Australian Sky & Telescope - April 2018

42 AUSTRALIAN SKY & TELESCOPE April 2018

BINOCULAR HIGHLIGHT by Jonathan Nally

Theta’s hot, blue

radiance

I

s there anything better than grabbing your binoculars and scanning

through the star clouds of Carina? Doing so takes me back to

my youth, when all I had to guide me was the famous National

Geographic map of the whole sky and my mother’s 8×30 binoculars.

I remember being dazzled by the seemingly endless variety of star

groupings, with sparkling gems galore in every field of view.

One of the prettiest of those groups is IC 2602, also known as the

Southern Pleiades, located about four degrees south of Eta (η) Carinae.

Described by Abbe Lacaille in the 18th century from South Africa, it’s

one of the closest open clusters, around 480 light-years from Earth. Its

70-or-so stars have an average age of about 40 to 50 million years.

We’ve mentioned IC 2602 a number of times in these pages, but it’s

the binary star at its centre that draws my attention this issue. Hardly

anything is known about the secondary star of the Theta (θ) Carinae

pair, but the primary is a magnitude 2.7, hot, blue star (spectral class

B0). Its diameter is a whopping 5.5 times that of our Sun, its mass is 13

times, and its luminosity is a staggering 16,600 times that of our star.

Scientists think it might be a ‘blue straggler,’ a type of giant star formed

when two smaller stars merge. Being the mass that it is, Theta will some

day explode as a supernova.

Just five arcminutes from Theta lies the variable star V518 Carinae,

another member of IC 2602. It too is a B-type star and also possibly a

blue straggler. At magnitude 4.8 it is quite easy to find. It’s brightness

varies by about 0.2 magnitudes with what seem to be two periods,

around 100 days and 970 days.

■ JONATHAN NALLY has progressed from 8×30s to 10×50s, but still loves

starhopping through Carina.

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Star

magnitudes

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USING THE

STAR CHART

WHEN

Early March 10pm

Late March 9pm

Early April 8pm

Late April 7pm

These are standard times —

add Daylight Savings if it applies

to your location.

HOW: Go outside within an hour

or so of a time listed above. Hold

the map above your head with

the bottom of the page facing

south. The chart now matches

the stars in your sky, with the

circular perimeter representing

the horizon and the centre of the

chart being the point directly over

your head (known as the zenith).

FOR EXAMPLE: Look at

the chart, and you’ll see that the

bright star Achernar at the end

of the constellation Eridanus

(The River) is about one-third of

the way from the southwestern

horizon and the middle of the

chart. So if you look to the

southwest, you’ll find Achernar

about one-third of the way up

from the horizon.

NOTE: The map is plotted for

35° south latitude (for example,

Sydney, Buenos Aires, Cape

Town). If you’re much further north

of there, stars in the northern part

of the sky will be higher and stars

in the south lower. If you’re further

south, the reverse is true.

15

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ONLINE

You can get a real-time sky chart

for your location at

skychart.skyandtelescope.com/

skychart.php

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