NOVEMBER 2019 OBSERVING
Going Deep
ESO^ (4 )58 NOVEMBER 2019 • SKY & TELESCOPE
Massive stars burn brightly and use
their fuel at prodigious rates, often
becoming unstable and variable in
brightness toward the end of their lives.
Eta Carinae in our galaxy, for example,
underwent a brightening event in
the mid-1800s during which it briefl y
became the second-brightest star in our
night sky, despite being around 7,500
light-years from Earth. Only nearby
Sirius (8.6 light-years away) outshone it.
Stars can be bright because they’re
close, or hot, or large, or all three.
Greater surface area increases luminos-
ity (for a given temperature), and RW
Cephei is big, really big. Some estimates
put its radius at more than 1,500 times
that of our Sun’s, and if placed at the
Sun’s position it would extend beyond
Jupiter’s orbit. An orange hypergiant, it’s
a variable star and at its brightest bor-
ders on naked-eye visibility — amazing
for its distance, which some estimates
place at 10,000 light-years or more.
Stepping farther out, the globular
cluster NGC 2419 is one of the largest
in the Milky Way’s halo. It orbits at a
distance of 300,000 light-years (5–6
galactic disk radii) from both the center
of our galaxy and the solar system. From
the perspective of M31 it would be the
most visible and prominent Milky Way
globular, similar to how we view G1
(Mayall II) in M31. The cluster is 9th
magnitude, and its brightest stars may
be 17th magnitude, potentially visible
in a 16- to 18-inch telescope. Amateur
astronomer Paul Alsing reported in 2014
that several dozen stars in NGC 2419
were visible using the 82-inch instru-
ment at McDonald Observatory.
Moving outward, we next see the
irregular galaxy NGC 6822, or Bar-
nard’s Galaxy, at a distance of 1.7
million light-years in Sagittarius. Many
of its nebulae and clusters are visible in
amateur equipment, and I have spot-
ted both a supernova remnant and a
planetary nebula. An 18.6-magnitude
star just off the northwest edge of theBubble Nebula (Hubble 1925 I) in the
northwestern reaches of the galaxy,
should be visible in larger refl ectors.
The Andromeda Galaxy, at 2.5 mil-
lion light-years from us, had long been
touted as the most distant object vis-
ible to the unaided eye. Edwin Hubble
forever changed our understanding of
the universe in 1929 with his study of
Cepheid variable stars in M31, proving
it was an external galaxy and offering
the most robust estimates to date of its
distance. Two stars are readily visible
within our sister spiral to amateurs
with medium-size refl ectors.
In the 1990s, former S&T editor
Stephen O’Meara claimed a successor to
M31’s title in Centaurus A. At a probable
distance of around 12 million light-
years, its sighting extended our visible
universe by more than fi ve times. I was
thrilled to duplicate that observation
from the mountains of central Chile in
the spring of 2017.Determined Distances
Wolf-Rayet (WR) stars are an intrin-
sically bright class named after two
researchers who identifi ed them in the
19th century (see Steve Gottlieb’s articlefor more detail on WR stars and their
nebulae, S&T: Aug. 2019, p. 28). Astrono-
mers categorize these stars depending
on the relative abundances of carbon
(WC), nitrogen (WN), or oxygen (WO),
the latter being the rarest type. WR stars
are characterized by strong stellar winds
that can carve a bubble out of the sur-
rounding medium. Two of the most spec-
tacular WR nebulae in the sky are the
Crescent Nebula in Cygnus (NGC 6888)
and Thor’s Helmet (NGC 2359) in Canis
Major. Seen through a large refl ector
in a dark sky using an O III fi lter, these
magnifi cently detailed extrusions take
one’s breath away.
How does all this apply to an ama-
teur seeking the farthest star visible
through the eyepiece? I picked WR stars
because they can be identifi ed in other
galaxies due to their prominent emis-
sion lines (and thus their distances are
better constrained).
In perfect conditions I can see to at
least 20th visual magnitude with myWolf-Rayets in NGC 300
Object Type Size Mag(v) RA Dec.
NGC 300 Sc 21 ′ × 13 ′ 8.1 00 h 54.9m –37° 41′
NGC 300 #20 — — 17.6 00 h 54 m 52.6s –37° 41′ 49 ′′
NGC 300 #56 — — 17.7 00 h 55 m 13.5s –37° 41′ 38 ′′
NGC 300 #4 WN — — 00 h 54 m 42.8s –37° 43′ 02 ′′
NGC 300 #38 WN — 19.5 00 h 55 m 04.1s –37° 43′ 19 ′′
Visually, an object’s size is often smaller than the cataloged value and varies according to the aperture
and magnifi cation of the viewing instrument. Right ascension and declination are for equinox 2000.0.uTHE FARTHEST STAR? The author de-
tected these four WR stars using his 32-inch f/4
refl ector. They’re a challenge, but the satisfac-
tion of snagging them is worth the effort. Each
image is approximately 5′ × 5 ′.#20