the original articles should be consulted.
• An Applet computing the Chemical Evolution of galaxies with detailed nucleosynthesis developed by Céline Bonilla, Marie-Hélène Annat, Julien Faivre and Benjamin Gonzales as their second-year project of the ENSPS school of physics. This is a fully workable version, which however shall undergo some more revisions and cosmetics.
• In Exponential Infall we compute the evolution of gas, stars, and a primary element in a galaxy represented by a single-zone model into which gas can fall in with a rate decreasing exponentially in time. The user specifies the law for star formation, the initial gas mass, and a factor to take into account gas loss by winds.
• In the Simple Infall Model we compute the chemical evolution of primary and secondary elements in gas and stars in a galaxy represented by a one-zone model with gas infall following a user-specified arbitrary rate (Old version)
• In the One Zone Model we compute the chemical evolution of elements in gas and stars in a galaxy represented by a single-zone model, with user-specified arbitrary rates for the star formation, gas infall and gas outflows, and with user-defined metallicity dependent yield for the chemical elements. Also, the histogram of the stellar abundance can be shown.
• Flows in a Galaxy computes the evolution of gas, stars, and metals in a one-zone model for the chemical evolution of a galaxy, under the influence of a (user-specified) completely arbitrary inflow of metal-poor gas. This Applet accompanies the paper by Köppen & Edmunds (1999) Monthly Not.Royal Astron.Soc. 306, 317
• Oxygen and Nitrogen computes the evolution of gas, stars, and the abundances of these two elements in a one-zone model for the chemical evolution of a galaxy, given the nucleosynthesis prescriptions which the user can enter and modify. The model takes into account the finite life times of the stars of different masses, so that the effects of delayed primary and secondary production of nitrogen can be studied.
• Galaxies computes the evolution of gas, stars, and metallicity of galaxies of various total masses, given some recipes how the star formation rate, the infall time scale, and other perameters depend on the total mass of the galaxy. The one-zone models neglect the finite lifetimes of the stars.
• The observed age-metallicity-relation (AMR) is used by Inversion of AMR to compute the history of star formation and gas infall in a simple galactic evolution model. Depending on the user-given data, a solution may be possible or not at all!
• The model for the evolution of a Galactic Disk shows how the densities of gas and stars and the metallicity of the gas changes with time and radius. Stars are formed from the gas following a simple recipe. Metal-free gas falls into the disk with a time- and radius-dependent prescription, and radial flows carry gas towards the galactic centre. (under development)
• Stellar Population is a Monte Carlo simulation of a population of stars, born according to some specified star formation history. It also shows the computed evolutionary paths of any star between 0.8 and 120 solar masses in the Hertzsprung-Russell diagram, as well as the isochrones, i.e. the locus of stars of any given age. It allows also to show the contributions to the total number, mass, luminosity, brightnesses, and colours by each part of - say - the Hertzsprung-Russell-diagram (NEW version with simulated noise in the magnitudes).
• My version of the Toomres' simulation of the Collision of Disk Galaxies is available here. The two galaxies are modeled by a number of stellar particles circling the centre in the shape of disks, and during the collision only the tidal forces are taken into account. The user specifies the parameters of the collision course as well as the masses and orientations of the disks. This classical simulation proved that extended arms observed in some galaxies are due to the tidal interaction between the two galaxies.
• Here are my 'analytical' considerations to estimate the Ram pressure stripping which can be responsable for the removal of gas from a galaxy as it travels through the hot gas in a cluster of galaxies. (no explanations provided, experts know what things mean ;-)
• More dynamics Applets are found in Dynamical Astronomy Javalab at Case Western Reserve University'

more Astrophysics

• The Onedimensional Hydrodynamics simulation shows how the gas which had initially been confined to the left part in a tube, fills the entire volume, once its partition is opened; the formation of a shock and a rarefaction wave can be followed. Both the isothermal and the adiabatic case can be done. The user can also enter graphically by mouse clicks any arbitrary initial profiles of density, velocity, and temperature.
• Collapse of a spherical selfgravitating gas cloud shows the evolution of density, velocity, and temperature as such as the gas in such cloud moves towards the centre first in free-fall but then modified by pressure forces. Both isothermal and adiabatic cases can be done. Also, the user can enter graphically by mouse clicks any arbitrary initial profiles of density, velocity, and temperature.
• The Smoothed Particle Hydrodynamics in One Dimension simulation shows also the isothermal shock tube, as above, but treated with the SPH method in which a large number of particles are moved in space in such a way that the smoothed average of their density behaves as the density of a real fluid. So far, the user can choose the initial density contrast in the two tube sections ...
• The Ginzburg-Landau Equation comes up in the context of convectively unstable gas flows ... This applet allows to explore the physical and numerical instabilities in a 1D flow, having terms of advection, diffusion, and exponential growth/decay ...

