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M 97   Planetary Nebula

Mnemonic name:  Owl Nebula  

Other names - NGC 3587

Location:         Constellation: Ursa Major

Distance:          uncertain  1,600 to 12,000 ly

Magnitude:      9.9 (visual)  12.0 (photographic)

Angular Size:  3.5 arcminutes         age: 6,000 years

Equatorial Coordinates:  11h 14m 47.7s  +55⁰ 01’ 08”

Galactic coordinates: 57.00, 148.50

Planetary nebula will typically have several events in which a shell of gases is thrown off.   However, they are widely separated by many thousands of years and typically we will see only one of the shells.  By the time a new pulse from the star produces a new nebula, the previous one has usually dispersed beyond the distance where it's gases can be ionized by the host star's ultraviolet radiation.  M 97 is unusual in that we can actually see the shells of 2 successive pulses.  The inner shell is the one that gives the nebula its distinctive shading.  It is enveloped by another fog-like shell with lower ionization than the inner shell. 

The central star of M 97 is a 16^th magnitude white dwarf. There are several background objects in this image some of which are certainly galaxies, and some probably galaxies.  Above and to the left is PGC 34279, which is partly eclipsed by a foreground star.  Below and to the right is PGC 249029,1 an edge on spiral galaxy and nearby are several fainter probable galaxies.

A cumulus cloud on a summer afternoon will have 10,000,000,000 (10 billion) particles in a cube 1/8 inch on a side.  The density of planetary nebulas varies, but a density of 25 particles in that same 1/8” cube is one that is typically found.  This low density has some interesting consequences for subatomic quantum mechanical processes.  There are a fixed number of defined energy states that electrons can occupy.  When dropping from a higher to a lower state, energy is given off, often as a photon.  In plasmas produced in earth environments, there have always been what are called ‘forbidden states’, certain transitions from a higher to a lower state that do not give off a photon because collisions with other nuclei absorb the energy that would otherwise go into producing the photon.  In nebulas, the gas atoms are so sparse that these nuclear collisions do not take place and emissions are made at the forbidden wavelengths.  This will typically be a photon given off by doubly ionized oxygen at a wavelength of around 5007 Angstroms.  90 to 95% of the visible light produced by a planetary nebula is at this frequency.  Because this is the wavelength of green light and is near the most sensitive frequency for the human eye, these nebulas are often rated a couple of magnitudes brighter visually than photographically.

To the left the image of M97 has been reprocessed to bring out contrast between the inner and outer shells.  The current interpretation of the inner shell is that is a cylindrical torus (donut) like M 57, but viewed from an angle so that the dark spots are the poles where we are looking through less gas and hence the emissions are less bright.