Sky & Telescope - June 2021 - 57

Going Deep by Steve Gottlieb

The Footprint, the Egg,
and the Butterﬂy
You may need to stay up late to explore all of these
summertime pre-planetary nebulae.

EGG NEBUL A A ND STA R: BER ND KOCH / STEFA N BINNE WIES / JOSEF PÖPSEL
/ CA PELL A OBSERVATORY; ZOO M: ESA / HUBBLE / N ASA; DSS: POSS-II /
STSCI / CA LTECH / PA LO M A R OBSERVATORY

efore producing the beautiful glowing shells of planetary nebulae,
low- to intermediate-mass stars (0.8
to 8 solar masses) eventually exhaust
their core fuel supply of hydrogen and
helium. The star then ascends the
asymptotic giant branch (AGB) of the
Hertzsprung-Russell diagram, where
helium burning continues sporadically
in a thin shell surrounding an inert carbon-oxygen core. Hydrogen also burns
periodically in an outer shell. However,
this double-shell conﬁguration is thermally unstable. Helium ashes from the
hydrogen shell build up, increasing the
pressure and temperature of the helium
shell. Then the AGB star experiences
episodes of short but strong bursts of
helium burning, known
own as a thermal pulse or helium shell
hell ﬂash.
Thermal pulses dredge
redge up
substantial amountss of

carbon to the surface and trigger shock
waves in the atmosphere. A signiﬁcant
outﬂow of gas and dust (stellar wind)
escapes as an expanding circumstellar
envelope, though the central star is too
cool to ionize its surrounding envelope.
An ephemeral (a few thousand years)
phase bridges the end of the AGB and
an ionized planetary nebula. This poorly
understood metamorphosis - a key
link in late stellar evolution - produces
a pre-planetary nebula (PPN). Of the
hundred or so known PPNe, most are
visible only in the infrared, but the
summer Milky Way is home to three of
the best targets for backyard telescopes.

One Tiny Footprint
In 1946, German-American
Germa
astronomer Rudolph Minkowski began a
survey of planetary nebulae
using objective-prism survey

plates at Mount Wilson Observatory. He
described M 1-92 (the 92nd object on
his ﬁrst discovery list, see image p. 58)
as follows: " Binuclear nebula without
central star. The north preceding mass
has a diameter of about 3″ and is separated from the south following fainter
mass by a dark lane. . . . The nebula is
probably not an emission nebula, but
a reﬂexion nebula which obscures the
peculiar star by which it is illuminated. "
George Herbig, known for his spectroscopic studies of young stars and
the discovery of Herbig-Haro objects,
assigned the nickname Minkowski's
Footprint in 1975. The footprint
comprises two sections, the " sole " and
" heel, " divided by a slight gap. Herbig
found both lobes exhibited a stellar
continuum superposed by a variety of
emission lines.
He proposed that a ﬂattened dust disk
or torus shrouded the central star and
a much larger dust shell of low density
surrounded this disk. The star illuminates the two lobes through transparent holes at the poles, though the disk
partially obscures the southeastern lobe.
Herbig speculated that certain emission
lines formed near the star and scattered
off dust in the lobes, while other lines
resulted from photoionization.
Ground-based telescopes have studied M 1-92 at optical, near-infrared,
WT EGG NEBULA Hidden by the dusty disk
is the star powering this nebula. We don't
observe the star directly, but we see its light
streaming through holes in the disk as " searchlight beams. " Find this target in Cygnus at
RA: 21h 02.3m Dec: +36° 42′.
HD 200530

~30′ x 30′

HD 200389
82

HD 200597
85

HD 200371
80
CRL 2688

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Sky & Telescope - June 2021

Table of Contents for the Digital Edition of Sky & Telescope - June 2021