46 ASTRONOMY • DECEMBER 2018
Barnard’s Star, it’s a “fast” mover — the
fastest visible to the naked eye. With a pri-
mary of magnitude 5.2 and a secondary
that glows at magnitude 6.0, it’s hard to
pick out with the naked eye under an
urban sky.
Observing tip: The stars are separated by
a generous 2 8", so it’s easy to resolve them
through a small telescope and distinguish
their orange hue.Open cluster
Hyades ( 153 light-years)
The famous saying about “not seeing
the trees for the forest” can also apply to
groups of stars called open clusters. Just as
a foreground tree can blend into a forest
when viewed from afar, a foreground star
can blend into a open cluster when they are
both viewed along the same line of sight.
Such is the case for the Hyades.
The Hyades, which forms the head of
Taurus the Bull, is the nearest open cluster.
Because it’s only 153 light-years from us, it
has an apparent diameter greater than 5°.
Its true diameter exceeds 20 light-years.
Even after an estimated 625 million years,
the stars are still moving together.
Aldebaran, the brightest star in Taurus, is
only 65 light-years away. However, its
apparent association with the Hyades is
simply a coincidence.
Observing tip: The best views of the
Hyades usually come through binoculars
or a small rich-field telescope.Planetary nebula
Helix Nebula (690 light-years)
In about 5 billion years, the Sun will shed
its outer layers of gas to become a planetary
nebula, leaving behind a white dwarf in
the center. Through the same process, an
ancient star expelled varying densities of
ionized gas — especially oxygen — to give
this next nebula its name.
The Helix Nebula (NGC 7293) is an
older planetary, so the gas has traveled far
enough to appear as a ring (because the
greatest density of gas in the sphere we see
is along the edge) about 2.5 light-years in
diameter. Its name comes from the irregu-
lar brightness that gives it the appearance
of a helix (like a spring) when viewed from
the end. With a telescope, it appears as a
ghostly smoke ring in Aquarius.Observing tip: The Helix Nebula has a
low surface brightness, so a dark sky is
essential. This is a rather large object
(slightly smaller than the Full Moon),
visible through binoculars, where it looks
like a disk. The darker interior is visible
with small telescopes.Emission nebula
Orion Nebula
( 1 , 3 00 light-years)
Spiral galaxies like the Milky Way are full
of light-emitting gas clouds, called emis-
sion nebulae, that eventually condense to
form clusters of stars. These swirls of ion-
ized hydrogen, other elements, and dust
are stellar nurseries, resembling the Sun’sbirthplace some 4.5 billion years ago.
We see the nearest emission nebula, the
Orion Nebula (M42), because it’s illumi-
nated by the Trapezium, a tight group of
luminous, hot stars that have formed
within it. Their intense radiation makes the
gas glow, like a f luorescent lightbulb.Observing tip: M4 2 , located in the sword
of Orion, is visible to the naked eye. If you
view through a large telescope, the nebula
may appear greenish. The vibrant colors in
beautiful photos of this object are too faint
for the human eye to register.Ty p e O bj e c t D i s t a n ce
Red dwarf Barnard’s Star 6 light-years1
White dwarf Sirius B 8. 6 light-years
Double star 6 1 Cygni 11.4 light-years2
Open cluster Hyades 153 light-years
Planetary nebula Helix Nebula 6 90 light-years
Emission nebula Orion Nebula 1,300 light-years
Supernova remnant Veil Nebula 1,470 light-years3
Globular cluster M4 7,000 light-years
Large galaxy Andromeda 2 .5 million light-yearsThe table above lists the nearest object that is visible from mid-northern lati-
tudes for each type of object; however, some types of objects have closer
counterparts deep in the southern sky. For those, see the footnotes below.(^1) Proxima Centauri (4. 2 light-years)
Alpha Centauri (4. 2 light-years)
3Vela supernova remnant (815 light-years)
NORTHERN NEIGHBORS
Helix Nebula (NGC 7293)
Orion Nebula (M42)
JOHN A. DAVIS
ADA
M BLOCK/
MOUNT LE
MM
ON SKYCENTER/UNIVERSITY OF ARIZONA
ADA
M BLOCK/NOAO/AURA/NSF