Physics Photo of the Week

September 28, 2013

M27 - The Dumbell Nebula
The Dumbbell Nebula is one of the brightest of its type.  This is also the first image of a planetary nebula photographed by the recently completed College View Observatory near Warren Wilson College.  The designation "planetary nebula" for this kind of object was given by Sir William Herschel mainly because its image resembled a disk-like planet in the primitive telescopes of the 18th century.  

Such a nebula will probably be the ultimate fate of the Sun in about 6 billion years from now.  When a star like the Sun uses up all its nuclear fuel (Hydrogen, Helium, Carbon, ... all converging to Iron), compression by the star's strong gravitational force no longer leads to nuclear fusion into heavier elements, and the nuclear process shuts down.  The star collapses to form an extremely dense star at the center called a white dwarf (the star at the center of this nebula).  Much of the outer envelope of the former sun-like star has rebounded and is expanding over time.  The white dwarf is about the size of the Earth, (1/100 the solar diameter) and it contains about the mass of the Sun.  This collapse results in a  volume about 1/millionth the original star, hence the white dwarf is about a million times more dense than the Earth (and Sun).  At these densities, the matter in a white dwarf is unlike ordinary matter.  The electron orbits in the atoms of the white dwarf have all collapsed from the extreme pressure.  The electrons in white dwarf matter are said to be "degenerate".  One electron cannot be localized from another.  All the electron orbitals that we learn in elementary Chemistry are collapsed.

The nebular material (the glowing halo) consists of gas from the parent star that is expanding out into space.  The estimated distance from the Earth to this nebula is about 1200 light years.  The angular size in the photographs predict the physical size of the expanding gas to be about 3 light years in diameter.  That is about 90,000 times the diameter of Earth's orbit around the Sun!  Thus when the Sun "dies" into a planetary nebula, all the planets will be engulfed by the nebula.  Life on Earth would be terminated - not so much by the expanding Sun's atmosphere - but by the sheer ultimate brightness of the white dwarf compared to the Sun.  The Sun's surface temperature is about 6000 deg K.  In contrast a white dwarf is about 30,000 deg K - about 5 times hotter.  The radiation emitted, proportional to the 4 power of the absolute temperature, will be about 54 = 625 times greater - incinerating all life on the Earth as we know it.

Keep tuned.  Shortly we plan to obtain an image of another planetary nebula - the ring nebula - at the College View Observatory.  This image was made with a DSLR camera mounted on a 14 inch aperture telescope given to Warren Wilson by alumnus Gary Starkweather (WWC class of 1978). 


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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