Observation and Instruments

• Gravitational Lenses by Pete Kernan.
• Gravitational Lens shows the effects on the images of a star produced by the bending of the light rays in the gravitational field of a point mass on the line of sight. It also allows to compute the gain in flux received from the star with and without the lens.

• Coordinate Transformations permits to change between Equatorial, Horizontal, and Galactic cooordinates of a celestial source, for a given location, date, and time. It shows the correction from the measured radial velocity to that with reference to the Local Standard of Rest (LSR).

• ImageBlurring shows how a source of a given angular size appears as seen through a telescope of given angular resolution (in one dimension).

• Interfering Waves simulates how the waves emitted by two spatially separated transmitters are interfering with each other to setup a stationary pattern. The user may change the positions of the sources as well as the frequencies of the emissions.

• Refracting and Diffracting Waves demonstrates Huygens' principle by simulating that when a plane wave enters a medium of different refractive index the refracted wave is the superposition of many spherical elementary waves emitted at the border between the media. We can also restrict the aperture, thus showing the diffraction pattern of a single slit. The user may change the orientations of the incident wave and the medium interface, the width of the slit, the number of sources for the elementary waves, and the frequency of the wave.

• Reflecting Waves demonstrates Huygens' principle for the reflection of a light beam by a plane mirror. The outgoing wave is the superposition of many spherical elementary waves emitted at the mirror's surface. For better clarity, we use a light beam of finite width. We can also restrict the mirror's aperture, thus showing the diffraction pattern of a single slit. The user may change the orientations of the incident wave and the mirror, the number of sources for the elementary waves, and the frequency of the wave.

• An interactive tool to Model Images and Spectra of 3-D Objects allows the simulation of observational data which would be obtained from three-dimensional objects, such as gas clouds, gaseous nebulae, star clusters, and galaxies. It helps in the interpretation of data cubes which are a set of images taken in differnt wavelengths, and thus contain the data on the spatial distribution of matter and the velocity field in such an object. This applet was designed and programmed by Sophie Bresson, Bertrand Leriche, Cindy LeLoirec, and Marc-Olivier Sercki in 2002/03 as a second-year project of the ENSPS school of physics. (The present version is undergoing cosmetics, some more debugging and changes - accompanying web-pages and help screens in English will be constructed from the original version in French)

• Airglow Spectrum depicts a typical observed optical spectrum of the night sky; this must always be subtracted from the raw observations of a star, nebula or galaxy before one gets its true spectrum.
• The Virtual Radio Interferometer is a modified version of the original applet written by Nuria McKay, Derek McKay and Mark Wieringa. It shows the operation of an interferometric array of radio telescopes, whose configuration you can change and then observe how an object in the sky would look like. It also is a nice tool to demonstrate two-dimensional Fourier transformations. Great stuff!
• Optics Applets written by Tim McIntyre, shows the operation of a Fabry-Perot and Michelson Interferometers, Diffraction gratings
• Thin Lens Applet written by Fu-Kwun Hwang shows imaging of thin lenses and mirrors
• Radiation Patterns of Aperture Antennas shows the diffraction pattern of reflector antennas whose aperture of rectangular shape is illuminated uniformly or in one of several other ways. This simulation is based on Huygen's principle. The intensity can be computed not only far away from the antenna, but also at closer distances, thus showing both Fraunhofer and Fresnell diffraction patterns.

Other Stuff ....

• Lorenzian Chaos is an oscilloscope-like display of the never-ending variation of a chaotic solution on the system of differential equations by Lorenz which describe some very simplified model of the dynamics of the atmosphere.
• Dynamical Systems shows typical behavioural features of systems of nonlinear equations, which can be displayed in their time evolution as well as by their phase-space portrait: Volterra's system describes how populations of two competing species oscillate about an equilibrium point. The amplitude of the Van der Pol oscillator is limited by nonlinearities and thus has a limit-cycle. The systems of Rössler and Lorenz shows chaos.
• Here is just another pocket calculator.
• Cashflow is an experimental demonstration for the use of barchart diagrams to show the cashflow in a project