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Cosmic Challenge: Alcor and Mizar


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Cosmic Challenge: Alcor and Mizar

 

 

June 2021

 

 

 

Phil Harrington

 

 

This month's suggested aperture range:

 

Target

Type

RA

DEC

Const.

Mag

Size

Alcor and Mizar

Double
star

13h 23.9m

+54° 55.5'

Ursa
Major

2.2, 4.0

11.8'

 

Is there any constellation in the sky more universally known than Ursa Major, the Great Bear? Most of us learned of it as a child, perhaps from a relative or friend, or possibly as a Scout working our way toward a merit badge in astronomy. The seven brightest stars in the group, known in North America as the Big Dipper or in England as the Plough, always draw our attention, especially in the spring when they ride highest in our sky.

 

Many cultures incorporated the stars of Ursa Major into their myths and legends.  Ancient Hindu stargazers, for instance, saw the stars in the Big Dipper asterism as Rishi, or "seven scholars."  In Scandinavia , they were known as Thor's Wagon, and also the Wagon of Odin, Thor's father.  In ancient Egypt , the Dipper's seven bright stars were seen as a bull's thigh, while in China , they were symbolic of the government.  Some Arabs and Hebrews alike saw the Dipper stars as a bier, or coffin, while early Christians called it the Bier of Lazarus.

 

Above: Evening star map. Credit: Map adapted from Star Watch by Phil Harrington.


Above: Finder chart for this month's Cosmic Challenge

Credit: Chart adapted from Cosmic Challenge: The Ultimate Observing List for Amateurs by Phil Harrington.
Click on the chart to open a printable PDF version.

 

 

Who exactly was the first person to see the seven Dipper stars as well as the fainter surrounding points as forming a huge bear is long lost to history, but it is interesting to note that this interpretation bridged many cultures that presumably didn't know the others even existed. We find references to a great bear in such diverse sources as ancient Greek and Roman texts and among Native American tribes. Was this strictly happenstance? Or is the common meaning indicative of a shared origin, perhaps with roots in ancient Asia?

There are many legends surrounding our skybound bear. Ancient Greek legend had it that Ursa Major was the heavenly incarnation of Callisto, a beautiful young woman with whom Zeus himself fell in love. Fearing the jealous rage of his goddess wife Hera, Zeus transfigured Callisto into a bear and placed her in the sky for safety. He also placed their son, Arcas (after the Greek word arktos, or "bear") near her side. Today, we know them as Ursa Major and Ursa Minor, respectively.

 

Half a world away and before European influence could be felt, many Native American peoples had myths of celestial bears portrayed among the same stars. My favorite comes to us from the Iroquois and Algonquin tribes of eastern North America. They each tell a story of how, every spring, a vicious bear left its den to wreak havoc on their tribes by killing many people and eating their food stores. Finally, in a desperate attempt to rid them of this menace, the tribal leaders sent their three mightiest Indian warriors out after the bear. In this story, the stars in the Dipper's handle represent those warriors rather than the Bear's unusually long tail so often depicted in drawings. The Indians continue to chase the bear across the sky throughout the summer. Then, in the autumn, as the Bear grows weary, one of the Indians injures it with an arrow. Blood drips from the wound and colors the leaves of the forest in pastel shades of red, orange, and yellow. What finally happens to the Bear? Apparently, the wound is not fatal, as both it and the relentless hunters return to the sky every spring.

 

Above: The stars of the Big Dipper near the northwestern horizon from Stellafane.

 

If you look carefully, you can even see that the middle star in the Dipper's handle is joined by a nearby companion star.  The Algonquins saw this faint cohort as the large pot that one of the hunters was carrying on his back to cook the bear in once they captured it.  Other tribes knew the two close-set stars as a horse and rider, and often used them as a vision test for their own hunters, just as Greek warriors did.

 

Today, we know the horse and rider by their given Arabic names, Mizar and Alcor.  Mizar, the brighter of the two, shines at magnitude 2.3, while Alcor is magnitude 4.0, or about five times fainter. To ancient Chinese stargazers, this was Foo Sing, the Supporting Star, while in Latin, it was called Eques Stellula, or Little Starry Equestrian.

 

Although it is not the tell-all check that ancient star-watchers thought, seeing the Little Starry Equestrian by eye remains a fun test to try from just about any spot in the northern hemisphere. The pair is separated by 11.8 arc-minutes, well within the resolution limit of most peoples' eyes.  Both should be visible even through only marginally dark suburban skies.  Look for Alcor to the east-northeast of Mizar.

