Leo, Cancer, Sextans
L
eowas the mythological Nemaean lion which became
one of Hercules’ many victims, but in the sky the
Lion is much more imposing than his conqueror, and is
indeed one of the brightest of the Zodiacal constellations.
The celestial equator cuts its southernmost extension,
and Regulus, at the end of the Sickle, is so close to the
ecliptic that it can be occulted by the Moon and planets –
as happened on 7 July 1959, when Venus passed in front
of it. On that occasion the fading of Regulus before the
actual occultation, when the light was coming to us by
way of Venus’ atmosphere, provided very useful informa-
tion about the atmosphere itself (of course, this was well
before any successful interplanetary spacecraft had been
launched to investigate the atmosphere of Venus more
directly).
Regulus is a normal white star, some 78 light-years
away and around 125 times as luminous as the Sun. It is a
wide and easy double; the companion shares Regulus’
motion through space, so that presumably the two have
a common origin. The companion is itself a very close
double, difficult to resolve partly because of the faintness
of the third star and partly because of the glare from the
brilliant Regulus.
About 20 minutes of arc north of Regulus is the dwarf
galaxy Leo I, a member of the Local Group, about
750,000 light-years from us. It was discovered photo-
graphically as long ago as 1950, but even giant telescopes
are hard pressed to show it visually, because its surface
brightness is so low; it is also one of the smallest and least
luminous galaxies known. Even feebler is another member
of the Local Group, Leo II, which lies about two degrees
north of ‰.
There is a minor mystery associated with Denebola or
‚Leonis. All observers up to and including Bayer, in
1603, ranked it as being of the first magnitude, equal to
Regulus, but it is now almost a whole magnitude fainter.
Yet it is a perfectly normal Main Sequence star oftype A, 39 light-years away and 17 times as luminous
as the Sun – not at all the kind of star expected to show
a slow, permanent change. It is probable that there has
been a mistake in recording or interpretation; all the same,
a certain doubt remains, and naked-eye observers may
care to check on it to see if there are any detectable
fluctuations. The obvious comparison star is Á, which is of
virtually the same brightness and can often be seen at the
same altitude above the horizon.
Áis a magnificent double, easily split with a very
small telescope. The primary is orange, and the G-type
companion usually looks slightly yellowish. The main
star is 60 times as luminous as the Sun, and the companion
is the equal of at least 20 Suns; the distance from us is
91 light-years. Two other stars, some distance away, are
not genuinely connected with the bright pair.
Two fainter stars (not on the map), 18 Leonis (magni-
tude 5.8) and 19 Leonis (6.5), lie near Regulus and are
easily identified with binoculars. Forming a group with
them is the Mira variable R Leonis, which can reach
naked-eye brightness when at maximum and seldom falls
below the tenth magnitude. Like most stars of its type, it
is very red, and is a suitable target for novice observers,
particularly since it is so easy to find.
There are five Messier galaxies in Leo. M65 and M66,
which lie more or less between ıand ÈLeonis, can be seen
with binoculars, and are only 21 minutes of arc apart, so
that they are in the same field of a low-power telescope.
Both are spiral galaxies; M66 is actually the brighter of
the two, though M65 is often regarded as the easier to see.
Unfortunately, both are placed at an unfavourable angle to
us, so that the full beauty of the spiral forms is lost. They
are around 35 million light-years away, and form a true
pair. Another pair of spirals, M95 and M96, lies between
Úand ıLeonis; close by is the elliptical galaxy M105,
which is an easy object. Leo contains many additional
galaxies, and the whole area is worth sweeping.ATLAS OF THE UNIVERSE
Two Zodiacal
constellations are shown
here, Leo and Cancer: Leo
is large and prominent,
Cancer decidedly obscure.
Both are at their best during
evenings in northern spring
(southern autumn). Leo
is distinguished by the
‘Sickle’, of which Regulus
is the brightest member,
while Cancer contains
Praesepe, one of the finest
open clusters in the sky.
Sextans is very barren
and obscure. The equator
crosses this map, and
actually passes through
the southernmost part
of the constellation of Leo.MagnitudesVariable starGalaxyPlanetary nebulaGaseous nebulaGlobular clusterOpen cluster–1
0 1 2 3 4 5COMA BERENICES
URSA MAJORLEO MINOR
GEMINI
LEO
CANCER
CANIS MINOR
HYDRA
SEXTANS
CANIS MAJOR
MONOCEROS
CRATER
VIRGO
RegulusM44 PraesepeÁ‰‚ıÈÛÚÌ Â ̇ Á Ë ·È
ÎÁ
Ë
‰ ı·
‚M65
M66
M105
M96 M95 R M67
R‚ ·
‰ÁÓ̄ProcyonÔÏ‚·‰̇
Â
‰
Ë Û
ı
È Ù̇ÂÏ
‰Â
ıÂ
Ô
Ó‚‰Á ËÔ‚
·
·‚·ÙÍGa Atl of Univ Phil'03stp 2/4/03 7:36 pm Page 226