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ASTRONOMY 130
CONSTELLATION STUDY - FALL SKY
PURPOSE: To be able to identify stars in the sky for the particular season of the year.
PROCEDURE: The stars and constellations will first be observed in the planetarium and then
in the actual sky.
CONSTELLATION SYSTEM:
The Greeks recognized the impossibility of attempting to learn much about the heavens
without first organizing their information about the vast number of stars in some systematic
system. The geometric arrangement of some stars provided the Greeks with a natural
organizational system which we call constellations. By connecting the stars with imaginary lines,
the ancients saw pictures in the sky. These constellations have names and a star may be identified
as being in a certain constellation. Although the modern astronomer no longer employs these
constellations as the ancients did for mythological or astrological purposes, he does use them for
quick reference purposes.
Originally, the constellations did not encompass all of the stars, but in 1933 the
International Astronomical Union divided the entire celestial sphere into 88 constellations using
regular north-south or east-west boundaries so that all stars (and areas of the sky) are now
assigned to a constellation.
Almost everyone is familiar with a few constellations, such as Ursa Major, Orion, and
Scorpio. One could identify the star Alkaid as being the end star in the tail of the constellation
called URSA MAJOR. See Figure 1.
GEOMETRIC DESIGN OR ASTERISM SYSTEM:
In the geometric design system we again connect stars by imaginary lines. Since this
system originated after the constellations were established the geometric designs (asterisms) do
not carry the prestige of the constellations. Nevertheless, they are easily identifiable and quite
useful. They sometimes are comprised of only part of the stars of a constellation or some stars
from two or more constellations. For example, the three stars Vega, Deneb, and Altair form the
geometric design of a 30-60-90 triangle known as the Summer Triangle. One would identify the
star Alkaid as being the end star in the handle of the asterism called the BIG DIPPER. See
Figure 2.
Alkaid
CIRCUMPOLAR CONSTELLATIONS:
It is easier to locate a celestial object if some prominent constellation or asterism can be
found and used as a reference point. Ursa Major (the Big Dipper) serves as a good reference
point for the north circumpolar constellations. Locate the Big Dipper. Begin with the star at the
tip of the handle, this is Alkaid. Continue down the handle, the next star is Mizar. Look
carefully at Mizar with the naked eye and then through one of the telescopes that are set on
Mizar. Note your observations. The next star down the handle is Alioth. Alkaid and Alioth are
almost of equal brightness. Now jump to the lip of the cup. The lip star is Dubhe and star just
below, the corner of the cup, is Merak.
Mentally, draw a line connecting the two stars Merak and Dubhe and extend this line
approximately 5 lengths (25o), and it will reach Polaris. Polaris, the pole star, is the end star in the
tail of Ursa Minor or the end of the handle of the Little Dipper. On a sheet of paper record the
orientation of the two dippers with respect to each other and the horizon. Make sure the
dippers are drawn to scale.
Return to the Big Dipper. Extend the line of the two pointer stars (Merak and Dubhe)
toward Polaris but continue the line an equal distance to the other side of Polaris. This points to a
" W " shaped constellation called Cassiopeia. The star at the sharper tip, the tip that extends
slightly farther south than the other, is Schedar. The bright star at the end of the " W ", the one
near Schedar is known as Caph. Include Cassiopeia in your previous drawing. The husband of
Cassiopeia, Cepheus, is located counterclockwise from Cassiopeia. Cepheus, the King, looks like
a drawing of a house made by a child. The gable or peak of the roof points towards Ursa Major.
Once Cepheus has been located, record its position with respect to the other constellations
on your drawing. Be sure to label directions as well as date and time.
Winding its way between the two dippers is an irregular line of stars bearing the name of
Draco (the dragon). The tail is located between the cups of the dippers and winds clockwise
toward Cepheus. The head of the dragon is located between the cup of the Little Dipper and the
bright star, Vega. This concludes a look at the circumpolar constellations. Before class meets
again, make another drawing showing the location and orientation of these five
constellations at a time at least four hours later than the time of your first observation. Be
sure to labe directions as well as date and time.
CONSTELLATIONS OF THE FALL:
Turn to face the southern horizon and the region of the sky overhead. You should find
three bright stars that form the apexes of a large right triangle. The brightest of the three is Vega
(in Lyra), which is farthest west and at the right angle of the triangle. The other two stars are
Deneb (in Cygnus) and Altair (in Aquila). These three constellations make good reference points
for the rest of the fall sky. Note in the eastern sky a large area defined roughly by four stars in a
great square. This is the body of the constellation of Pegasus, which is connected to Andromeda.
Make a drawing showing the relative size and location of the following
constellations: Lyra, Cygnus, Aquila, Hercules, Scorpio, Sagittarius, and Libra. Be sure to
label directions as well as date and time.
Locate the stars Vega and Altair, compare their colors. What do you think the colors
might indicate? There will be several telescopes set up on the observing deck. Observe the
following objects and make a sketch of what you observe. Beta Cygnus (Alberio), Epsilon
Lyra, M-57, M-13, and any of the planets that are visible. Record on your drawing of the
constellations the location of all the objects observed through the telescope.
Questions:
1. What is a constellation? Name 8 of them.
2. What is an asterism? Name 5 of them.
3. How many constellations are located on the celestial sphere? Can all of these constellations
be observed from earth at one time? Explain.
4. Describe how to locate the north star.
5. Describe how to locate your zenith, celestial meridian, and the cardinal points.
6. How are stars named scientifically? What are the scientific names for each of the bright stars
pointed out in this exercise? What is the "common" name for the star Alpha Scorpio?