Australian Sky Telescope MayJune 2017

http://www.skyandtelescope.com.au 49

Taking the pulse of T Cen

A southern superstar that’s perfect for 50-mm binoculars.

W

e first met T Centauri four years
ago, but this unusual and easy-
to-observe star demands closer
attention. T Cen is a pulsating giant whose
exact classification has proved tricky.
Despite differing sources and opinions,
120 years of light curve data says that
it is an RVb star. ‘RV’ stars, after RV
Tauri, show alternating deep and shallow
pulsations that can vary in depth, even
swap around, with amplitudes reaching
3 to 4 magnitudes, and periods between
adjacent primary minima in the range of
30 to 150 days. ‘RVb’ means that the mean
magnitude itself varies over time.
While it’s reasonably well established
that the RV stars represent a transitional
stage between the red giant and the
planetary nebula stage, no one really
knows why these stars vary in just
this way. One possibility is interacting
multiple pulsation modes (if musically
minded, think of two closely out of
tune strings audibly beating). Another
is a more chaotic interaction. These
ideas are not exclusive; red giants are
unstable, comprising a very small,
hot and dense core inside a hugely
distended, cooler and vacuous envelope.
T Cen varies between 5th and 8th
magnitude (sometimes!) and that

makes it perfect for 50-mm binoculars.

The three bright stars shown on the

finder chart above are visible to the

naked eye as a faint triangle between

Iota and Theta Cen (and those two are

to the north of the globular cluster

Omega Cen). The all-sky chart in this

centre pages is sufficient to find the

S T Centauri is located at 13h 41m 45.56s –33° 35" 50.6" (epoch J2000). This chart (courtesy

of the AAVSO) is approximately 4 degrees wide and has visual magnitudes shown with decimal

points omitted to avoid confusion with faint stars — so 70 denotes a magnitude 7.0 star.

by Alan Plummer VARIABLE STARS

field. If you’re interested in contributing

to T Cen’s on-going data set, please

email me for information.

■ ALAN PLUMMER observes from the

Blue Mountains west of Sydney, and

can be contacted at alan.plummer@

variablestarssouth.org