Page 74 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

OBSERVING

VARIABLE STARS

NOT ALL STARS APPEAR TO SHINE AS BRIGHTLY ALL
OF THE TIME – SOME APPEAR TO WAX AND WANE

At fi rst glance, or even after
a prolonged stare, it can seem
like the starlit night changes
very little. Apart from the slow
movement of the sky caused
by Earth’s rotation and the
odd meteor, nothing much
else appears to happen.
However, if you know when
and where to look, even the
seemingly fi xed stars can take
on a life of their own. After a
little investigation, you’ll see
that the night sky is, in fact,
constantly changing. This
is because of variable stars,
stellar wonders that change
in brightness over time. Some
do so in just a few hours, while
others take several years.
Variables come in many
forms, the main types being
intrinsic and extrinsic
ECKHARD SLAWIK/SCIENCE PHOTO LIBRARY, TIL CREDNER/WWW.ALLTHESKY.COM, KEITH SCHLOTTMAN/
variables. If you’re looking
WWW.XANADUOBSERVATORY.COM, PETE LAWRENCE X 2, JOHN CHUMACK/SCIENCE PHOTO LIBRARY
for action, then look out for
an intrinsic variable. The
changes in these objects
happen within the star itself.
It may pulsate in and out,
for example, getting brighter
and fainter as it goes.
One type of intrinsic is the
long-period variable. These
tend to be unstable older stars
fi ghting internal battles with
gravity and pressure, resulting
in them growing and shrinking
over long periods of time. They
make good observing targets:
some can be seen with the
naked eye when they are at
their brightest, yet they can
then dim to a point where
only a pair of binoculars or
Algol, the middle star in the row of three just below centre, is an eclipsing binary with a period of nearly three days a telescope will reveal them.
74 skyatnightmagazine.com 2012