Sky & Telescope - November 2020 - 56

NOVEMBER 2020 OBSERVING
Deep-Sky Wonders

Σ2813

NOVEMBER 2 02 0 * SK Y & TELESCOPE

u Compare this infrared
image of IC1396A, the Elephant Trunk Nebula, to the
visible-light picture on page
55. Dense clouds of gas
and dust absorb light from
stars and re-emit the energy
as heat, so they're dark in
visible light and bright in
infrared.

respectively. The eastern star is the
double Σ2813, a nearly matched duo of
yellow-white stars 10″ apart. Σ2813 pins
the northern edge of one of the darkest
patches. To the west, it tapers to a little
wisp that passes into obscurity south
of the 8th-magnitude star. To the east,
the patch narrows, widens again as it
engulfs a 9th-magnitude star, and then
ends in a 4′ bulb. A hazy glow near the
bulb's center cocoons a faint star.
The Elephant Trunk is a globule of
dense dust sculpted by ﬁerce stellar
winds from the intensely hot primary
star of Σ2816. Many such structures
are sites of star formation. The faint
star in the bulb is only a million years
old, quite young for a star, and illuminates the little nebula surrounding it.

The IC 1396 complex is about 2,500
light-years distant.
Our ﬁnal target is the little-known
planetary nebula Preite-Martinez
1-333, which resides 23′ west-northwest
of Mu Cephei. Through my 10-inch
scope at 187×, the nebula is faintly
visible with averted vision, but a narrowband ﬁlter makes it fairly easy. It
appears perhaps 40″ across and brighter
in the southeast. My 15-inch scope at
216× shows a couple of extremely faint
superposed stars, and adding an O
III ﬁlter nicely emphasizes brightness
variations in the nebula.
¢ Contributing Editor SUE FRENCH
wrote this column for the November
2010 issue of Sky & Telescope.

Treasures of the Celestial King
Object

Type

Mag(v)

Size/Sep

Dec.

δ Cep

Variable/double star

3.5-4.4, 6.1

41″

22h 29.2m

+58° 25′

Double star

9.9, 11.4

1.7″

22h 28.0m

+57° 42′

25.3m

+57° 49′

Krüger 60
NGC 7281

Open cluster

8.2

12′

22h

NGC 7261

Open cluster

8.4

7′

22h 20.2m

+58° 07′

22h

16.1m

+57° 29′

21h 43.5m

+58° 47′

39.1m

+57° 30′

36.1m

+57° 28′

21h 41.0m

+58° 59′

M 2-51

Planetary nebula

13.5

47″ × 38″

μ Cep

Variable star

3.4-5.1

3.5

2.8° × 2.3°

21h
21h

IC 1396

Nebula/cluster

IC 1396A

Dark globule

18′ × 3′

PM 1-333

Planetary nebula

105″ × 50″

Angular sizes and separations are from recent catalogs. Visually, an object's size is often smaller than the
cataloged value and varies according to the aperture and magnification of the viewing instrument. Right
ascension and declination are for equinox 2000.0.

N ASA / JPL- CA LTECH / WILLIA M RE ACH

the central star, which glimmers at a
wretchedly dim 20th magnitude.
Our next stop is Mu (μ) Cephei. In
1783, William Herschel wrote, "It is of
a very ﬁne deep garnet colour . . . and
a most beautiful object, especially if we
look for some time at a white star before
we turn our telescope to it." Today, it's
often called Herschel's Garnet Star.
Mu is a red supergiant and one of the
largest and brightest stars in our galaxy.
It's approximately 100,000 times more
luminous than our Sun. If Mu took
the Sun's place, its outer edge would lie
between the orbits of Jupiter and Saturn. Like most red supergiants, Mu is
unstable. It pulsates in size, brightness,
and color with a semiregular period
of roughly 860 days and a magnitude
range of 3.4 to 5.1. To my eyes, Mu
seems to vary from deep orange to a
striking red-orange.
Mu burnishes the northern
extremes of the emission nebula and
star cluster IC 1396. Through my 130mm refractor at 23×, the cluster covers
about 1½° and presents more than 100
stars. Near its center, the bright triple
star Σ2816 shows three blue-white stars
in a 30″-long arc. About 13′ east-northeast, the double Σ2819 has a 7.4-magnitude yellow-white primary with an
8.6-magnitude yellow attendant 13″ to
its east-northeast.
The 2½° nebula engulﬁng the cluster
is irregular in outline and brightness,
with a fairly large dimmer region in the
center. A narrowband ﬁlter accentuates details in the nebula and around
its edges. These include several intricate
dark nebulae from Edward Emerson
Barnard's 1927 catalog: B160 through
B163, B365, and B367.
The most famous object within IC
1396 is IC 1396A, the Elephant Trunk
Nebula, a subtle lane of darkness trending east-northeast. Look 25′ west of
Σ2816 for an east-west pair of widely
spaced stars, 9th and 8th magnitude,

Sky & Telescope - November 2020

Table of Contents for the Digital Edition of Sky & Telescope - November 2020