Sky and Telescope - June 2018 - 45
Under the Stars by Fred Schaaf
This asterism is all about
pairs of stars, and pairs
A K IR A FUJII; G A ZELLE: BLUELEL A / G E T T Y IM AG ES
ne of the less famous asterisms
of spring is nevertheless one of
my favorites. Therefore, I'm going to
devote this entire column to this single
asterism whose name is derived from
medieval Arabic lore: the "Three Leaps
of the Gazelle."
Taking the three leaps of the
gazelle. Between the Big Dipper and
Leo shines a line of stars composed of
three naked-eye pairs of similarly bright
stars that belong to Ursa Major. Originally imagined as six gazelle footprints,
the pairs are now often depicted as the
three visible paws of the Great Bear (the
fourth paw is hidden behind one of the
other ones in drawings of Ursa Major).
The overhead leaps. The three stellar pairs are located at declinations of
about +47°, +42°, and +32°, and thus a
pair passes exactly or nearly overhead as
seen from anywhere in the 48 contiguous states of the U.S. The doubles are at
right ascensions of about 9h, 10h 20m,
and 11h 18m. From northwest to southeast, from front of the Great Bear to
the back, the three pairs are Iota (ι) and
Kappa (κ) Ursae Majoris (magnitudes
3.1 and 3.6); Lambda (λ) and Mu (μ)
Ursae Majoris (magnitudes 3.5 and 3.1);
and Nu (ν) and Xi (ξ) Ursae Majoris
(magnitudes 3.5 and 3.8).
But the Arabs saw the leaps of the
gazelle going the other direction,
southeast to northwest: thus Nu and
Xi Ursae Majoris are the footprints of
the ﬁrst leap. Alula means "ﬁrst leap"
or "ﬁrst spring" in Arabic and hence
these two stars are commonly called
Alula Borealis and Alula Australis (the
northern Alula and the southern Alula).
Tania means "second," and so Lambda
and Mu are known as Tania Borealis
and Tania Australis, respectively. Talitha
means "third" - however, only one
of the two stars of the third leap, the
northern star, Iota Ursae Majoris, gets
called this, but without the "borealis"
addition: It's just Talitha.
There doesn't seem to be an accepted
proper name for Kappa Ursae Majoris.
But Iota gained an additional name that
was used quite a bit in recent decades:
the strange-looking "Dnoces." This was
later revealed as a new name coined
by Gus Grissom for one of several
navigational stars to be used on the
fateful Apollo 1 mission. Dnoces is for
America's ﬁrst spacewalker, Edward H.
White II: "Dnoces" is "second" spelled
backwards. It's ironic that a star named
"second" backwards is one already
called Talitha, or "third."
Contrasts of the leap stars. The
considerable contrasts in true nature
between the members of each of the
three pairs is fascinating because the
stars in each pair are separated by
only about 1-2° in the sky. A beautiful visual juxtaposition are the colors
of Tania Borealis and Tania Australis,
the former being an A2 white or bluewhite star, the latter an M0 red (or very
Alula Borealis is about 400 lightyears distant while Alula Australis is
only around 26 light-years away. And
Alula Australis is one of the most interesting double stars in the sky. The ﬁrst
double star whose orbit was determined,
it has a period just shy of 60 years. Its
two main components, Xi Ursae Majoris
A and B, shine at magnitudes 4.4 and
4.9. They're currently about 2″ apart
and separating rapidly. Xi UMa A and
Xi UMa B (the two brightest lights in
what turns out to be at least a ﬁve-star
system) are also marvelously Sun-like,
belonging to spectral classes G0 and G5,
respectively. Xi UMa A has a luminosity,
radius, and mass calculated to be 1.1,
1.0, and 1.0 that of the Sun, while the
corresponding numbers for Xi UMa B
are 0.7, 0.9, and 1.0.
The false fourth leap. But beware:
There are several pairs of stars in the
region of the Three Leaps that could be
mistaken for one of them. The most
outstanding pair, however, is composed
of the magnitude-3.1 star Alpha (α)
Lyncis and the
star 38 Lyncis.
Editor FRED SCHAAF
founded the South
Jersey Astronomy Club
the spring of 1989.
sk yandtelescope.com * J U N E 2 018