Accretion - The Evolution of a Solar System

The interactive graphic shown below models the concept of accretion in the evolution of a solar system - like ours. The graphic uses HTML5 - Canvas, which may not work with older web browsers such as IExplorer V8 and earlier.

After a star spends most of its nuclear energy converting hydrogen and helium into heavier elements, those with atomic numbers ranging up to that of iron, it collapses under its own weight, expands, and finally explodes or goes Nova... meaning the creation of something new. In this process, some of the star's material is upgraded to heaver elements like gold and even uranium. The resulting debris field is filled with chunks of randomly moving material that soon begins to collapse upon itself due to mutual gravitational attraction. Often, this is referred to as happening in a stelar nursary, All of this may take several billion years.

As the debris collapses, pieces merge with one another and growth results. This is the accretion process and it is noted by red colored flashes as chunks smash into one another. These combined larger pieces acquire speeds and travel in directions that depend on the relative mass and velocity of the accreted smaller chunks plus the gravitational tug of all of the surronding mater. After some time, most of the debris coalesces into one or two large pieces. The largest ignites, due to the massive internal pressure and temperature, and becomes a new star- like our Sun.

A few small planetary sized chunks or remnants sometimes have sufficient speed and a location history that allows them to escape from being accreted into the new star. Some chunks become planets when their orbit is close-in and nearly circular. Those with highly elliptical orbits become comets.

After running the Accretion Graphic several times with randomized debris fields, you'll see how truly fortunate it is to end up with one or maybe two planets that circle the new sun star. Note that the planets travel in straight lines between each time step rather than along a curved eliptical path. Hence, there is a built-in energy loss mechanism that eventually causes most planets to crash into the larger survivors.

Operating Instructions and a Hot-Key list are shown below the Graphic. Here's an exciting hint! Check the box that opens the Input Panel. Enter 50 for [Vave] and 100 for [Angular] (percent of angular momentum). Press the [Start] button and watch the creation of a binary star system.

  Number   Gravity   Mdist   Vave   Vdist   Angular   Time Step   

Display Panel instructions

[Start] begins the accretion process with a newly randomized position-mass-velocity distribution of the stellar debris.

[Pause] freezes time and then turns into a [Resume] button. The [space bar] key toggles Pause / Resume.

[Step] advances the time by one Delta-Time step.

[Replay] the current Solar System's evolution using the current runs position-mass-velocity distribution.

[Black Hole] converts the sun, that forms after accreting 30% of the total mass, into a black hole. The Black Hole has the old Sun's mass but with zero radius. Normally, planet's radius is proportional to the cube root of the mass.

[Tails] checkbox adds streaming comet tails to each planet. The tail Length is proportional to planet's velocity.

[Center Sun] checkbox keeps the sun or black hole centered in the window.

[Input Panel] checkbox opens a panel where you can enter initial conditions for the evolution of the solar system.

Hot Keys

Hot keys change the window size, making the initial debris field oval or circular in shape. You can pan in all four directions and zoom in and out in real time. A lifetime trace path can be generated for a single planet. Planet information can be displayed by clicking on it with a mouse. Most of the Hot Key operations are best preformed while the display is paused, as the action may be too fast to allow successful clicking.

[Space Bar] - Pause / Resume
[Enter] - Single Time Step
[Ctrl] + click on planet - Begin Planet Trace Path
[Ctrl] + click off planet - End Planet Trace Path
[Shift] + click on planet - Planet Information
[Shift] + click off planet - Search for Planet
[Home] - Center on initial location
[Ctrl] + [Arrow] - Change Window Size
[Ctrl] + [PgUp] - Increase Time Step
[Ctrl] + [PgDn] - Decrease Time Step
[Arrow] - Pan left-right-up-down


Drag in graphic - Pan all Directions
Wheel in graphic - Zoom In / Out

Input Panel instructions

[Fill Screen], when checked, fills the zoomed screen with debris, thereby allowing for larger or smaller initial debris fields. Otherwise, the size of the debris field will match the screen size at zoom level = 1.

[Number] is the initial quantity of random sized stellar debris chunks.

[Gravity] is a percent of the gravitational constant. 200 means gravity is twice as strong as usual. A negative value yields repulsive gravity.

[Mdist] is the percent distribution of mass about the mean. Zero forces all of the debris chunks to start out with the same size. 100 means chunks range in size from zero to twice the average mass.

[Vave] is the average particle speed in the initial debris field. Half move faster, the remainder slower.

[Vdist] is the distribution of speeds as a percent of Vave. Zero means all particles have the same Vave speed. For speeds ranging from 0.3*Vave to 1.3*Vave, enter 30. Note: Speed is the magnitude of velocity and ignores direction. Initially, particle travel direction is random.

[Angular] momentum is the tendency of the debris field to rotate like a galaxy. Zero means no preferred rotation, while (+/-)100 makes everything rotate clockwise/counter-clockwise, still with random Vdist of speeds.

[Time Step] is the small delta-time step between each update of the computer's display (1 means 30 ms.). You can't directly enter a new value into the window but you can still change it in real time. Press [PageUp] for faster and [PageDn] for slower updating.

Dick Kostelnicek -

March, 9, 2012