http://www.skyandtelescope.com.au 49by Alan Plummer VARIABLE STARSMira’s rise to power
Follow this famous star’s climb to peak light this summer.
M
any of the brightest stars that
make up the constellations are
giants of one sort or another:
blue Beta Centauri, yellow Canopus,
orange Arcturus and red Gamma
Crucis among them. But of the more-
evolved pulsating red giants, only a few
ever reach naked-eye brightness. Of
these, the class prototype of the Mira
variables, Mira itself, is the brightest.
Mira was the first star to be identified
(in the 1600s) as a periodic variable, yet
despite centuries of observation it is still
very much worth studying.
Mira is a binary system about
300 light-years from Earth, with the
companion (Mira B) being a white
dwarf. Scientists have determined that
Mira A has a hugely distended gas
envelope 500 times the diameter of the
Sun. The star ‘shakes’ as it pulsates,
wobbling like a balloon full of water
thrown into the air, on the way to itself
eventually becoming a white dwarf
surrounded by an expanding cloud of
gas and dust. Maybe a lovely planetary
nebulae will one day grace its location
in the sky.
It’s possible that the distorted gas
envelope is due to the proximity ofW Mira is located
at 02h 19m 20.79s
–02° 58 ́ 39.5 ̋
(epoch J2000). This
chart (courtesy
of the AAVSO)
is approximately
10 degrees wide
and has visual
magnitudes shown
with decimal points
omitted to avoid
confusion with
faint stars — so 60
denotes a magnitude
6.0 star.the Mira B companion as much as the
inherent instability of Mira A itself. This
makes the system interesting on another
level; it’s the closest ‘symbiotic binary’ in
the sky, albeit a weakly interacting one.
Mira’s magnitude range is about
3.5 to 9.0, and the next maximum is
expected for January 11th... so we’ve
planned this column in time for you tofollow its rise to peak light. To locate
the star, use the all-sky chart on the
centre pages and the chart provided
here. And don’t worry if you can’t see it
the first time you look; keep an eye out
— this wonderful star will appear!■ ALAN PLUMMER can be contacted at
[email protected]