Page 31 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

NEED TO KNOW

Some of the more distant
red galaxies in this Hubble
picture are nearly 13 billion
lightyears from Earth

VARIABLE STARS

Most of the stars we
see shine away quite
happily – their brightness
in the night remains
reassuringly constant.
These, thankfully, include
the Sun. However there
are some that, for various
reasons, appear to get
brighter and fainter
over time. These are the
variable stars.
First catalogued in
the early 17th century,
variables have played
an important part in
measuring the Universe.
The most useful are called
Cepheid variables. What
makes them so splendid
is that the time they take
to change from bright
to dim and back (their
period) is very regular,
and related to how truly
bright they are. If you
MEASURING know how bright a star is
(its absolute magnitude),
then by simply measuring
its brightness in the sky
(apparent magnitude) you
SPACE can calculate its distance.
So in this way, we can
work out how far distant
galaxies are just by
looking for variable stars

THERE’S MORE TO HOW ASTRONOMERS CALCULATE in them. Easy!
THE SPACE BETWEEN OBJECTS IN THE NIGHT SKY

So now you know all about distances in space, and That’s all well and good, but the missing
how our Earthly metres and feet are not much use, ingredient is how do you measure space and how
as they turn into impractically large numbers with much space have we actually found out there?
lots of zeroes, like 56,000,000,000,000,000,000km This brings us to the things that fundamentally
– that’s 56 quintillion km. Now, a ‘quintillion’ altered our understanding of space – the telescope, The M100 galaxy’s Cepheid
doesn’t mean anything to me; add on a few subsequent associated measuring devices and variables tell us that it’s 56
million lightyears away
squillions and it’s all uncharted territory. You know the use of non-visual wavelengths like infrared
that to make life easier when measuring distances and ultraviolet. Without these we would still be
across the Solar System, we use the astronomical oblivious to the nature of the Universe.
unit (AU), which is the mean Earth to Sun distance.
And, for the next stage (all the way to the ‘edge’ EXPLOSION OF KNOWLEDGE
of the Universe, in fact) we use the lightyear. One The fi rst explosion of knowledge came with
lightyear is, of course, simply the distance that light precision telescopes that enabled us to view the
travels in one year. minute shifts of nearby stars against the more
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