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Constellations and Asterisms: Ursa Major
Kamara Wright
A constellation is a group of stars with small visual angular separations.
Constellations are anciently and culturally recognized as many cultures looked to the skies
for answers to questions about life on earth, or even just for entertainment.
There are two sets of constellations: an ancient set, and a more recent set. Ancient
constellations are in the shapes of mythological beasts and great heroes; there are 48 of
these in total that are still used today. The more modern set includes 40 constellations in
the shapes of microscopes and telescopes. As you can probably infer, these sets are so
drastically different in shape from each other reflecting who was looking up into the sky.
The early constellations were most likely seen by the naked eye by cultures wanting to see
these creatures mapped in the sky to have an explanation for occurrences here on earth.
The later set of constellations were mostly viewed during the time in technology increase,
where microscopes and telescopes were coming in to being, and viewed by scientists or
astronomers.
Today we will look at the constellation Ursa Major. This is an ancient constellation
first noted by Greek astronomers. Abbreviated UMa, it is the 3 rd largest constellation in the
sky, and located in the northern hemisphere. More precisely, it is located at 10.67h right
ascension, and +55.38 declination, covering about 1280 square degrees of the sky.
If you look at the diagram of Ursa Major, you will see that it resembles a four-legged
animal. Traditionally, Ursa Major is called “the Great Bear” of the northern sky, and this
stems from the various legends told about the constellation. However, before we get to
these stories, let us look at the individual components of the constellation.
Ursa Major is made up of 21 stars, along with 7 other messier objects. Most of the
stars are named; only 6 are not. All of the stars, though, are labeled with a Greek letter
according to magnitude. The magnitude scale labels the stronger, brighter objects with low
and sometimes negative numbers, and the weaker, dimmer objects with high numbers.
(Take a really bright star we will call A and a dimmer star we will call B. A is obviously
brighter than B, but on the magnitude scale A will be a smaller number than B. Perhaps A
will be magnitude 1 and B will be magnitude 12.) The Greek letter labeling system starts
with assigning the first letter of the Greek alphabet, alpha (α), to the highest magnitude star
(which also has the lowest number on the magnitude scale), the second letter of the
alphabet (beta, β) to the next highest magnitude star, and so on until all of the stars of the
constellation are labeled. This means that Ursa Major has stars labeled from alpha to chi, as
omicron and pi are the same star. Here is a list of the stars and their names:
Alpha: Dubhe
Beta: Merak
Gamma: Phecda
Delta: Megrez
Epsilon: Alloth
Zeta: Mizar
Eta: Alkaid
Theta: Sari Banat Al Na ash
Iota: Talitha
Kappa: unnamed
Lambda: Tania Borealis
Mu: Tania Australis
Nu: Alula Borealis
Xi: Alula Australis
Omicron, Pi 1/2: Muscida
Rho: unnamed
Sigma: unnamed
Tau: unnamed
Upsilon: unnamed
Phi: unnamed
Chi: Al Kaphrah
On a side-note, this constellation contains two notable stars: Mizar and Alula
Australis. Mizar was the first double-star discovered through a telescope in 1662, the first
star to be photographed in 1857, and the first star identified as a spectroscopic binary in
1889. Alula Australis was the first binary star to have its orbit calculated in 1928.
It was also said earlier that there are 7 messier objects in the Ursa Major
constellation area. Messier objects are non-stellar objects visible in the northern sky today
that were also visible to Messier who catalogued all of his noted objects and published the
catalogue in 1774. These objects are denoted by the letter M and a number.
M40 is called Winecke 4 and is a double star.
M81 is Bode’s Galaxy or Bode’s Nebula, and is a large 10’ x 4’ spiral galaxy oriented
NNW-SSE. It has a bright core with a stellar nucleus and spiral arms.
M82 is Cigar Galaxy, an irregular galaxy, roughly 10’ x 3’. It is oriented NE-SW with
tapering ends and a large amount of mottling.
M97 is the Owl Nebula, a planetary nebula around 3’ in diameter. It looks like a gray
puff of light with a slightly brighter center. At times, the ‘eyes’ of the owl can be seen as two
slightly darker spots.
M101 is called the Pinwheel spiral galaxy. It is a large, 7’ in diameter, face-on spiral
with low surface brightness but a brighter core.
M108 is an unnamed spiral galaxy extended ENE-WSW. It is fairly large, 10’ x 3’, with
an evident central bulge and stellar nucleus, as well as tapering ends.