 

That raises an interesting, and wildly disputed, question: what is the angular resolution of the human eye?  Depending on the study, numbers can vary greatly.  Part of the problem has to do with the conditions under which the tests are conducted.   The human eye has two basic types of photo-receptors: cones, which respond to bright-light conditions, and rods, which respond to low-intensity light.  It turns out that the resolving ability of rods is approximately 7 arc-minutes, only a fraction of the maximum resolution of the cones.

 

Of course, your personal results may vary depending on your own visual acuity.  With sharp vision, a person may easily exceed this number, while a myopic observer may not come close.  To judge your own eyes' angular acuity, try this simple test.  Draw two black dots separated by 0.0625 inch ( 2 mm ) on a white index card and tape the card to a wall in a properly lit room.  Close one eye and back away from the card slowly.  Stop when the two black dots can no longer be resolved, but instead merge into one.  Measure your distance to the card to the nearest foot.  Plug that distance into the table below to find your eye's angular resolution value.

Angular Resolution Test

Distance to card (feet)

Angular resolution (arc-minutes)

1

27

2

13.5

3

9

4

7

5

5.5

6

5

7

4

8

3.5

9

3

10

2.5

 

Don't discount the resolving ability of the human eye.  Today we rely on telescopes and attached instruments to determine the positions of sky objects to a small fraction of an arc-second, but early astronomers used only their eyes to make some startling discoveries.  Perhaps the most amazing of all was Johann Kepler's (1571-1630) three laws of planetary motion, which he based largely on naked-eye observations of planetary positions by his mentor, Tycho Brahe (1546-1601).  Brahe created a clever device that coupled a transit instrument with a giant protractor of sorts, and placed it in an observatory that he called Uraniborg.

 

Above: Tycho Brahe sits in his Uraniborg observatory. Unknown author, Public domain, via Wikimedia Commons

 

Brahe's sole mission was to record the times and angles when each of the naked eye planets appeared in a narrow reference window in his observatory.  He did so meticulously for more than two decades.  After Brahe's death, Kepler tried to use these observations to confirm the Copernican model theory that placed Earth and the other planets in circular orbits around the Sun.  Try as he might, he could not get the numbers to agree with the observations.  Finally, Kepler was forced to conclude that, while the planets did indeed orbit the Sun, they did so in elliptical paths.  Millennia of misconception had been corrected all because of the precision of the human eye.

 

Good luck with this month's challenge! And be sure to post your results in this column's discussion forum.

 

Remember that half of the fun is the thrill of the chase.  Game on!



About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy, including Cosmic Challenge: The Ultimate Observing List for Amateurs.  Visit www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2021 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.

 


  • okiestarman56, John O'Hara, Tyson M and 4 others like this


11 Comments

An excellent target for people new to astronomy.

I had a couple friends over the other night while I was letting my scope cool down so I decided to show them mizar.

Almost everyone knows what the "dipper" looks like so it was easy to direct them to the middle star in the "handle".

I asked them to look closely and see if they could make it out as a double, they could.

Then they looked through a small refractor at 12x (my finder scope) and they both clearly saw mizar & alcor along with hd116798 making its nice triangle. Then I dropped an EP into my 10" and let them look at 80x, they both said something along the lines of "wow I had no idea you could get so close".

They both had to leave so I didn't go all esoteric and try to split mizar for them, that will have to wait for another day :)

Anyways a great target for showing people the power of a scope and getting people interested. I could be crazy but I could have sworn I saw their 8th magnitude friend between them with my naked eye that night.
    • PhilH, Karl15, Eclipsed and 2 others like this
I must admit to having never been able to easily split these two stars without optical aid. Nowadays my eyes are certainly past their prime, having had retina surgery twice in my right eye (which was my better eye) and then cataract surgery on both eyes a couple of years later. But, this is an interesting challenge and I’ll give it another shot as soon as a good clear evening comes along!
    • PhilH and piperwhite62 like this
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vrodriguez2324
Jun 02 2021 11:01 AM

I do indeed have fond childhood memories of looking up at the night sky many nights and being able to identify the big dipper and the little dipper. Although now I know that what I once thought was the little dipper is actually the Pleiades. I enjoy observing the sky with the naked eye to look for interesting targets. I make a conscious effort to scan slowly and look for faint stars. In my experience the longer I look at a portion of the sky, the more it shows me. From my light polluted backyard I am limited to the brighter stars so naturally the Big Dipper is a target I frequently visit.  I started this hobby last summer after Comet Neowise paid us a visit so this season is the first time I have paid closer attention to the "Plough". Last month as I directed my vision between and around the stars I noticed something in the middle of the handle, a nice naked eye double that I could easily resolve. Mizar and Alcor, how could I have missed you so many times in the past? I then reached for my binoculars to increase the magnification 10 fold. The view was even better. I then aimed my telescope towards the middle of handle at 46x. To my pleasant surprise one of the stars in the pair was actually also a double! A dark velvet sliver of sky separating the bright pair, absolutely beautiful. 

