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Astronomical Observatory: Cool Images

Images: U Gem -- A Dwarf Nova Outburst

Contents: The image at right was made with the Calvin telescope on the night of 5 April 2002. By clicking on the "Outburst" button beneath it, you can see how the field changed looked on 31 May. Clicking on "Blink" will toggle back and forth between the two images.

Notice that all stars remain essentially unchanged but one. The exception is the star U Geminorum (U Gem for short), which has increased its brightness by a factor of 50 (4.2 magnitudes)!

What is the historical significance of a nova? The word nova comes from the Latin phrase for "new star", as indeed a nova would seem to be if before outburst it was too faint to be noticed. The enormous change in brightness over such a short time is dramatic evidence of the ongoing dynamic nature of the universe around us. It is a direct refutation of Aristotelian science, which supposed everything in the heavens to be eternal and unchanging. Tycho Brahe's observations of a nova in 1572 (a supernova, not a dwarf nova, but he didn't know the difference) was one of the first observations widely recognized to undermine Aristotle's view.

What is a dwarf nova? A dwarf nova is actually a binary star system consisting of a small, normal star (a "red dwarf") orbiting rapidly around a collapsed, electron degenerate star (a "white dwarf"). Over time the orbit decays and material in the atmosphere of the red dwarf falls toward the white dwarf, forming an accretion disk. In the case of U Gem, it takes just 4.25 hours for a complete orbit (see our scale model movie of the U Gem system).

Why does it go into an outburst? In the absence of viscosity, material in the accretion disk simply orbits the white dwarf as the Earth orbits the Sun. However, when the disk heats up, the viscosity increases and material spirals in. The energy released by spiralling in is enormous. It makes the disk shine brightly (the outburst we see) and keeps it hot (keeping the spiral in process going). Eventually the disk is depleted of material and it goes into a low brightness state while it is gradually replenished by material from the red dwarf star. In the case of U Gem, it typically takes just one day for an outburst to reach full brightness, twelve days for it to burn itself out, and four months for the disk to be replenished.

Processing: The images were obtained by Prof. Molnar and by Calvin student Phil Ammar. The quiescent image is the average of eight 105-second exposures obtained on 5 April 2002 between 10:43 and 11:50 pm EDT. The outburst image is the average of forty-two 15-second exposures obtained on 31 May 2002 between 10:27 and 11:10 pm EDT. All images were made with the clear filter, and were dark subtracted and flat fielded.

Orientation and scale: North is to the left and East is down. The image is 8.2x4.8 arcminutes in size. U Gem's celestial coordinates are 07h55m05s,+22d00'06" (epoch 2000). This is in the constellation Gemini, 6 degrees southeast of the bright star Pollux. U Gem has a magnitude of approximately 13.8 in the quiescent image and 9.8 in the outburst image.

Content updated 8/15/2



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