101 SKY OBJECTS
WWW.ASTRONOMY.COM 5786 The Hyades
Crisp winter nights bring us a glorious naked-eye
treasure: the Hyades. This V-shaped gathering of
suns has been recognized as a celestial bull since
at least 4000 B.C., when the Sun resided among its
stars during the spring equinox — a hopeful union
that heralded the return of life and agricultural
activity to Earth after a barren winter. Classically,
the Hyades represented the mythical seven
daughters of Atlas, half-sisters of the Pleiades;
together they formed the 14 Atlantides.
To modern astronomers, however, the Hyades
mark the bright core of the Taurus Moving Cluster,
which, at a distance of 150 light-years, is the clos-
est star cluster to our Sun.
Formed about 625 million years ago, the
Hyades appears to share a common origin with
the Beehive Cluster (see #94) — their ages and
motions through space are remarkably similar.
The Taurus Moving Cluster is now rifling through
space at 29 miles per second (46 kilometers per
second), toward a point a few degrees east of
Betelgeuse. It passed closest to our solar system
more than a million years ago, and in 50 million
years it will appear only about ½°-wide through
our telescopes.
Fortunately, today we can revel in the cluster’s
majesty, as the central Hyades stars span about
5.5°, or about 15 light-years. Adding to its splen-
dor is the warm light of the orange giant star
Aldebaran, at the tip of the V’s southeast branch;
however, it is not a part of the Hyades proper.
ESA’s Hipparcos satellite has confirmed that
Aldebaran is a foreground object 65 light-years
distant, while the center of mass of the Taurus
cluster lies 151 light-years from Earth.
The smallest of binoculars will transform
the celestial V into a stunning starscape about
10 light-years across. The outlying regions cover
at least twice that distance. But do run your
binoculars over the face of the Bull, as there are
ample stellar pairings that make the sight feel
“homey.” Most notable are the two Theta stars (θ^1
and θ^2 Tauri), which shine at 4th and 3rd magni-
tude, respectively, and are separated by about 6'.
The Sigma stars (σ^1 and σ^2 Tauri) are even more
appealing, appearing as two chalk-white beacons
of near-equal intensity (magnitude 5) separated
by 7'. — S.J.O.The Trifid Nebula (M20) is the trifecta
of deep-sky objects: It combines a blue
reflection nebula with an emission nebula
glowing in red Hydrogen-alpha (Hα) light,
all divvied up by opaque lanes of dark
nebulosity threaded throughout.
For all its spectacle in photos, however,
the Trifid Nebula is visually faint. That
explains why, when Charles Messier
discovered it June 5, 1764, he portrayed
this 20th entry in his catalog as only a
“cluster of stars.” William Herschel was
the first to describe the object as “three
nebulae, faintly joined, form a triangle.
In the middle is a double star.” It was this
appearance that led Herschel’s son John
to later christen the object Trifid, from the
Latin trifidus, meaning split into three.
A quick glance at M20 through a scope
will usually show only a 7th-magnitude
star surrounded by a faint glow. With
careful observation, that glow will appear
cleaved by dark nebulae. The dark lanes
all seem to diverge away from the central
star, HD 164492.
The energy irradiating the Trifid, whichJOHN CHUMACKMAICON LOPES NUNESis roughly 5,000 light-years from Earth
and spans some 15 light-years, comes
from a young star cluster buried within
it. HD 164492 is the brightest member of
this cluster. High magnification reveals
HD 164492 is a multiple star system, too.
How many stars can you resolve? There
is a magnitude 8.7 star 11" to the primary
star’s south and a 10th-magnitude com-
ponent 6" to its north. The former is also
joined by two close-set stars, and one
more hidden in the system makes this a
sextuple star system.
Since the Trifid never gets very high
in the sky from mid-northern latitudes,
for the best view, you will need a clear
southern horizon free from interfer-
ing light pollution. Averted vision will
certainly help, as will using a narrow-
band filter. With a double-digit aperture,
the full shape of the Trifid blossoms into
view. More than two dozen stars appear
sprinkled across the partitioned emission
nebula. Faint tufts of the reflection nebula
extend farther to the north, toward and
beyond 7th-magnitude HD 164514. — P. H.85 The Trifid Nebula