| Quoted From SEDS Dicovered by Pierre Méchain in 1780.
Planetary Nebula M76 was discovered by
Pierre
Méchain on September 5, 1780, who reported it to
Charles
Messier, who observed it on October 21, 1780, determined its
position and
added
it to his catalog. While Méchain found it to be a nebula without
stars, Messier thought it was composed of small stars with some
nebulosity, probably being fooled by foreground or background stars.
Lord Rosse
erroneously suspected to have detected some spiral structure in this
nebula. In 1866,
William
Huggins, the pioneer of spectroscopy, found its spectrum to be
gaseous, showing Nebulium lines. Pioneer astrophotographer
Isaac
Roberts found that this was not a double, but a single nebula, and
first suspected it might be a broad ring seen edgewise. In 1918,
Heber D.
Curtis correctly classified it as a planetary nebula for the first
time.
M76 is among the fainter Messier objects. It is known under the names
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
William
Herschel, who
numbered the "second component" H I.193 on November 12, 1787. 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 42x87", the `wings'
157x87"), this nebula is surrounded by a faint halo covering a region of
290 arc seconds in diameter (Millikan, 1974);
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] (see our
Planetary Nebulae
page).
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
0.34x0.72 and 3.1x6.4 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 0.68x1.44, 2.6, and 4.8
light years, while with Kenneth Glyn Jones' distance, the cork is
1.7x3.5, the wings 6.2, and the extensions 11.5 light years). |
