Physics Photo of the Week

November 9, 2007

IP Pegasi Eclipsing Star
George Keel, a senior at Warren Wilson College, photographed this variable star in the fall of 2006.  This star has remarkable properties: it is a binary star that looks like a single star, and every three hours the brighter component star is eclipsed by the fainter star, so its brightness dramatically decreases as the animated photo shows.  This star is too faint to be seen without a telescope.  However, George captured this star with a small portable telescope and an inexpensive CCD camera.

IP Pegasi is a class of very interesting and complicated variable stars called "cataclysmic variables".  A cataclysmic variable star consists of two stars in a very close orbit to each other.  One of the stars is a very dense white dwarf star at the end of its evolutionary life cycle.  The very close proximity
of the two stars has two consequences: the orbital period is very short (3 hours in this case), and the extreme density of the white dwarf (a million times the density of the Earth) pulls material from the secondary star.  As a result there is a constant stream of material flowing from the larger, ordinary secondary star onto the white dwarf.  Because the two stars are rotating very rapidly, the angular momentum of the streaming material is preserved and the transiting material forms a narrow disk that rotates around the white dwarf.  See the model drawing at the right (drawn by George Keel).  The model shows the accretion disk surrounding the white dwarf.  Another important consequence of the accretion disk is the intersection of the stream of material from the donor star.  Because the disk is spinning very fast, the streaming material makes many inelastic collisions with the disk material.  A hot spot develops near the edge of the accretion disk where this intersection takes place.  Imagine a merge lane of bumper-to-bumper cars intersecting a freeway containing bumper-to-bumper cars at high speed.  Collisions will occur or tempers will flare!

Examination of the light curve (the brightness as a function of time) shows the major eclipses as the white dwarf rotates behind the dimmer donor star.  Closer examination of the light curve demonstrates that the hot spot is on the edge of the accretion disk and dominates the brightness of the system.  The assymetry of the light curve between eclipses is attributed to the fact that the hot spot is off-center.  If there were no perimeter hot spot, the light curve would peak between eclipses.  George's data show the peak in the light curve to occur very soon before entering eclipse.  From Earth's perspective, both the hotspot and the white dwarf enter eclipse at the same time, whereas they emerge at different times.  George has produced a poster that explains the phenomenon in greater detail.  People are welcome to view the poster in Spidel Hall - the location of the Physics Department.

Students Tina Milne, Jessica Harris, Chelsea Maier, Carla Cao, Eri Watanabe, Eleanor Vena, Andrew Jones, and Matt McDaniel participated in the analysis of the observational data.  The project was supported by a student-initiated Yarbrough Grant from the North Carolina Academy of Sciences and a Small Projects Grant from the American Astronomical Society.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

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