April
2013 Constellation and Asterism of the Month – Canis Minor and the Winter
Triangle
After the spectacular night sky of winter,
with many bright stars forming easily-discernible constellations, comes the comparatively faint night sky
of spring. In our previous blog posts,
we've covered the prominent constellations that are visible in April soon after
sunset. These include winter
constellations which are now in the western part of the winter sky -- such as
Orion, Gemini, Canis Major, and Auriga – a couple of spring constellations –
Leo and Bootes – and one circumpolar constellation -- Ursa Major.
There is, however, one very bright
star which is not included in any of these.
It is the 5th brightest star which can be seen in the
Northern hemisphere – Procyon of the constellation Canis Minor ("Small Dog"). In mythology, Canis Major and Canis Minor are
the hunting dogs of Orion the Hunter.
One star from each of these three constellations form the asterism
called the "Winter Triangle".
(An asterism is a shape made of stars, which is not itself a
constellation). The 3 stars are Procyon,
Sirius in Canis Major, and Betelgeuse, which marks the left shoulder of Orion. The Winter Triangle looks almost perfectly
equilateral – 3 equal sides, 3 equal angles.
It is very rare to find such regularity in the sky.
Procyon and Sirius appear very bright to us
because they are so close – 11 and 8.5 light years respectively. They are only 2 to 3 times as far away as the
star which is nearest to us, Alpha Centauri.
Betelgeuse, on the other hand, appears so bright because it is
extremely bright. Betelgeuse is 650
light years away.
Canis Minor basically consists of
Procyon and one other, much fainter, star northeast of it. That one is called Gameon or Beta Canis Majoris. (Procyon is Alpha). Gameon is a 3rd magnitude star,
which means it is fainter than all but one of the stars in the Big Dipper. (The Big Dipper is itself an asterism which
forms part of the larger constellation of Ursa Major.)
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