Background Reading: Stars & Planets,
p. 5 to 10 (Constellations, Star names, and Star brightness).
Additional readings for individual constellations are listed below.
For thousands of years, people looking at the night sky have grouped stars into constellations. The stars making up a constellation often seem to trace recognizable patterns. For example, the stars in Orion outline a man wearing a sword, and the stars in Maui's Fish-hook follow the shape of a fishing hook. The stars making up a constellation usually have very little to do with each other; some may be relatively close, while others are much further away. But because stars move so slowly through space, the patterns we see today have hardly changed since the dawn of history. Some of the constellations we know today were first defined by our ancestors thousands of years ago.
The pattern of stars in the sky is basically random, much like the pattern made by spattering droplets of ink on a sheet of paper. If you look at a random pattern of dots for a while, your mind will start to group dots together, and some groups might even seem like pictures of things you know. Another person looking at the same pattern might come up with some of the same groups.
It may surprise you to learn that professional astronomers don't use constellations to locate objects in the sky; instead, they use celestial coordinates. To point a modern observatory telescope at a particular object, you just give the object's celestial coordinates to a computer, and the machines do the rest. In this class we will use simple telescopes with manual controls, and a knowledge of the constellations will be helpful in finding things to observe.
Once you've found a constellation, turn to the individual constellation charts in Stars & Planets for more detail. These charts show every star you are likely to see with your naked eye. They label stars with Greek letters (and some with numbers). These Bayer letters (see Stars & Planets, p. 8) provide standard names for stars; for example, the bright star Vega is also called alpha Lyrae. You will need to be familiar with Bayer letters to locate the stars we discuss in this class. The charts in Stars & Planets also show the brightness of each star, using a system described below, and employ special symbols to indicate double and variable stars, star clusters, nebulae, and galaxies.
|star A has apparent magnitude 1.0||mA = 1.0|
|star B has apparent magnitude 3.5||mB = 3.5|
|star C has apparent magnitude 6.0||mC = 6.0|
|star A appears 10 times brighter than star B||mB - mA = 2.5|
|star B appears 10 times brighter than star C||mC - mB = 2.5|
|star A appears 100 times brighter than star C||mC - mA = 5.0|
To give some specific examples, the 'dog star' Sirius, the brightest star in the night sky, has apparent magnitude -1.4, the brightest star in Orion has apparent magnitude 0.2, and the brightest star in Cancer has apparent magnitude 3.5. With the naked eye, the faintest stars visible from Honolulu have apparent magnitudes of about 4.5 to 5.0, and the faintest stars visible from a really dark observing site have apparent magnitudes of 6.0 to 6.5.
Knowledge of apparent magnitudes is useful in making observations. For example, you might want to know how much of the constellation of Orion you can expect to see. The stars making up Orion's body have magnitudes between 0.2 and 2.2, the star representing his head has apparent magnitude 3.5, and the stars outlining his club and shield have apparent magnitudes of 4.0 or more. Thus Orion's body is easily visible, and his head is not too hard to see, but his club and shield will be harder to see unless you are looking from a really dark location. The constellation charts in Stars & Planets show stellar magnitudes by using dots of different sizes; in addition, magnitudes are usually included when individual stars are discussed in the text.
|Fig. 1. A "handy" measure of angular separation. At arm's length, the angle between your outstretched thumb and pinkie is about 20°.|
You can estimate smaller angles with your hand as well. For example, your fist, held at arm's length, defines an angle of about 10°. A pinkie finger, at arm's length, is about 1° wide and a thumb about 2°.
To measure angular separations more accurately, we will use a device called a cross-staff, which is basically a stick, 57.3 cm long, with a centimeter ruler mounted on one end. The length of the stick was deliberately chosen; if the ruler is 57.3 cm from your eye, 1 cm on the ruler defines an angle of 1°. (Note: if you know trigonometry, 57.3 = 1/tan(1°).) It's fairly easy to use a cross-staff; close one eye and place the end of the stick without the ruler just under the other eye. Sight along the stick towards the two stars you want to measure and adjust the markers on the ruler to line up with these stars. Finally, read off the positions of the markers on the ruler; the difference between them is the angular separation between the two stars.
Stars & Planets
The Sky Tonight
All of these constellations are easy to see; you may even
know some of them. Cassiopeia, Perseus, Taurus, and Orion are part of a
chain of bright constellations along the Milky Way. Taurus and Gemini lie
along the ecliptic, which is the path the Sun, Moon, and planets take across
the sky. In observing Gemini and Perseus, take special note of the brightness
of the stars beta Persei and zeta Geminorum by comparing them with other
stars in these constellations.
Stars & Planets
The Sky Tonight
|Canis Major||p. 98||2-South|
|Ursa Major||p. 248||3-North|
Canis Major extends the chain of Milky Way constellations
listed above; it contains the brightest star in the sky, Sirius. Cancer
and Leo lie along the ecliptic. Auriga and Ursa Major are bright constellations.
Stars & Planets
The Sky Tonight
|Ursa Minor||p. 252||4-North|
Ursa Minor, Booties, and Virgo are relatively faint, but
each contains one fairly bright star of special significance. Virgo follows
Leo along the ecliptic. Centaurus and Crux are spectacular constellations
which can be seen from Hawaii but not from most of the continental US.
Use your text and the smaller field map of the Orion sky area below to gain some experience reading a star map and to answer the questions below. Turn paper copies of this chart in with your lab report. Your lab report should include all relevant observing information (date, time, location, name, etc.) and your sketch work and answers to any lab questions.
FULL SKY MAP
Last modified: January 25, 2005