addition to the constellation, rivalling the brightest member of Cetus, a
star known as Menkar. However, at its dimmest, Mira disappears from
view, dropping down to the limits of binocular visibility. No wonder then
that the name Mira literally means “wonderful”.
Another example is Mu Cephei in the far north of the sky, a star also
known as Herschel’s Garnet Star. Mu varies between magnitude 4
and 6 so it is generally a naked eye object and always within binocular
range. Use a telescope and it appears like a glowing coal because its
temperature is so much lower than our own Sun. To compensate Mu
Cephei is immense and would fill our Solar System out as far as Saturn. It
will explode as a supernova this year, next year, or a hundred years, or a
million years hence – we cannot know.
We can have far brighter examples of star colours. In the main winter
constellation of Orion, the two leading stars are Betelgeux, an orange
red supergiant, and Rigel, a very bright powerful blue star well over 40
thousand times more luminous than our Sun. Both these giants evolve
much more quickly than our Sun, though of course one cannot see any
detectable change over periods of a good many lifetimes.
To find the various objects mentioned here it is merely a question
of time and patience, using the star maps at the back of this book. In
addition to the "fixed" stars of the night sky, there can be unexpected
visitors like novae which appear as "new" stars. In fact a nova is not a new
star at all but the white dwarf companion of a normal star. The white dwarf
pulls material away from its companion, increases its brightness and then
fades back to obscurity. Some novae may be very brilliant. Nova Aquilae
in 1918 surpassed every star in the sky apart from Sirius, but sadly did not
last very long and has now become very faint indeed.Mulitple star systems abound in the night sky, with binaries, triples,
quadruples, quintuples, sextuples and even a possible septuple, a seven
star system known as Nu Scorpii.Variable Stars
We have already talked about double stars, binary systems and star
colours. Even a small telescope will show plenty of examples of what we
mean. In addition, many stars show variation in light output and these
are what we refer to as variable stars. The variation may come about
because of external or internal processes. An example of an external
process is the so called eclipsing binary. One of the best examples of
this type of star is Algol in the constellation of Perseus. Also known
as the “Winking Demon” or Beta Persei, to the naked eye Algol dips in
brightness every 2 days 20 hours and 49 minutes, remaining dim for
around 10 hours. The dimming is due to the fact that Algol is a binary
star system. One component is bright, the other dim and from Earth
one star appears to cross the face of the other, partially covering its light
from view. When the bright star covers the dim one, the dip in brightness
is only detectable using specialist equipment. When the dim one covers
the bright one however, the dip is appreciable and it’s this which makes
the Demon “wink”. There are many examples of eclipsing binary stars.
Other stars vary because of internal processes. Old, large red stars
for example, vary because they are running out of fuel. This leads to
instability and changes in luminosity. One of the finest examples is the
star Omicron Ceti, also known as Mira, in the constellation of Cetus,
the Whale. This remarkable star varies in brightness over a period of
332 days. At its brightest, it can easily be seen with the naked eye as an[8] Dark absorption lines provide spectral finger prints for stellar classification. [9] A misty sky helps bring out the colours of stars in the constellation of Orion, the Hunter.8Astronomer Book