SECRETSKY
BY STEPHEN JAMES O’MEARA
Snow White
and the seven
dwarfs
Replace Happy and Doc with
the seven dwarfs of the sky.
T
his month, I’m chal-
lenging you to go
after some diminu-
tive objects that
spice up the night
sky with their hidden beauty.
I have selected seven dwarf
objects you can search for.
First up is the white dwarf
central star of planetary nebula
IC 2149, in the northern part of
the constellation Auriga the
Charioteer. Lying just 40' west-
northwest of Pi (π) Aurigae,
IC 2149 is an “inside out” plan-
etary nebula. Visually, planetar-
ies usually appear as gaseous
rings or butterflies with nearly
impossible-to-spot central stars.
But IC 2149 is a planetary neb-
ula with a magnitude 11.3 cen-
tral star surrounded by a tiny
and almost inconspicuous neb-
ula 8.5" in extent.
You’ll find our next target,
newly designated dwarf planet
1 Ceres, in the northeastern
part of Cancer the Crab. Ceres
reached perihelion April 22,
when it shined at magnitude
7.8 and sailed about 1° north-
east of Iota (ι) Cancri. On
May 1, the minor world is
within 30' of the magnitude 6.7
star 70 Cancri. It then heads
southeast toward magnitude
4.5 Kappa (κ) Leonis on the
15th, and about 1° northwest of
magnitude 4.3 Lambda (λ)
Leonis on the 31st.
The dwarf spheroidal galaxy
Leo I appears as a magnitude
10 glow only 20' north of
Regulus (Alpha [α] Leonis), the
Lion’s brightest star. Glare
from the luminary hampers
The trick to observing Leo I (top)
is to move the brilliant blue star
Regulus just outside the field of view
of your eyepiece. BERNHARD HUBL
the view. Doubling the diffi-
culty, the galaxy’s light covers
an area measuring 12' by 9', so
a dark sky and moderate mag-
nification are required. At a
distance of 800,000 light-years,
Leo I may be the farthest satel-
lite system orbiting our galaxy.
We now proceed to an enig-
matic object: the dwarf nova
TV Corvi (Tombaugh’s star).
Lowell Observatory astrono-
mer and discoverer of Pluto
Clyde Tombaugh found this
star on plates he took in 1931,
during his trans-Neptunian
planet search. He considered it
a nova. Not until Astronomy
contributing editor David Levy
researched Tombaugh’s discov-
ery in 1989 did we learn that it
has repeated outbursts.
TV Corvi, in fact, appears to
be composed of two dwarf
objects: a white dwarf and a
substellar brown dwarf donor.
The variable is characterized
by the appearance of long and
bright “super-outbursts” that
can last weeks. At minimum,
TV Corvi lies dormant at 19th
magnitude before an outburst
brightens it 250 times to 13th
magnitude (or brighter) in
roughly two days, making it
visible in backyard telescopes.
You can plot a finder chart at
tinyurl.com/y8o8ee2y.
Our next target, magnitude
7.5 Lalande 21185 in
Ursa Major (R.A. 11h03m,
Dec. 35°58'), is the brightest
red dwarf star north of the
celestial equator and the third
brightest in the entire sky. A
mere 8.3 light-years distant, it
sails through the background
stars at nearly 5" a year.
Red dwarfs are the smallest
stars in our galaxy. Lalande
21185 possesses only about half
the Sun’s mass and spans only
about 40 percent of its diam-
eter. It appears red because the
star has a cool surface tempera-
ture of 6,400° Fahrenheit
(3,800 kelvins). In 2017, astron-
omers discovered an exoplanet
orbiting Lalande 21185. They
christened it Lalande 21185b. It
is several times larger than
Earth, but several times smaller
than Neptune.
Sailing northward along the
Milky Way into the constella-
tion Cepheus the King, we
encounter a cosmic rosebud,
NGC 7129 — a dwarf nebula
(with a diameter less than 10'
when photographed through a
blue filter) 2.6° northwest of
magnitude 4.5 Xi (ξ) Cephei,
the heart of the celestial king.
This evolved HII (star-form-
ing) region belongs to a com-
plex molecular cloud some
3,300 light-years distant that
skirts the upper regions of the
Cepheus Bubble — a giant dust
ring with an apparent diameter
of about 10°. NGC 7129 is itself
a little bubble of hot gas created
by the three B stars within its
cavity. Visually the 7'-by-7'
nebulosity centers on a trap-
ezoid of stars, which causes the
nebula to appear patchy.
As star expert Jim Kaler
tells us in his online column
BROWSE THE “SECRET SKY” ARCHIVE AT www.Astronomy.com/OMeara.
14
A ST R O N O M Y • MAY 2018
The Small Cluster Nebula (NGC 7129)
combines a star cluster with three
nebulous regions. A 6-inch telescope
will reveal the cluster. To spot the
nebulae, move up to at least a 10-inch
scope. KEN CRAWFORD
Stars of the Week, “Rare is the
naked-eye star that has a lumi-
nosity and mass less than that
of the Sun.” But with a magni-
tude of 4.5, the yellow/orange
dwarf star Xi Boötis can be
spied under a dark sky without
effort. You’ll find it nearly a
fist-width due east of the
orange giant star Arcturus
(Alpha Boötis). I call it “yellow/
orange” because Xi is an easily
accessible double star (at a dis-
tance of about 22 light-years,
one of the closest to our Sun)
consisting of a magnitude 4.6
yellow dwarf and a magnitude
6.8 orange dwarf companion
some 7" away. So you get two
dwarfs for the price of one!
I’ll finish with Snow White.
Well, actually, you’ve probably
already seen her — the Milky
Way. Astronomers investigating
the color of our galaxy used
Sloan Digital Sky Survey data
collected on hundreds of galax-
ies resembling ours and found
its color resembles that of
freshly fallen snow seen about
an hour after dawn. So, it seems
the Milky Way is aptly named.
As always, let me know how
you fare by sending emails to
[email protected].
Stephen James O’Meara
is a globe-trotting observer
who is always looking for the
next great celestial event.