Hi everyone, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences,
welcome to The Night Sky.
If you pay any attention at all to the sky then you’ll have no doubt noticed that it’s
always in motion, always changing.
As one season gives way to another we see a given set of constellations disappear only
to be replaced by others.
And from night to night we see stars rise in the east, move across the sky, and then
set in the west.
Of course, all this apparent motion is due to the Earth moving, and not the stars.
The seasonal changes in our sky occur because the Earth is orbiting the Sun and the nightly
motions are due to the Earth’s spin upon its axis.
But here in the northern hemisphere we can see a section of the sky that, while always
in motion, in some ways stays pretty much the same.
A quick way to see this is to look at time lapse photos or videos that show “star trails”.
A star trail is a long exposure photograph of stars that captures the apparent motion
of the sky in the form of long streaks of star light.
Facing towards the south we see the stars appear to rise in the east and then travel
in great arcs across the sky before setting in the west.
However, looking towards our northern sky, we see the trails form a set of concentric
circles of star light.
These stars never rise or set as seen from middle northern latitudes and we refer to
them as being “circumpolar”.
The southern hemisphere has its own set of circumpolar stars that never rise or set as
well.
If you happen to be directly at either the north or south pole then all the stars you
see at night are circumpolar, standing along the equator and there's
no such thing as circumpolar stars in your sky.
It’s only in the mid latitudes that you have a group of stars that never rise or never
set.
In this episode of The Night Sky we’re going to look at the northern hemisphere’s most
famous and most important circumpolar stars and they reside in the constellations of the
big and little bears, Ursa Major and Ursa Minor.
But you might know them better as the Big and Little Dipper.
Aside from the big and little bear, other northern hemisphere circumpolar constellations
include: Cassiopeia (the vain queen of Greek mythology), her husband Cepheus, Draco the
Dragon, and the little known and quite faint, Camelopardalis (which is said to be a giraffe
and was first introduced in the 1600’s by a Dutch theologian for …well, reasons that
still elude most of us).
While everyone knows the “Big Dipper” and “Little Dipper” what you may not know
is that they are not official constellations.
They're called “asterisms”, these are unofficial star patterns formed from the stars found in one
or more of the constellations that are officially designated by the International Astronomical
Union, the governing body in astronomy when it comes to standardizing the names of things
in the night sky.
While you and I see a dipper when we look at these stars not everyone else does.
In Ireland and the United Kingdom these stars are seen as a plough, while in northern England
some folks recognize it as a cleaver.
Throughout the British Isles and parts of western Europe other people see a wagon, or
“wain”.
This has an old tradition that goes as far back as Norse legend with Scandinavian people
referring to it as Odin’s Wain.
In Chinese sky lore, we see a celestial bureaucrat in a cart being followed by two loyal petitioners
while in Burma other folks look up and see a giant prawn in the heavens.
According to Greek mythology, the Big Bear was once a beautiful woman named Callisto
who catches the eye of the King of the Gods, Zeus.
From their union comes a baby boy named Arcas.
But Jupiter’s wife, Hera, finds out about her husband’s indiscretion and, in a fit
of jealous rage, she changes Callisto into a bear.
Years later, Callisto, while roaming the forest in her bear form, sees a young man armed with
with a bow and arrow hunting in her part of the woods.
She recognizes the youth as her own son, Arcas, and raises up on her hind legs and walks towards
him with arms spread wide to embrace him.
Arcas only sees a large, ferocious bear coming towards him and he notches his bow and fires off an arrow at the bear
Fortunately, Zeus sees what’s happening and stops the arrow in mid-flight.
Once Zeus explains the situation Arcas demands that his mother be turned back into a human
once more.
Unfortunately, Zeus cannot undo Hera’s magic and to protect mother and son he transforms
Arcas into a bear as well.
Seizing the bears by their tails he slings them around and around and then hurls them
into the sky where they can live out the rest of their lives among the stars.
When you sling a bear by the tail into the sky something must give and this explains
why it is that when you look at depictions of Ursa Major and Ursa Minor you’ll notice
that both bears have very long tails and not the short, stubby ones that we see in all
other bears.
Now Hera was still not happy about things, she arranged it so that while the bears circle
around the sky they would never be able to refresh themselves by swimming in the oceans
of the Earth.
This part of the story explains why it is that from most parts of the northern hemisphere
we never see the bears dip below the horizon.
Ursa Major, the big bear, is the third largest constellation in our night sky, occupying
some 1,280 square degrees upon the sky (that’s about the size of two full hand widths held
out at arm’s length).
It is one of the original 48 constellations described by Greek astronomer Ptolemy back
in the second century A.D. and is even referred to in Homer’s “Odyssey” and the Bible.
But some scholars think that the origins of Ursa Major go as far back as 50,000 years
when various bear cults were thought to have existed among different Paleolithic cultures
from around the northern hemisphere.
Curiously, there have been other cultures widely separated from the Ancient Greeks in
both time and location who also saw a bear in this part of the sky.
The Micmac Indians of New Brunswick and Nova Scotia, along with many other Native American
cultures also saw a bear in the pattern of stars we call the Big Dipper.
The Micmac, like many other people whose lives are intimately tied with nature, noticed that
the Great Celestial bear traveled around the sky in circles each night and that its orientation
upon the sky was also determined by the seasons.
For example, during the spring the Bear/Dipper is high up in the sky but during the fall
it’s very low upon our horizon.
During the summer and winter, it’s placed intermediately upon the sky.
To explain the seasonal wanderings of the celestial bear and how it’s related to seasonal
changes in nature, the Micmac have a story that uses a hunt as a metaphor.
