Boötes, Corona Borealis, Coma Berenices
B
oötes. A large and important northern constellation,
said to represent a herdsman who invented the plough
drawn by two oxen – for which service to mankind he was
rewarded with a place in the heavens.
Of course the whole area is dominated by Arcturus,
which is the brightest star in the northern hemisphere of
the sky and is one of only four with negative magnitudes
(the other three are Sirius, Canopus and ·Centauri). It is
a light orange K-type star, 36 light-years away and with a
luminosity 115 times that of the Sun; the diameter is about
30 million kilometres (about 19 million miles). It is too
bright to be mistaken, but in case of any doubt it can be
located by following through the tail of the Great Bear
(Alioth, Mizar, Alkaid).
Arcturus has the exceptionally large proper motion of
2.3 seconds of arc per year, and as long ago as 1718
Edmond Halley found that its position relative to the
background stars had shifted appreciably since ancient
times. At the moment it is approaching us at the rate of
5 kilometres per second (3 miles per second), but this will
not continue indefinitely; in several thousand years’ time it
will pass by us and start to recede, moving from Boötes
into Virgo and dropping below naked-eye visibility in half
a million years. It is a Population II star belonging to the
galactic halo, so that its orbit is sharply inclined, and it is
now cutting through the main plane of the Galaxy.
In 1860 a famous variable star observer, Joseph
Baxendell, found a star of magnitude 9.7 in the field
of Arcturus, at a P.A. of 250 degrees and a separation of
25 minutes of arc. Within a week it had disappeared, and
has never been seen again, though it is still listed in cata-
logues as T Boötis. It may have been a nova or recurrent
nova, and there is always the chance that it will reappear,
so that amateur observers make routine checks to see
whether it has done so.
Â, the second brightest star in the constellation, is a
fine double; the primary is an orange K-type star, whilethe companion looks rather bluish by contrast. No doubt
the two stars have a common origin, but the revolution
period must be immensely long. The primary is 200 times
more luminous than the Sun, so that it is more powerful
than Arcturus, but it is also further away – around 150
light-years. The semi-regular variable W Boötis is in the
same binocular field with Â, and is easily recognizable
because of its orange-red hue.
̇is a binary, with almost equal components (magni-
tudes 4.5 and 4.6) and an orbital period of 123 years, but
the separation is never more than 1 second of arc, so that
a telescope of at least 13-centimetre (5-inch) aperture is
needed to split it. There are no Messier objects in Boötes,
and in fact no nebular objects with integrated magnitude
as bright as 10.
One constellation which is still remembered, even
though it is no longer to be found on our maps, is
Quadrans Muralis (the Mural Quadrant), added to the sky
by Bode in 1775. The nearest brightish star to the site is
‚Boötis (Nekkar), magnitude 3.5. Like all the rest of
Bode’s groups, Quadrans was later rejected, but it so
happens that the meteors of the early January shower
radiate from there, which is why we call them the
Quadrantids. For this reason alone, there might have been
some justification for retaining Quadrans.
Corona Borealisis a very small constellation, covering
less than 180 square degrees of the sky (as against over
900 square degrees for Boötes), but it contains far more
than its fair share of interesting objects. The brightest star,
·or Alphekka (also known as Gemma), is of the second
magnitude, and is actually an eclipsing binary with an
unusually small range; the main component is 50 times as
luminous as the Sun, while the fainter member of the pair
has just twice the Sun’s power. The real separation
between the two is less than 30 million kilometres (19
million miles), so that they cannot be seen separately. The
distance from the Solar System is some 78 light-years.ATLAS OF THE UNIVERSE
The mapis dominated
by Arcturus, the brightest
star in the northern
hemisphere of the sky;
it is sufficiently close to
the celestial equator to be
visible from every inhabited
country, and is at its best
during evenings in northern
spring (southern autumn).
The Y-formation made up
of Arcturus, Âand ÁBoötis,
and Alphekka (·Coronae)
is distinctive. The rejected
constellation of Quadrans
is now included in Boötes,
near ‚; it is from here that
the January meteors radiate,
which is why they are known
as the Quadrantids.MagnitudesVariable starGalaxyPlanetary nebulaGaseous nebulaGlobular clusterOpen cluster–1
0 1 2 3 4 5DRACO URSA MAJOR
CANES VENATICI
COMA BERENICES
VIRGO
CORONA
BOREALISHERCULES
BOÖTES
AlphekkaArcturusı ÎÏÓ̉‚
ÁÂÛÚ
̇·Ë
Ù
̆‚·Á
ı·‚ÁSERPENS CAPUT
‰ÂÈSR
T WRRM51
TU
Y
M106M63 M94
M3
M53
M64
M88
M100
M98
M99
È·‚LEO
Ë̇ Â
Á‚Â
Ô
ÓË
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Á·È Î Á ‚ ‰ Ï ·41ËÛ ‰ ı ‚ È ÎGa Atl of Univ Phil'03stp 2/4/03 7:36 pm Page 224