Physics Photo of the Week

October 11, 2013

Nova Delphinus 2013

Astronomers got a rare treat this past summer as the first "naked-eye" nova appeared for many years.  This nova was soon designated Nova Delphinus 2013, that means it appeared in the small constellation Delphinus in 2013.  It was first noticed on August 14, 2013 by   Koichi Itagaki of Yamagata, Japan.  I had good weather (away from Western North Carolina) and obtained my first image with the DSLR camera on August 17.  This image is my digital camera image on Aug. 28 from a residential section of Ann Arbor, Michigan.  The nova (indicated by arrow) is almost the brightest image in the photo.  I proceded to obtain a series of DSLR images (from various places, including my back yard near Warren Wilson College). 

A nova is a cataclysmic explosion on a white dwarf star.  The white dwarf, a very dense star the size of the Earth, but the mass of the Sun, is orbited by a small, ordinary Sun-like star in a close orbit.  The white dwarf's mass is very concentrated, about 1 million times more dense than the Sun.  The stars are so close that they orbit each other in a period of about 2 hours.  Compare that with the orbital time of the Earth.  The orbit of the ordinary star is so close to the white dwarf, that the huge gravity gradient near the small white dwarf pulls material out of the ordinary star onto the surface of the white dwarf.  However, because of the rapid orbital motion, the conservation of angular momentum means that the star stuff (hydrogen) falling into the white dwarf forms a rotating disk close to the white dwarf.  In a nova type of cataclysmic star, the disk becomes thicker and thicker, eventually falling onto the surface of the white dwarf and compressed to initiate thermonuclear fusion - as happens in the core of sun-like stars.  The whole surface of the white dwarf blows up in a tremendous explosion becoming about 100,000 times brighter.  This is not to be confused with a type Ia supernova, an extremely rare process in which the whole mass of the white dwarf blows up becoming billions of times brighter than the Sun.  The nova fades back to its previous brightness (usually invisible) in a matter of weeks or months.

The animation below shows a succession of 9 DSLR images that I obtained since mid August.  On the right side of the animation is a graph showing the measured brightness magnitude of the nova.  On August 17, my first image, the nova is clearly the brightest star in the image (just right of the center).  In both the image and the graph the nova becomes dimmer by more than 5 magnitudes.  The logarithmic magnitude change means the star has become more than 100 times fainter in about 60 days.  The magnitude scale for astronomy is also backwards: bigger magnitude numbers mean fainter stars.

Soon we will publish some results from the telescope in the College View Observatory.


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature 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 dcollins@warren-wilson.edu.

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