    • PhilH, jim kuhns and piperwhite62 like this
Photo
piperwhite62
Jun 02 2021 12:55 PM

I do indeed have fond childhood memories of looking up at the night sky many nights and being able to identify the big dipper and the little dipper. Although now I know that what I once thought was the little dipper is actually the Pleaides. I enjoy observing the sky with the naked eye to look for interesting targets. I make a conscious effort to scan slowly and look for faint stars. In my experience the longer I look at a portion of the sky, the more it shows me. From my light polluted backyard I am limited to the brighter stars so naturally the Big Dipper is a target I frequently visit.  I started this hobby last summer after Comet Neowise paid us a visit so this season is the first time I have paid closer attention to the "Plough". Last month as I directed my vision between and around the stars I noticed something in the middle of the handle, a nice naked eye double that I could easily resolve. Mizar and Alcor, how could I have missed you so many times in the past? I then reached for my binoculars to increase the magnification 10 fold. The view was even better. I then aimed my telescope towards the middle of handle. To my pleasant surprise one of the stars in the pair was actually also a double! A dark velvet sliver of sky separating the bright pair, absolutely beautiful. 

I also started around the time of Neowise. Mizar and Alcore was one of the first doubles I looked at. Mizar and Alcore are about 1 light year apart and are both spectroscopic binaries each with their own dwarf star orbiting them, so you're really looking at six stars. Alcor's dwarf star is too close to resolve with a normal telescope but you should be able to see the tiny dwarf star next to Mizar at low magnification.

 

Clear skies

    • PhilH, jim kuhns, Eclipsed and 1 other like this

We finally had some above average seeing and average transparency last night in my suburban Chicago backyard.  Easily split Mizar and Alcor naked eye.  This happens maybe once per season around here.

 

So happy to finally complete one of these challenges.

 

-Mike 

    • PhilH and piperwhite62 like this

When I grow up I want to be as amateurish as Phil Harrington, the SUPER AMATEUR!

 

    amateur-astronomer-looking-through-a-tel

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Dave Mitsky
Jun 03 2021 09:33 PM
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JamesDuffey
Jun 03 2021 10:01 PM

I recommend Alcor and Mizar  to beginners as one of the first things to look at. It is easily found and placed in the FOV of the scope, giving the newcomer confidence that they can find stuff. It is a different view of a common object. As one increases magnification, more and more is seen, a good experience in changing eyepieces and seeing old things in a new way.

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ChrisCharlesJax
Jun 04 2021 12:46 PM

I learned yesterday that this is actually a system of six stars. Pretty cool.

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Astro Canuck
Jun 04 2021 01:12 PM

In the before time -- when we had observing sessions on the university roof after planetarium shows I would ask people to look at Mizar and what do you see?   never say 2 stars as the psychological factor will play.. some saw the fainter one  - Alcor and some did not .. the age

range was interesting, some not all younger people/students 10-25 years old could not see it, some older people 40-55 saw it in a snap..!     was interesting.. use to tell the Native American version.. Horse and Rider...  will we ever do these sessions again...not for 1-2 years till virus is killed better.  frown.gif  

Mizar and Alcor are one of the favorite targets for our public outreach programs.  I'm a member of the National Capital Astronomers club and we hold our outreach programs in Rock Creek Park in Washington DC.  For anyone who is at all familiar with Washington, you'll quickly realize that this is not a dark sky location by any stretch of the imagination.  At best you can pick out 3rd or 4th magnitude stars.

 

But this is where the people are!

 

As a consequence, we are limited to only the brightest objects.  But people are thrilled to get a peak at the planets, the moon, and some of the easier to find bright objects.  Many have never had a look through an astronomical telescope.

 

With Mizar and Alcor, I usually start by pointing out the Big Dipper and having them look at the center star in the handle.  Many can see that there's a faint companion.  (I haven't kept statistics, but most younger people can see the pair easily.  Many of the older people are aware that it's a binary, so it's hard to tell if they can really pick them apart or not.)

But then we switch to a telescope, usually my 4" Meade semi-APO refractor.  And that catches their attention.  All of a sudden there are Mizar and Alcor on a velvety black background.  And then they realize that Mizar is itself a double star.

 

At this point, we frequently get into comparisons with the fictional double "sun" of the planet Tatooine from Star Wars along with the fact that each of these three stars are in fact doubles themselves.

 

P.S.  Based upon the double dot test, I can resolve somewhere between 2.5 and 3 arc-minutes with one eye, slightly less with the other.  But I have problems with my contact lenses drying out at night and blurring my vision without me noticing it immediately.



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