Page 75 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO SEE

FIVE FAMOUS

VARIABLES

DELTA CEPHEI
Type: Pulsating variable;
it’s also the prototype of
all Cepheid variables
Range: Mag. +3.9 down
to mag. +5.0
Period: 5 days 9 hours
The action really starts to one of the stars moves in Best time to see: Autumn
Constellation: Cepheus
hot up with another group of front of the other.
Equipment: Naked eye
intrinsic stars called cataclysmic Another extrinsic variety
– or explosive – variables, is the rotating variable. These
which pull gas off their close stars spin so fast that their MIRA
neighbours. The pile-up of new light output is actually affected: Type: Long-period red giant;
gas leads to nuclear explosions, if we could see them they would the fi rst of its type, its variability
was discovered in 1596
called novae, which in turn have a squashed appearance.
Range: Mag. +2.0 down
result in a dramatic, sudden Professional astronomers have to mag. +10.1
increase in brightness. The discovered extrinsic variables Period: 332 days
dying explosions of old stars, at incredible distances in Best time to see: Autumn
called supernovae, are also the Large Magellanic Cloud. Constellation: Cetus
part of this group. The variability of these stars Equipment: Binoculars
Add in the eruptive variables, is controlled by a light-
which include stars whose bending phenomenon called RASALGETHI
surfaces fl are up from time to gravitational microlensing. Type: Massive semi-regular
time, and it’s clear how active That may be a little far old red supergiant
the intrinsic type really is. away for our equipment, but Range: Mag. +2.8 down
watching out for the fl uctuating to mag. +4.0
OUTSIDE INFLUENCE light of variable stars is a Period: About 3 months
Best time to see: Summer
Meanwhile, extrinsic variables perfect project for amateur Constellation: Hercules
owe their changeable nature astronomers. With a relatively Equipment: Naked eye
to an external element in the inexpensive telescope, you
mix. Take an eclipsing binary, can add useful observations
for example – this is where to the knowledge bank that ALGOL
the orbits of two close stars are professional astronomers
Type: Eclipsing binary
such that, from our perspective may use to study how the Range: Mag. +1.6 down
on Earth, one appears to move Universe works. There are to mag. +3.0
in front of the other as they go so many variable stars out Period: 2 days 21 hours;
around. The amount of light there that they would never brightens over 10 hours
Best time to see: Autumn
we see coming from the system be able to look at them all
Constellation: Perseus
as a whole changes whenever without your help.
Equipment: Naked eye

STARRY MEASURING STICKS RS OPHIUCHI
Type: Recurrent nova
Cepheids are intrinsic variable stars that are useful to astronomers Range: Mag. +5.0 down
because they have a very regular period of light change. Some to mag. +12.5
change just once day, while others take a month or more to Period: Around 20 years
complete their cycle. The period is linked precisely with the true Best time to see: Summer
brightness of the star – so a Cepheid with a fi ve-day period near Constellation: Ophiuchus
to us is the same true brightness as a fi ve-day Cepheid in a distant Equipment: Telescope;
galaxy. As we know exactly how light diminishes with distance, naked eye when bright
we can work out how much farther away the distant Cepheid is,
using it to help measure distances in space.

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