Page 32 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

distant ‘fi xed’ stars, which in reality is caused by its centre is about 30,000 lightyears away in the
Earth’s movement around the Sun. It’s like looking direction of Sagittarius; the whole kit and caboodle
at your fi nger with one eye and then the other, is over 100,000 lightyears across. So 500 lightyears
noticing how your fi nger’s position has changed is still in our back garden.
relative to the background. Parallax, as the system
is known, was fi rst successfully used in 1838 when NEAREST NEIGHBOURS
Friedrich Bessel calculated the distance to the Our Galaxy is part of a Local Group of some 30
naked-eye star 61 Cygni (in Cygnus, the Swan) to galaxies – some large, such as our own and M33,
be just over 11 lightyears. but many far smaller, such as the Wolf-Lundmark-
Today, of course, astronomers and scientists Melotte Galaxy in Cetus, or the Magellanic Clouds.
don’t need to look through a telescope in the cold The closest galaxy is the Andromeda Galaxy,
night; they employ spacecraft in order to do all visible as a smudge to the unaided eye, sitting at
of the measuring stuff for them. Up above the a distance of around 2.8 million lightyears. That’s
wobbling, swirling impreciseness that Earth’s still peanuts. The whole Local Group is around 10
atmosphere gives us, spacecraft can ‘see’ much million lightyears in size. Still peanuts, but only
better and with much more precision. For example, just. Farther out (and there’s still a long way to go)
from 1989 until 1993 the European Space Agency’s our Group belongs to the Virgo Supercluster – a
Hipparchos mission measured the parallax of over large rounded curtain shape that stretches over
118,000 stars out to nearly 500 lightyears from 200 million lightyears.
Earth. Try that with a telescope down here and You may be wondering how we can measure
you’ll be lucky to get out to 100 lightyears, and even these vast distances. Well, it’s by using special
then you’d doubt the accuracy of the outer stars. kinds of variable stars, some of which do
Plus, there’s no way you can get through as many spectacular things (see ‘Variable stars’, p31). Plus,
stars as a robotic spacecraft – not without endless on large scales, we see that most galaxies are
cups of tea, or getting incredibly bored. ‘moving away’ from us, and the further we look,
the faster they seem to be travelling. This is all
Five hundred lightyears, eh? That’s not a very
PAUL WOOTTON big bag of peanuts as far as space is concerned. a part of the famous ‘redshift’ (see below), which
has enabled us to calculate the size of our visible
In fact, it’s like going for a short jog with your dog
to the garden gate and back. Look at our Galaxy:
Universe as 13.7 billion lightyears across.

REDSHIFT

How the ‘stretching’ of space helps us to measure it more accurately

Imagine a very big rubber band, happening to the cars? They reality it is the space in-between stretched out as it zooms away.
as wide as a road and as long as are beginning to move apart, stretching. This has an effect on In the case of waves of light, this
you fancy. Onto this rubber band, not because they themselves are the light we get from the distant stretching turns the light more and
at some distance apart, place moving, but because the rubber galaxy – it is being stretched too. more red (visible red wavelengths
two cars. Now with two helpers between them is stretching. This is much like hearing a are the longest), hence the name
at each end of the band, start to This is exactly what happens police car siren sound ‘dropping’ redshift. And simply (with maths
pull. You can envisage what will in space, on a much larger scale. as it passes us by (the famous again!), the greater the degree
start to happen: the fabric of the We see galaxies appearing to Doppler effect). That’s caused of redshift, the further away the
band will start to stretch. What’s move away from us, when in by the sound wavelength being galaxy in question must be.

Direction of expanding space

Light from distant galaxy
Astronomer Light waves from is shifted into the red end
on Earth distant galaxy of the spectrum
spread out
Distant galaxy
moving away from
astronomer on Earth

32 skyatnightmagazine.com 2012