Orbits Around A Compact Mass

Click to place stars. Press and drag to draw velocity vectors.
Press "Start" to watch the stars orbit the black hole.

Orbits Around A Compact Mass written by Paige Junghans, Dr. Melissa Hayes-Gehrke, and Dr. Alberto Bolatto in 2011-2012. We acknowledge support from a CAREER award by the National Science Foundation, grant AST-0955836, and from a Cottrell Scholar award by the Research Corporation for Science Advancement, grant 19968.



Using the Applet

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Frequently Asked Questions

  1. One or more of my stars disappeared. What happened?
  2. What are the masses of the stars and black hole?
  3. Does this simulation use classical or relativistic mechanics?
  4. How do I get a stable orbit?
  5. Why don't all my stars simply fall into the black hole?
  6. How is this applet scientifically interesting or useful?
  7. It looks like my stars are going right through the black hole. What is happening?

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  1. One or more of my stars disappeared. What happened?

  2. There are several possibilities. The star could have reached escape velocity and will never reappear. Alternatively the star's orbit could be too large for the screen and it will reappear eventually. A final possibility is that the star was too close to the black hole and was not able to escape its gravitational pull.


  3. What are the masses of the stars and black hole?

  4. The stars are each one solar mass. The mass range possible for the black hole is one million to one hundred million solar masses. A value of four million solar masses makes the black hole similar to the black hole in the center of our Milky Way. A solar mass is defined as the mass of our sun.


  5. Does this simulation use classical or relativistic mechanics?

  6. This simulation uses only classic Newtonian mechanics.


  7. How do I get a stable orbit?

  8. To obtain a stable orbit the velocity of a given star must be smaller than escape velocity yet large enough to keep the star from being pulled into the black hole. For a better chance at a stable orbit make the velocity tangential to the black hole. To see sample velocities that create stable orbits choose a mass and then press the "Sample" button.


  9. Why don't all my stars simply fall into the black hole?

  10. Outside of the event horizon the black hole acts like any other massive body in space. The event horizon is the distance from the black hole that after crossed, nothing can escape. As long as the star has a velocity that is large enough to create a stable orbit it will never fall into the black hole.


  11. How is this applet scientifically interesting or useful?

  12. In a real life situation, observing stars' orbits can help astronomers determine if a black hole exists, where the black hole is located and the mass of the black hole. Observations of specific stars in our galaxy led to the discovery of the black hole at the center of the Milky Way and the mass of that black hole. This link shows stars orbiting the black hole at the center of our galaxy.


  13. It looks like my stars are going right through the black hole. What is happening?

  14. The event horizon fits within 1 pixel. The sizes of the stars are larger than they should be on this size scale, so when it looks like they are going through the black hole, they are really passing very close to its event horizon.

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