Astronomy

BINOCULARUNIVERSE

BY PHIL HARRINGTON

66 ASTRONOMY • JULY 2018

T

wo of my favorite

summer star clusters

can be found scraping

the southern horizon

this month. M6 and

M7 trail after the stinger stars

that mark the tail of Scorpius

the Scorpion. Each would be a

lovely sight if viewed alone, but

when teamed together in such

rich surroundings, the scene

becomes singularly beautiful.

To spot them, place the

stinger on the southern edge of

your binoculars’ field of view,

and then look to the north. The

clusters should fit into the same

field. In fact, you might not even

need binoculars to spot them.

Both are bright enough to see

with the naked eye if the sky in

that direction is dark and clear.

M6 is the smaller of the pair.

At first glance, it strikes most

observers as sharply rectangular,

making it unique in a universe

populated with ovals and circles.

But take a closer look, and let

your mind play connect-the-dots

with the stars. See anything

besides a rectangle? Many

observers can trace the outline

of a butterf ly among the stars.

Look for two wings outstretched

from a centered body. The but-

terfly appears to be headed

toward the northwest.

M6 is about 1,590 light-years

from Earth, spans about 10

light-years, and is believed to

be between 90 million and

100 million years old. More

than two dozen cluster stars can

be seen using 50mm binoculars,

while 70mm and 80mm glasses

add another 10 to 15 fainter

points. Most are hot blue-white

infernos.

One of its most massive stars,

however, has left the main

sequence and evolved into an

orange stellar ember. That star,

set east of the cluster’s center

and known as BM Scorpii, is

an irregular variable that f luc-

tuates slowly and erratically

from 6th to 8th magnitude

across an average of 850 days.

Steer to the southeast of M6

to find the next member of

Charles Messier’s catalog. M7 is

larger and brighter than M6, so

it should be even more obvious

through binoculars. In fact, it is

one of the few deep-sky objects

that was known to the ancient

world. Ptolemy was first to

mention it in his epic volume

Almagest, published in a.d. 130.

Today, many refer to this as

Ptolemy’s Cluster, although he

had no way of knowing its true

nature. That revelation had to

wait 15 centuries. In 1654,

Italian astronomer Giovanni

Battista Hodierna published the

first telescopic observation,

recording 30 stars. In 1764,

Messier included it as object

No. 7 in his catalog, describing

it as “a cluster considerably

larger than the preceding” (M6).

One reason M7 appears con-

siderably larger is because it’s

considerably closer, some 980

light-years away. It also spans

twice the space — 23 light-years

versus 10 light-years. However,

both include about the same

number of stars.

At nearly 35° south of the

celestial equator, M7 is the

southernmost object in

Messier’s listing. With a clear

view, you’re in for a real treat.

Even through modest pocket

binoculars, M7 explodes into a

striking assortment of stars

covering an area larger than the

Full Moon. Eighty stars have

been identified as belonging

to the cluster, with dozens of

nonmembers either in the fore-

ground or lying beyond also

contributing to the scene. More

than 30 cluster stars shine

brighter than 10th magnitude,

and as such, should be visible

through 50mm binoculars.

Several also show delicate hues

of yellow and blue, with the

brightest being a yellow beacon

close to the group’s center.

I know it’s an illusion, but my

16x70 binoculars create a three

dimensional effect that makes

many of the brighter stars in M7

look like they’re f loating in front

of a field of fainter stardust. This

visual impact can never be

duplicated in a photograph.

This region of Scorpius also

holds a pair of asterisms visible

through binoculars. Both were

created by the imaginative mind

of Massachusetts amateur

astronomer John Davis, who,

sadly, passed away earlier

this year.

The first, which he chris-

tened the Hockey Stick, lies

about a 7° binocular field west

of M7. Look for a north-south

line of four equally spaced 7th-

magnitude stars. A fifth sun

southwest of the line forms the

stick’s heel. I can squeeze this,

plus both open clusters, into my

10x50s. When you place M7 at

the easternmost edge of the

field and M6 toward the north,

the hockey stick will be visible

along the western edge.

Another Davis creation is

about a binocular field north-

west of the hockey stick. He

dubbed this one the Garden

Trowe l. Three 7th-magnitude

stars point southward to form

the trowel blade, while another

four or so create its northward-

meandering handle. Most of

the Trowel’s stars are white, but

a couple of them shine with a

subtle golden glint.

We will explore more of

Davis’ asterisms in my next

column. Meanwhile, do you

have a favorite binocular target

that you’d like to share? I’d love

to hear about it and possibly

feature it in a future column.

Drop me a line through my

website, philharrington.net.

Until next time, remember

that two eyes are better than

one!

Scorpion

treasures

Scorpius contains

dazzling open

star clusters for

binocular users.

The Butterfly Cluster (M6) in Scorpius is beautiful through binoculars, with dazzling

stars and a curious shape. BERNHARD HUBL

The enormous open cluster M7 is a fine

naked-eye sight. In binoculars, it

explodes with bright stars.

Phil Harrington is a longtime

contributor to Astronomy and

the author of many books.

ALLAN COOK/ADAM BLOCK/NOAO/AURA/NSF