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Astronomical Observatory: Cool Images

Images: U Gem -- Scale Model Movie

U Gem scale model

Contents: This movie shows a precise scale model of the geometry of the dwarf nova U Gem as viewed from Earth. (Due to the enormous distance to U Gem actual images of the star system always appear as an unresolved point of light.)

In general, a dwarf nova is a binary star system consisting of a small, normal star (a "red dwarf") orbiting rapidly around a collapsed, electron-degenerate star (a "white dwarf"). In the scale model, the red object is the red dwarf star. Its shape is distorted (flattened with a knob on one end) both by its rapid rotation and by the tidal distortion of the more massive white dwarf.

The blue object is the accretion disk orbiting the white dwarf (which itself is too small to be seen). The relatively smaller side-to-side motion of the disk shows the white dwarf star is more massive than the red dwarf. The disk is modeled as being two-dimensional and appears long and skinny as we view it nearly edge on. Its shape deviates from circular (being shorter along the axis connecting the two stars than on the perpendicular axis) because of the tidal influence of the red dwarf star on it.

Finally, the orange line connecting them is the accretion stream, material flowing from the inner Lagrange point (the tip of the knob on the red dwarf star) onto the accretion disk.

Processing: The shape of the red dwarf was computed using the assumption of Roche geometry: the red dwarf fills its Roche lobe, the shape of which is determined by two point masses in a corotating frame. The shape of the disk was computed using the largest closed orbit that does not intersect the next annulus in (the tidal radius). The trajectory of the accretion stream is a ballistic trajectory beginning with a small perturbation from corotating motion at the inner Lagrange point.

Orientation and scale: The center of mass is the point (0,0). The vertical direction is the projection of the orbital axis. Linear distance is in units of solar radii. One full orbit of the animation lasts four seconds, a factor of 3800 faster than the actual system.

Content updated 8/7/2





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