M 76

Planetary Nebula M76 (NGC 650), type 3+6,

Little Dumbbell Nebula

Little Dumbbell Nebula (the most common), Cork Nebula,

Butterfly Nebula, and Barbell Nebula,

and it was given two NGC numbers as it was suspected to be a double nebula

with two components in contact, a hypothesis brought up by

who numbered the “second component” H I.193. NGC 651 is the North following

(East) part of the nebula.

The appearance of M76 resembles to some degree that of the Dumbbell Nebula

M27. Most probably, the main body (the bar, or cork)

is a bright and slightly elliptical ring we see edge-on, from only a few

degrees off its equatorial plane. This ring seems to expand at about 42 km/sec.

Along the axis perpendicular to this plane, the gas expands significantly more

rapidly to form the lower surface brightness “wings” of the butterfly.

While the bright part of the nebula is of about 65 arc seconds in diameter

(more accurately, the `cork’ is about 42×87″, the `wings’ 157×87″),

this nebula is surrounded by a faint halo covering a region of 290 arc seconds

in diameter; this material was probably ejected in the form of stellar winds

from the central star when it was still in the Red Giant phase of evolution.

Today the central star is of mag 16.6 and a high temperature of some 60,000 K,

which will probably cool down as a white dwarf over the coming tens of billions

of years.

As usual for planetary nebulae, M76’s visual magnitude is much brighter (9.6

according to Don Machholz’ personal estimate, 10.1 according to Hynes; the

present author thinks this is close to his own perception) than

photographically (most sources agree on 12.2 mag photographically). This

is due to the fact that most visual light is emitted in one spectral line,

the green 5007 Angstrom forbidden line of doubly ionized oxygene, [O III]

As is not unusual for planetary nebulae, the distance is poorly known, with

estimates between 1,700 and 15,000 light years (the latter value is from

Kaufmann’s Universe; Kenneth Glyn Jones has the value of 8,200).

Accordingly, the true dimensions of the cork is between 0.34×0.72 and 3.1×6.4

light years, while the wings extend up to between 1.3 and 11.3 light years,

and the faint halo reaches out to between 2.4 and 21 light years.

(Our 3400 light years yield 0.68×1.44, 2.6, and 4.8 light years, while with

Kenneth Glyn Jones’ distance, the cork is 1.7×3.5, the wings 6.2, and

the extensions 11.5 light years).