Page 13 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

NEED TO KNOW

JUDGING MAGNITUDES

Looking up at the sky, sometimes people say that biggest dots. There will generally be a key to the
a star is really big. Of course this isn’t dots and what magnitude they represent nearby.
the case, as all stars are so far away that they It’s a good idea to get acquainted with the faintest
only ever appear as points of light to your eye. It’s stars you can see in your usual night sky. Take a
what your eye does with that dot that can make it star chart and look for the smallest dots (the
seem bigger. And, in fact, drawing bigger dots is faintest stars) in a recognisable constellation and
the only way of showing the difference in star see if they’re visible in the sky. Then you’ll know
brightness on a chart. The brighter stars have the how much your viewing is affected by light pollution.

Mars is easily distinguished
by it ruddy hue, giving rise to
the nickname ‘the Red Planet’

Magnitude is represented on
star charts by size; you can see
Jupiter may not be the brightest here that Polaris is brighter than
planet in the Solar System, but Beta (`) Ursae Minoris
it’s certainly the largest

in the constellation of Ursa Minor – its magnitude up and made them stand in a line 5m in front of It’s a really
is just +2.0. With practice, it is possible for the you, you would easily be able to tell. WHAT worthwhile
NEXT getting to know
eye to spot differences of one-tenth of a magnitude Of course, 5m away is not going to work with a
the faintest stars
between stars – now there’s something to try star! Astronomers use a distance of 32.6 lightyears.
you can see from where you
of an evening. A star’s magnitude, as viewed from this calculated
live – in other words, the
Of course, stars are at different distances from distance, is its ‘absolute magnitude’. But how do you limiting visual magnitude of the
Earth, and come in many sizes and colours. So move a star 32.6 lightyears away? Not with some skies above you. Try this on
visual magnitude only tells us how the brightness strange gravity device, I can assure you. a few occasions, as you may
of one differs from another in the sky. It tells us To get the absolute magnitude for each star we fi nd the seeing conditions are
different each time. Any good
nothing about a star’s ‘absolute brightness’. just need to know its visual magnitude (easy: it’s
constellation guide will give you
What do we mean by absolute brightness? Well, in a book) and how far away it is (easy again:
the visual magnitude of stars
if you lined all the stars up at the same distance space telescopes have worked that one out for us). right down to mag. +6.0, so
from Earth, you would be able to truly see how We know that brightness diminishes with out you go. You’ll have fun and
bright each star is. It’s just like being in a fi eld distance, just as a bonfi re doesn’t appear to be may just learn something into
during a pretty bad party – there are only a few as bright if you look at it from a distant hill as it the bargain. Just don’t forget
your red torch.
people dotted around and no-one is talking to each does if you are standing next to it. If we know the
other. Wherever you stand, you aren’t able to judge distance to a star and how bright it looks from
who is the tallest. However, if you rounded everyone here, then we can work out its absolute magnitude.

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