Sky & Telescope - USA (2020-06)

M101

HD 116798

Alcor

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Mizar

URSA MAJOR

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WHEN TO

USE THE MAP

Late April 2 a.m.*

Early May 1 a.m.*

Late May Midnight*

Early June 11 p.m.*

Late June Nightfall

*Daylight-saving time

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URSA

MINO

Polaris

Spica

Virgo

Cluster

Regulus

Denebola

Sickle

M44

M67

Castor

Pollux

M3

Arcturus

Mizar& Alcor BigDipper

M51

M81

M82

Thuban

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June 2

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May 29

MoonMay 26

Binocular Highlight by Mathew Wedel

I

have shown Mizar and Alcor — the “Horse and

Rider” in Ursa Major, the Great Bear — to hun-

dreds if not thousands of people at star parties and

outreach events. And for just that reason it never

hurts to brush up on the fundamentals.

Mizar and Alcor are roughly 80 light-years

away, but the parallax distance measurements

have large margins of error. We are certain the two

stars — or rather, the two systems — are moving

together through space as members of the Ursa

Major association. But after almost four centuries of

study, it’s still unclear if they’re orbiting each other or

not. If they are gravitationally bound, it’s a long orbit,

taking 700,000 years or more to complete.

I mentioned that Mizar and Alcor are systems, not

single stars. Mizar is at minimum a quadruple-star

system and Alcor is at least double. You probably

already know about Mizar, since splitting the two

brighter and wider components is a favorite trick with

small telescopes or big binoculars. Mizar A and B are

separated by 14′′, which is doable with 20× tripod-

mounted binos but very tough with any less magni -

cation. If you haven’t split Mizar, grab your big glass

and see what you can do.

While you’re in the neighborhood, check out

HD 116798, a yellow-white star a bit south of the line

between Mizar and Alcor. At magnitude 7.6, it should

be visible without optical aid to the eagle-eyed and

an easy catch in binoculars. In 1722, German theolo-

gian and scientist Johann Georg Liebknecht named

the star Sidus Ludoviciana, thinking it to be a planet.

Liebknecht was mistaken about that, but he studied

both fossils and astronomy, so he’s okay in my book.

¢MATT WEDEL loves contemplating the night

sky on dinosaur digs in Oklahoma and Utah. He’s

probably out there right now.

The Horse and Rider

skyandtelescope.org• JUNE 2020 43