Page 11 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

NEED TO KNOW

Sirius, the Dog Star, is star is 2.5 times brighter than a second magnitude
the brightest star in the star, and so on, right up to around 100 times
night sky, making it brighter than a 6th-magnitude star.
easy to recognise
However, the scale doesn’t bottom out at one.
To make things even more interesting, a star can
How bright a have a magnitude of zero, which would be a pretty
star looks is called bright star; objects that are brighter still are given
its ‘apparent visual a minus number. For example, the planet Venus,
magnitude’. You may see when at its brightest, appears at mag. –4.7. This
this written as ‘apparent is why positive magnitudes are marked with a
magnitude’, ‘visual magnitude’ or just ‘magnitude’. ‘+’ sign, to remove any ambiguity.
You may also see it abbreviated to ‘mag.’, as we do Sliding back down the scale – getting fainter – we
throughout the Beginners’ Guide to Astronomy. return to mag. +6.0. This is typically the limit of what
What’s strange about magnitude scales is you can see with the naked eye; anything dimmer
that the numbering system is back to front – the and it’s likely that you’ll need a pair of binoculars
brighter the star, the lower the number it is given. or a telescope to see it. In actuality, this depends
So a star of mag. +2.0 is therefore brighter than on your eyes: some people have no trouble seeing
one that’s mag. +5.0. To understand why this is, we down to mag. +6.5 or lower.
have to cast our minds back more than 2,000 years So, what about the stars we know and love? MORE
and think about how the ancient Greeks tried to Well, the brightest star in the night sky is Sirius, ADVICE
make sense of the heavens. the leading star in the constellation of Canis Major,
the Great Dog. Its visual magnitude is a dazzling OVER THE
STARING INTO SPACE –1.5. Compare that with Polaris, the North Star, PAGE
If you could travel back to ancient Greece, the
best person to seek out would be an astronomer
and mathematician called Hipparchus. His initial WHY DO STARS TWINKLE?
thoughts about the night sky were probably the
same as yours: it’s immediately clear that not all Twinkle, twinkle, little star, how I wonder atmosphere, which is where all the
of the stars and other astronomical objects are what you are… Well, we don’t mean twinkling takes place.
the same brightness. to disappoint any children who might Here the light is refl ected, bent,
be reading this, but actually it’s not shimmered and shaken by all the tiny
Hipparchus called this varying in brightness
the star twinkling at all. bits that make up our atmosphere, until
‘magnitude’ and based on this he catalogued
The light from the star may have it makes it to your eye. Stars would not
the stars into six groups. The 20 brightest travelled for many millions of years appear to twinkle if we viewed them
stars were labelled magnitude 1, or the ‘fi rst though space – nice and steadily, all from outer space, or from a planet or
magnitude’. Slightly fainter stars fell into the way – and then it meets Earth’s moon that didn’t have an atmosphere.
magnitude 2, and so on. Hipparchus continued
down to magnitude 6, which were the faintest
stars he could see with his eyes.
Earth’s
Today, we use scientifi c equipment to classify
atmosphere
magnitude exactly, and we use it to measure the Light from
brightness of all objects in the sky – not just the a distant star
ones visible to the naked eye, and not just stars.
But Hipparchus’s structure remains. Our modern
Stars ‘twinkle’
system is, of course, much more accurate, with
because we view
the mathematical difference between one
their light through
magnitude and the next being about 2.5 times turbulent air
the brightness. This means that a fi rst magnitude

5 6 7 8
BETELGEUSE

PROCYON
RIGEL
ACHERNAR
Star: RIGEL Star: PROCYON Star: ACHERNAR Star: BETELGEUSE
Magnitude: +0.1 Magnitude: +0.4 Magnitude: +0.5 Magnitude: variable +0.6
Constellation: Orion Constellation: Canis Minor Constellation: Eridanus Constellation: Orion
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