Astronomy

BINOCULARUNIVERSE

BY PHIL HARRINGTON

68 ASTRONOMY • JUNE 2018

T

he most famous pat-

tern of stars north of

the celestial equator,

the Big Dipper dom-

inates this month’s

late-spring sky. Its high posi-

tion as the late-evening sunset

fades makes it a prime hunting

ground for our binoculars.

Let’s begin with the closest

open cluster visible from Earth,

cataloged as Collinder 285.

Nearly every resident of the

Northern Hemisphere has seen

it at least once, yet few know it

exists. If this sounds like a rid-

dle to you, in a way I suppose it

is. The five brightest stars in

Collinder 285 belong to a much

more famous asterism — the

Big Dipper itself.

If we could compare the

positions of the Dipper’s seven

stars 100,000 years from now to

how they appear today, we

would be hard-pressed to iden-

tify the familiar figure. But

even though the familiar bowl-

and-handle pattern will be lost

over that stretch of time, five of

the stars will still move with a

common proper motion.

Their shared movement

through space was first sus-

pected by Richard Proctor in

1869, and was confirmed three

years later by William Huggins.

Studies conclude that at least 16

stars belong to this weak open

cluster. The group is about 75

light-years away, and it is spread

across an area spanning 18 by

30 light-years. That translates

to an apparent diameter of over

23°. The more prominent mem-

bers include the Dipper stars

Merak (Beta [β] Ursae Majoris),

Megrez (Delta [δ] Ursae

Majoris), Alioth (Epsilon [ε]

Ursae Majoris), Phecda

(Gamma [γ] Ursae Majoris),

Mizar (Zeta [ζ] Ursae Majoris),

and Alcor (80 Ursae Majoris).

Other cluster members that

have struck their own path but

continue to show similar proper

motions include Alpha (α)

Coronae Borealis, Beta (β)

Aurigae, and brilliant Sirius

(Alpha [α] Canis Majoris).

Let’s examine one of the

prominent core members of the

group, 2nd-magnitude Mizar,

marking the central crook in

the Big Dipper’s handle. If you

have good eyesight and reason-

ably dark skies, you should be

able to detect without any opti-

cal aid that Mizar is accompa-

nied by a fainter companion to

the east. That’s 4th-magnitude

Alcor, another core member.

Both have been well known for

millennia. Arabic cultures, for

instance, imagined them as the

“Horse and Rider” galloping

across the sky.

Swing even the smallest

pocket binocular their way, and

both easily resolve into white

beacons. You might also see an

8th-magnitude field star

through binoculars that joins

Alcor and Mizar to form a f lat-

tened triangular pattern.

Given monstrous binoculars,

like my 25x100s, Mizar resolves

into two tightly packed points

separated by 14". The brighter

star is known as Mizar A, while

the dimmer is Mizar B. Mizar’s

duality was first recorded in

1617 by Ita lia n astronomer

Benedetto Castelli, a friend of

Galileo. Galileo went on to con-

firm his discovery.

Then 240 years later, on

April 27, 1857, Mizar became

the first binary ever photo-

graphed through a telescope.

That night, using the 15-inch

refractor at Harvard College

Observatory, photographer

John Whipple and observatory

director George Bond captured

Mizar A and B on a glass pho-

tographic plate.

Nearly half a century later,

studies showed that both Mizar

A and Mizar B are themselves

spectroscopic binaries, making

this a quadruple star system.

Until recently, astronomers

believed that while the stars

shared a common proper

motion, Alcor and Mizar were

too far apart to be true physical

companions. That changed in

2009, when two research teams

independently discovered that

Alcor is orbited by a dim red

dwarf companion. Examining

revised parallax data for Alcor

and Mizar, both studies

concluded that the red dwarf,

and Alcor itself, are in fact

gravitationally linked to Mizar.

The discovery turns Alcor and

Mizar into a sextuple star

system.

Oh, and that 8th-magnitude

field star visible in your binocu-

lar field that I mentioned ear-

lier? It holds its own interesting

footnote in astronomical his-

tory. In 1722, German math-

ematician Johann Liebknecht

thought he saw the star shift

against the background from

one night to the next. He con-

cluded that it was not a star at

all, but rather a new planet

orbiting the Sun. In his excite-

ment, he christened it Sidus

Ludoviciana (“Ludwig’s Star”)

after Ludwig V, then the king of

Germany. It eventually became

apparent that Liebknecht was

mistaken, but the star is still

called Sidus Ludoviciana nearly

three centuries later.

Have a favorite binocular

target that you’d like to share

with the rest of us? I’d love to

feature it in a future column.

Drop me a line through my

website, philharrington.net.

Until next time, don’t forget:

Two eyes are better than one!

Check out the

Big Dipper!

The closest moving group of associated stars

offers plenty for binocular gazers. The Big Dipper is perhaps the most easily recognizable star group in the sky. It also

constitutes a moving group of stars, with most of them physically linked in space. JEFF DAI

Alcor and Mizar

make up one of the

most beautiful

multiple star

systems, as seen in

this telescopic

exposure. Alcor

is the fainter star

between and just

below the brighter

twin suns of Mizar

A and B. GREGG RUPPEL

Phil Harrington is a longtime

contributor to Astronomy and

the author of many books.