2019-07-01_Australian_Sky_&_Telescope

42 AUSTRALIAN SKY & TELESCOPE July 2019

Centaurus clusters

T

his issue we take a look at a few objects in the region

surrounding Alpha Centaurus. The whole constellation of

Centaurus is a goldmine of deep sky objects and star fields, and

really rewards the time you spend sweeping around with binoculars.

Our first object is NGC 5617, an open star cluster situated very

close to Alpha. It contains about 200 stars of around magnitude 8

or fainter. It was discovered by John Dunlop in 1826; he described

it as “a cluster of small stars of mixed magnitudes, considerably

congregated towards the centre, 4’ or 5’ in diameter,” while John

Herschel saw a “Class IV object, very rich; irregularly round; pretty

much compressed in the middle but scattered at borders; 15’; there

are three stars of 10th magnitude, 5 or 6 stars of 11th magnitude;

the rest below 11th.” What do you find when you observe it?

Interestingly, nearby is the fainter Trumpler 22, another open

cluster and one that seems to be gravitationally bound to NGC 5617,

as they have similar stars and are moving in the same direction.

Astronomers think the two are about 70 million years old and have

described them as “a primordial binary cluster pair”. It’s thought

that perhaps 10% of Milky Way open clusters are double or multiple.

A little further away is tiny NGC 5606, a much younger open

star cluster — it’s thought to be only about 6 million years old. This

grouping is compact and faint and seems to have very few stars

compared to many other clusters.

■ JONATHAN NALLY would like to do something about a tree that’s

blocking his binocular view of the sky. h^21

SAGIT TA

A Q U I L A

CAPRICORNUS

PISCIS

AUSTRIN

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WHEN

EarlyJune 10 p.m.

LateJune 9 p.m.

EarlyJuly 8 p.m.

LateJuly 7 p.m.

These are standard times.

HOW

Go outside within an hour or

so of a time listed above. Hold

the map out in front of you

and turn it around so the label

for the direction you’re facing

(such as west or northeast) is

right-side up. The curved edge

represents the horizon, and the

stars above it on the map now

match the stars in front of you

in the sky. The centre of the

map is the zenith, the point in

the sky directly overhead.

FOR EXAMPLE: Turn the

map so the label ‘Facing North’

is right-side up. About halfway

from there to the map’s centre

is the bright star Arcturus. Go

out and look north nearly half-

way from horizontal to straight

up. There’s Arcturus!

NOTE: The map is plot-

ted for 35° south latitude (for

example, Sydney, Buenos

Aires, Cape Town). If you’re

far north of there, stars in the

northern part of the sky will be

higher and stars in the south

lower. Far south of 35° the

reverse is true.

ONLINE

You can get a real-time sky chart

for your location at

skychart.skyandtelescope.com/

skychart.php

USING THE

STAR CHART

–

Star

magnitudes











BINOCULAR HIGHLIGHT by Jonathan Nally

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