Physics Photo of the Week

 February 9, 2024

The Owl Nebula - and other planetary nebulae

Photos by Donald Collins - College View Observatory 

If we look with a telescope with some skill we can view the Warren Wilson College's mascot looking down on us.  The Owl Nebula is a class of nebulae that are called "Planetary Nebulae".  Planetary nebulae are the remnants of dying stars.  A star like our Sun, when its hydrogen fuel is used up by fusing into helium, the resulting lack of internal radiation pressure allows the star to shrink by its self-gravity.  However, while shrinking, the core of the star becomes dense enough and hot enough for helium gas to ignite from nuclear fusion to form even heavier elements (carbon, oxygen,etc).  This second ignition blows away the outer portions of the progenitor star into an expanding cloud that we can see with telescopes.

Planetary nebulae have nothing to do with planets.  The name originates because primitive telescopes revealed these objects to be small disks in images similar to the planets of the Solar System.

The Sun - about 5 billion years from now - is believed to evolve into a planetary nebula, meanwhile blowing or vaporizing all its planets.  People have been on the Earth for a few hundred thousand years.

The planetary nebula pictured at right (NGC 1501) is called "The Oyster Nebula" - perhaps on account of the central star's resemblance to a pearl inside an oyster shell.  I think it resembles the inside of an eggplant that has been cut open.  This nebula is displayed in monochrome.  Making color images of astronomical objects is very time-consuming using a monochrome CCD camera and a succession of red, green, and blue filters.

The Ring Nebula (M 57) at left can be found in Lyra - the Harp.  Notice all these planetary nebula have a central star.  The central star is extremely hot (up to 100,000 deg Kelvin).  (The Sun's surface is about 6000 deg Kelvin).  The hot central star emits copious ultra-violet radiation that ionizes the gas that is being driven away from the original star.  The ionized gas - atoms that have been stripped of their outer electrons - emits visible light.  The colors of the ionized gas come from the spectra emitted by the ions as electrons are re-captured.  The electrons fall down through the discrete energy levels governed by quantum mechanics.  The specific color emitted depends on the energy change of an electron as it descends across different energy levels.   Planetary nebulae emit some of the most vivid colors for astronomical objects.

The last planetary nebula in today's post is "The Dumbell Nebula" (M27).  Perhaps the brightest planetary nebula that is visible from Earth.  

From this small sampling of several hundred planetaries visible with a small telescope we see that there is quite a variety of appearance.  Since these nebulae representing the death throes of stars as they have cast off massive stellar winds of particles and represent somewhat violent deaths of stars, they are subject to lots of variations like bad-weather storms on Earth.  The central star has large variations in the emission of particles and is mostly rotating strongly causing the dust envelope to form a disc.  We see the two lobes of a thick disk of dust in the M27 - this photo - or a disk face-on disc with the Ring Nebula.  In the Owl Nebula the dusty disc perhaps has been enclosed in a sphere.  Finally the Oyster Nebula, with a number of cavities may have been formed from several dust emitting regions of the star.  The structures of planetary nebulae are all very complicated and elicit many studies by astrophysicists. 

When all the dust has blown away into interstellar space, the central star continues to evolve into a white dwarf - a star that is immensely dense - a star about the mass of the Sun, but the size of the Earth.  All nuclear fusion has ceased in a white dwarf, so the white dwarfs slowly cool down - but the cool-down times can last many millions of years.

Planetary Nebulae are rather short-lived phenomena.  They last only about 100,000 Earth years - which is short considering the progenitor stars have lasted about 10 billion years before their deaths as planetaries.  The planetary nebulae exist only 1 hundred thousandth the lifetime of the progenitor star. 

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. 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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