Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Tonight's Sky — Change location

OR

Searching...

Tonight's Sky — Select location

Tonight's Sky — Enter coordinates

° '
° '

Check out the Big Dipper!

Harrington
The most famous pattern of stars north of the celestial equator, the Big Dipper dominates this month’s late-spring sky. Its high position 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 riddle 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 identify 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.

ASYPH0618_02copy
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
Their shared movement through space was first suspected 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 members 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 reasonably dark skies, you should be able to detect without any optical aid that Mizar is accompanied 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 flattened triangular pattern.

ASYPH0618_01copy
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
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 Italian astronomer Benedetto Castelli, a friend of Galileo. Galileo went on to confirm his discovery.

Then 240 years later, on April 27, 1857, Mizar became the first binary ever photographed 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 photographic 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 binocular field that I mentioned earlier? It holds its own interesting footnote in astronomical history. In 1722, German mathematician Johann Liebknecht thought he saw the star shift against the background from one night to the next. He concluded that it was not a star at all, but rather a new planet orbiting the Sun. In his excitement, 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!

0

JOIN THE DISCUSSION

Read and share your comments on this article
Comment on this article
Want to leave a comment?
Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Login or Register now.
0 comments
ADVERTISEMENT

FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy.

ADVERTISEMENT
ADVERTISEMENT
Apollo_RightRail

Click here to receive a FREE e-Guide exclusively from Astronomy magazine.

Find us on Facebook