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

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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