Ursa Major, Canes Venatici, Leo Minor
U
rsa Major. There can be few people who cannot
recognize the Plough – alternatively nicknamed King
Charles’ Wain or, in America, the Big Dipper. The seven
main stars make up an unmistakable pattern, but in fact
only five of them share a common motion in space and
presumably have a common origin; the remaining two –
Dubhe and Alkaid – are moving through space in the
opposite direction, so that after a sufficient length of time
the plough-shape will become distorted. Of the seven, six
are hot and white, but Dubhe is obviously orange; the
colour is detectable with the naked eye, and binoculars
bring it out well.
It is interesting that Megrez (‰Ursae Majoris) is
about a magnitude fainter than the rest. In 1603 Bayer,
who drew up a famous star catalogue and gave the stars
their Greek letters, gave its magnitude as 2; but earlier
cataloguers ranked it as 3, and there has probably been no
real change. It is 65 light-years away, and 17 times as
luminous as the Sun.
Of course the most celebrated star in the Great Bear is
̇(Mizar) with its naked-eye companion Alcor. Strangely,
the Arabs of a thousand years ago regarded Alcor as a test
of keen eyesight, but today anyone with average eyes can
see it when the sky is reasonably dark and clear. A small
telescope will show that Mizar itself is double, but the
separation (14.4 seconds of arc) is too small for the two
stars to be seen separately with the naked eye, or even
binoculars. Between Alcor and the two Mizars is a fainter
star which was named Sidus Ludovicianum in 1723 by
courtiers of Emperor Ludwig V, who believed that it
had appeared suddenly. Ludwig’s Star can be seen with
powerful binoculars, and it has been suggested that it
might have been the ‘test’ referred to by the Arabs, but
certainly it would have been a very severe one – even if
the star is slightly variable.
Outside the Plough pattern is a triangle of fainter stars:
„, Ïand Ì. The two latter stars are in the same binocularfield, and make a good colour contrast. Ïis white, while
Ì, with its M-type spectrum, is very red.
ÍUrsae Majoris, close to Ó, was one of the first binary
stars to have its orbit computed. The components are equal
at magnitude 4.8, and the period is 59.8 years; but the
separation is currently only about 1 arc second, so that a
very small telescope will not split the pair. (Generally
speaking, a 3-inch or 7.6-centimetre refractor will be able
to divide pairs down to a separation of about 1.8 arc
seconds, assuming that the components are more or less
equal and are not too faint.) There are not many notable
variables in Ursa Major, but the red semi-regular Z, easy
to find because of its closeness to Megrez, is a favourite
test subject for newcomers to variable-star work.
There are four Messier objects in the constellation.
One of these, M97, is the famous Owl planetary nebula. It
was discovered by Pierre Méchain in 1781, who recorded
it as being ‘difficult to see’, and certainly it can be elusive;
the two embedded stars which give it its owlish appear-
ance are no brighter than magnitude 14, and the whole
nebula is faint. It lies not far from ‚or Merak, and it can
be seen with a 7.6-centimetre (3-inch) telescope when
the sky is dark and clear. M81 and M82 are within
binocular range, not far from 24; M81 is a spiral, while
M82 is a peculiar system which is a strong radio source.
Each is about 8.5 million light-years away, and they are
associated with each other. The other Messier object,
M101, was also discovered by Méchain; it forms an equi-
lateral triangle with Mizar and Alkaid ( ̇and ËUrsae
Majoris), and is a loose spiral whose surface brightness is
rather low. It is face-on to us, and photographs can often
show it beautifully.
Though all the main stars of Ursa Major are well
below the first magnitude, their proper names are often
used. There are, incidentally, two alternatives; Ëmay be
called Benetnasch as well as Alkaid, while Áis also known
as Phekda or Phecda.ATLAS OF THE UNIVERSE
Ursa Majoris the most
famous of all the northern
constellations, and can
be used as a guide to find
many of the less prominent
groups. The ‘Plough’ is
only part of the entire
constellation, but its
seven main stars cannot
be mistaken; they are
circumpolar over the British
Isles and parts of Europe
and North America. They are
always low over South Africa
and Australia, but only from
parts of New Zealand is the
‘Plough’ completely lost.
Canes Venatici and Leo
Minor adjoin Ursa Major;
Lynx is also shown on this
map, but is described with
Auriga (Star Map 18).MagnitudesVariable starGalaxyPlanetary nebulaGaseous nebulaGlobular clusterOpen cluster–1
0 1 2 3 4 524LEO MINOR
BOÖTES
DRACO
URSA MAJOR
Mizar LYNX
Alcor46213831191512·„
ÌÏÍ‚ÎÊ ı̆‚· ·CANES VENATICI
M82
R M81zRLEO
M97
ÈÎ
Ï‚ÁÏ‚ı
ÎM106
5195
M51
M63 M94
RTUYM3
COMA BERENICES
̇‚̄M101
·Ë  ‰ÁÓÈÔÛÙGa Atl of Univ Phil'03stp 2/4/03 7:35 pm Page 218