Sky and Telescope - July 2017 - 37
A BELL 70: MIK E DURK IN A ND LILY JOE / A DA M BLOCK / NOAO / AUR A / NSF; A BELL 72: JAY BA LL AUER /
A DA M BLOCK / NOAO / AUR A / NSF; A BELL 78: A DA M BLOCK / NOAO / AUR A / NSF
p DIAMOND RING The bright sparkle at the north-northwest edge of
Abell 70's nebulous ring comes from PGC 187663, a background radio
galaxy some 250 million light-years from Earth. Abell 70, an estimated
13,500-17,500 light-years distant, lies comparatively close to us.
p FAINT COMPANION Finding Abell 72 southeast of Delta Delphini
is probably challenge enough, but if you want to level up your galaxyhunting skills, try for the dim galaxy PGC 65491 just 2 arcminutes southsoutheast of the nebula's shell.
Aquila, 4.6° northwest of 3.8-magnitude Epsilon (ε) Aquarii.
At 175× Abell 70 appeared as a crisp-edged disc roughly 35″
across. When I upped the power to 283×, a slightly brighter
strip around the periphery formed a weak ring. The nucleus
of the 15.5-magnitude galaxy showed as a small bump along
the north-northeast edge, but with averted vision it extended
to a 20″ × 6″ sliver tilting west-northwest.
I've only viewed the planetary's dim nucleus through Jimi
Lowrey's 48-inch reﬂector. A study conducted in 2012 using
the Gemini South telescope identiﬁed the central star as an
ultra-rare barium-rich binary, consisting of a 17.8-magnitude G8-class star and a UV-emitting 20.4-magnitude white
dwarf. While still on the AGB, the (now) white dwarf polluted its main sequence neighbor with a dense stellar wind
including heavy elements like barium.
Abell 72 is a somewhat ragged bubble 2° southeast of
4.4-magnitude Delta (δ) Delphini and just 2′ east of a distracting 8th-magnitude star. My O III ﬁlter partially masked
the star and showcased a well-deﬁned 1.7′ disc with a slightly
darker interior. A 12th-magnitude star is attached to the
southwest side, and another is just beyond the east end. At
high power the 15.5-magnitude central star ﬂickered in and
out of view with the seeing.
PGC 65491, a wraithlike 16th-magnitude galaxy, is a scant
2′ south-southeast of Abell 72. I was unable to catch it in my
18-inch but saw it as a feeble smudge, just brighter than the
background sky, through my 24-inch.
Abell 75 is a snap to locate less than 1° east-northeast of
2.5-magnitude Alpha Cephei (Alderamin), the brightest star
in its constellation, and 15′ east-southeast of a 7.4-magnitude
star. Abell reported the photographic magnitude at a dismal
17.0, but the visual magnitude is closer to 14.5.
First discovered by William Herschel on October 15, 1794,
Abell 75 is also cataloged as NGC 7076. Abell missed the
news about that earlier discovery, however. In fact, even the
1992 Strasbourg-ESO Catalogue of Galactic Planetary Nebulae
and the 2000 version of Luboš Kohoutek's Catalogue of Galac-
p RENAISSANCE Abell 78 is considered a "Born-Again" planetary. In rare
cases, after the star at the center of a planetary nebula burns through its
helium and hydrogen and collapses, helium fusion reignites in the star's
outer layers. This fresh nuclear activity produces a second, stronger wind
that blows stellar material through the existing shell of the nebula.
tic Planetary Nebulae still identify the planetary as Abell 75
rather than NGC 7076.
The planetary appeared as a slightly elongated, moderately
faint disc 40″ to 45″ across. The shape was irregular at 225×
with a partial ring formed by a brighter eastern side. The ﬁeld
is rich in faint stars, with two 14th- to 15th-magnitude stars
lodged on the north and east edge and a couple of 14th-magnitude stars 1′ and 2′ north.
Abell 78 lies in the southeast corner of Cygnus, 3.5°
northwest of the close binary Mu (μ) Cygni and conveniently
centered between two 7.4- and 8.2-magnitude stars 8′ northwest and 8′ southeast. A 1.5′ diameter shell was easily seen
at 115× surrounding a 13.3-magnitude central star. The disc
appeared slightly uneven but was otherwise featureless.
Abell 78 is a member of the rare class of "Born-Again" planetaries. A rebirth of sorts occurred for the nebula after hydrogen and helium burning had ceased in its core and its visual
outer envelope was ejected. When the nebula's progenitor star
descended onto the white dwarf track, a shell of helium reached
s k y a n d t e l e s c o p e .c o m
* J U LY 2 017