According to Micmac tradition, the four stars in the bowl of the dipper represents the bear
while the three stars of the dipper’s handle are the hunters.
But these aren’t human hunters, they are three birds.
The star closest to the handle is Robin, the middle star is Chickadee, while Moose Bird
(otherwise known as the Grey Jay) brings up the rear.
Our story begins in the spring time when the celestial bear awakes from hibernation and
crawls forth from its winter den.
The three friends begin their hunt and finally, by the time fall arrives, have managed to
catch up to the bear.
Robin shoots the bear with an arrow and during the struggle he gets a lot of blood on
his feathers.
He shakes most of it off and it falls to the Earth only to stain the fall foliage with
red.
Try as he might, Robin cannot get rid of the red that stains his breast.
The bear dies from its wound and the three friends cook the bear in a great pot.
As the fat bubbles over the sides of the pot it too falls from the sky, and stains the
leaves yellow.
By winter the skeleton of the bear lies upon its back in the sky but the spirit of the
bear now inhabits the body of another bear and when it wakes up and leaves its den the
hunt is going to begin all over again.
The constellation of Ursa Major is made up of about 20 stars but there are only seven
that you will be able to see from most city locations and they make up the asterism of
the Big Dipper: 3 stars for the handle, 4 for the bowl.
Starting with the tip of the handle and working our way inwards the names of the stars are
Alkaid, Mizar, and Alioth.
The bowl stars’ names are: Megrez, Phecda (sometimes known as Phad), Dubhe, and Merak.
The stars in most constellations and asterisms are physically unrelated to one another but
not so with the stars of the Big Dipper.
Except for Alkaid and Dubhe, the stars that form the dipper are part of an open star cluster
that are all loosely joined together by gravity.
This assemblage of stars is known as the “Ursa Major Moving Group” and contains a total
of 14 stars, 13 of which are found in Ursa Major.
Stars are formed in “litters” or “clusters” within vast clouds of hydrogen gas and dust.
Given time, they slowly disperse from their nurseries and spread themselves out amongst
the galaxy.
This is a slow process.
The stars that make up the Ursa Major Moving Group all formed about 300 million years ago,
at that time the planet Earth was dominated by two supercontinents and the early reptiles
were just beginning to diversify into the ancestral lines that would give rise to turtles,
lizards and snakes, dinosaurs, and the mammals.
If you come back in another 100,00 years the Big Dipper will still look much the same but
with some noticeable differences.
Go out some night and take a good look at Mizar, the middle star in the handle of the
Big Dipper.
Do you see just one star or two?
Well, unless your skies are badly light polluted or you just have terrible vision, you should
see that Mizar has a faint companion right next to it.
This is Alcor and to the ancient Arabs Mizar and Alcor were collectively referred to as
“the horse and rider” and being able to split the two easily was said to be a test
of just how good your vision was.
Oh, and remember that pot the birds used to cook the bear in with that story from the
Micmac?
It was being carried by Chickadee and Chickadee corresponds to the star Mizar while Alcor
is the pot he was assigned to carry during the hunt.
Mizar and Alcor were long thought to be optical binaries, two stars that just appear close
together by line of sight and not physically related to one another but we now know that
they are indeed connected gravitationally to one another.
What’s more, this isn’t just a binary system.
With a small backyard telescope you can see a third star in this group.
But wait, there’s even more!
In recent years astronomers have determined that Mizar has two unseen companions while
Alcor has one, making the Mizar-Alcor association a sextuplet system of stars.
Now that I’ve got you outside looking at Mizar and Alcor I want you to now turn your gaze
to the two stars making up the front of the bowl: Merak and Dubhe.
These two are known as the “pointer stars” and they can lead us to the star Polaris,
tip of the handle of the Little Dipper and the tip of the tail in the Little Bear in
the constellation of Ursa Minor.
With your mind’s eye draw an imaginary line in between the two pointer stars and extend
it for about 5 times the Merak/Dubhe distance and you will come upon Polaris.
Polaris is one of the most famous stars in our night sky because it marks where the north
celestial pole is upon the dome of the sky, the pivot point around which all the other
circumpolar stars seem to rotate, and it points our way north should we ever get lost, hence
the name, “The North Star”.
Being such an important star in our sky many people think that it is also the brightest,
but sadly, this just isn’t the case.
Polaris is not even in the top 10 list of brightest stars.
Nor will you find it among the top 20 or even the top 40.
Nope, Polaris comes in at number 46 among the brightest stars in our night sky with
a magnitude of +2.
Polaris has been assisting mariners and lost travelers for time immemorial by not only
pointing them northwards but also by telling them their latitude upon the globe.
Yes, the altitude of Polaris upon your sky also corresponds to your latitude.
For example, if you are standing at the North Pole, at a latitude of 90 degrees, then Polaris
is directly overhead, 90 degrees relative to your horizon.
Standing here in Little Rock you are at 35 degrees north latitude while Polaris is positioned
at 35 degrees upon the sky relative to the horizon.
One of my favorite stories about how the Big Dipper and Polaris have been used to aid travelers
comes to us from the American Civil War.
According to the story there was a mysterious figure in the Underground Railroad known as
Peg Leg Joe who had a song known as “Follow the Drinking Gourd” that he taught to various
people and which was then disseminated among slaves in the South.
The drinking gourd of course is the Big Dipper and the lyrics of the song are a very clever
code that points out various topographical and celestial landmarks that could lead slaves
in Mobile, Alabama northwards and eventually to freedom.
The dipper has meant many things to many people throughout history but as a symbol of freedom
from slavery and tyranny is perhaps my favorite of all.
Until we meet again I urge you to visit our web site for more astronomy content and, as
always, please take just a little bit of time to step outside and look up in both awe and wonder.