Ursa Major was recognized by two far different cultures as a constellation: the
Greeks and Native Americans. Both groups recognize the constellation as a bear, but there
are not many more similarities between the two.
Greek mythology tells the tale of Callisto, who was the daughter of King Lycaon.
Artemis, who was Apollo’s sister, patroness of childbirth and protector of babes and
suckling animals, highly prized chastity and was granted eternal virginity. She chose and
gathered nymphs to follow in her ways and take vows of chastity as well. One of these was
Callisto. Zeus seduced Callisto, just as he had many others, and Callisto became pregnant
with his child. When Artemis discovered this, she turned Callisto into a bear to take
revenge. Artemis planned that the bear would be hunted and killed. However, Zeus took
pity and sent Callisto to the heavens as the great bear. Her child, when born, then became
Ursa Minor.
Native Americans do not have any exact stories regarding the origination of the
great bear in the sky, however a large number of tribes associate Ursa Major with a big
bear, and this is suggested to be from the ideology that only a prodigious bear could live in
such a northerly climate. This would make sense as the Native Americans would have not
had much other contact with large animals much like Ursa Major other than the bear.
Take a look to the northern night sky and try to pick out Ursa Major. Can you? The
answer is most likely not. Partially due to light pollution on earth, it is terribly difficult view
all the stars of Ursa Major as many of the stars either don’t have enough luminosity to stand
out in our night sky or they look too much like the ones around them. In fact, even if you
can make out the pattern in the stars in the upper left corner of the constellation, it is still
rather difficult to draw the lines to the other stars.
For that reason, people are more likely to say that the Big Dipper, the brightest 7
stars of Ursa Major located in the upper left corner of the constellation, is actually the
constellation in this area, though they are mistaken. The Big Dipper is an asterism, a pattern
of stars most modernly recognized and usually a part of another constellation. Because the
88 constellations currently recognized span the entire visible sky, it makes little sense to
add any more as constellations are their own set, containing no stars that are part of
another constellation, and adding more constellations would mean beginning to create
doubles or duplicates.
As said before, the stars of the Big Dipper are the brightest 7 stars of Ursa Major (αζ are of magnitude 2, η has magnitude 3). Let’s look closer at these stars. Alpha (α), called
Dubhe, is a yellow giant 25 times the size of the sun and 86 light years away. Beta (β)is
Merak, gamma (γ) is Phecda , and delta (δ) is Megrez; all three are white A-type stars within
100 light years distance. Epsilon (ε), Alloth, is an alpha-cv type variable at roughly 360 light
years distance. Mizar, zeta (ζ), is at a distance of 78 light years. And Eta (η), Alkaid, is a
bluish star at 95 light years distance. As you can tell, most of these stars are at fairly close
distance, with all except one closer than 100 light years away.
Known to us as the Big Dipper, this asterism goes by different names in different parts of
the world. In Europe it is called a plough, Charles’ (Charlemagne’s) Wain, or the Big Wagon.
Hindus say it represents the Sapta Rishi or seven holy ancient sages. Romans regard to it as
Septentriones (the seven plowing oxen). An Arabian story has the bowl as a coffin, with the
three stars in the handle as mourners, following it and the Chinese say it is the government.
There are two American tales regarding the Big Dipper. The first comes from Native
Americans, who have widely agreed that the bowl of the Big Dipper is actually a big bear,
and the three stars of the ladle are either its’ three cubs, or three hunters. Following the
three hunters claim, a group of Native Americans supposed that as the asterism is low in the
autumn evening sky, the hunters had injured the bear and its blood caused the trees to
change color to red. The other tale comes from the Underground Railroad. Called the
Drinkin’ Gourd by those on the Underground Railroad, legend has it that the asterism was
used by fugitive slaves in the United States as a point of reference to the north, as the alpha
and beta stars point up to Polaris, the North Star. A folk song supposedly taught to these
fugitive slaves carries these lyrics:
“When the sun comes back and the first quail calls,
Follow the Drinkin' Gourd.
For the old man's waitin' for to carry you to freedom
If you follow the Drinkin' Gourd.”
The Big Dipper has been in roughly its present shape for the last 50,000 years.
Unfortunately, the stars of the Big Dipper are not moving in the same direction, and over
time the asterism will dissolve. As the stars move apart, the shape will become more and
more plow-like with Dubhe moving farther in front of the rest, as well as a little down.