Astronomy

14 ASTRONOMY • MAY 2018

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 butterf lies 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.

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 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

SECRETSKY

Snow White

and the seven

dwarfs

Replace Happy and Doc with

the seven dwarfs of the sky.

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

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.

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].

BY STEPHEN JAMES O’MEARA

BROWSE THE “SECRET SKY” ARCHIVE AT http://www.Astronomy.com/OMeara.

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 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

Stephen James O’Meara

is a globe-trotting observer

who is always looking for the

next great celestial event.