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.
