Constellations

Constellations are useful because they help us find our way around the night sky. There are 88 constellations which cover the entire sky. About 80 of them can be seen from Hawaii, but not all will be visible during the fall semester. We will study 10 or so of these constellations; most have brighter stars and are fairly easy to recognize.

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.

READING STAR-CHARTS

It is fairly easy to recognize constellations using the sky charts in The Sky Tonight. These charts show how the sky actually appears from Hawaii at various times. Despite their circular shapes, these are not all-sky charts; each shows a wide-angle view looking north, east, south, or west. For example, chart 9-North shows the view looking north at about 20:30 (8:30 pm) in late September. The translucent overlay for each chart shows the outlines of the constellations and names some bright stars.

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.

APPARENT MAGNITUDES

A star's apparent magnitude is a number which indicates how bright the star appears in the sky. Bright stars have small apparent magnitudes, and faint stars have large apparent magnitudes; this may seem backward, but it made sense to astronomers thousands of years ago and we've been stuck with it ever since. The difference in the apparent magnitudes of two stars tells you the ratio of their brightnesses; a difference of 2.5 magnitudes implies a brightness ratio of 10:1. (Note: if you know about logarithms, you may recognize that the magnitude system employs a logarithmic scale.) To show what this means, suppose we have three stars, called A, B, and C:

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
Here we are using the symbol m for apparent magnitudes; the letter written below the m indicates which star this value refers to. Then,
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 three bright stars making up the ``Summer Triangle'' are Vega (magnitude 0.03), Altair (magnitude 0.76), and Deneb (magnitude 1.2). The brightest of these - that is, the one with the smallest magnitude - is Vega. The constellation of Lyra, which includes Vega, is defined by several rather dim stars; the faintest is zeta Lyrae (magnitude 4.4). With the naked eye, the faintest stars visible from Kapiolani Park have apparent magnitudes of about 4.5, and the faintest stars visible from a really dark observing site have apparent magnitudes of 6.0 to 6.5.

Knowledge of apparent magnitudes helps in planning observations. For example, you might want to know how much of the constellation of Cygnus will be visible. The five stars outlining the basic shape of this constellation have apparent magnitudes between 1.2 and 3.2; these are all easy to see. However, the chart for Cygnus in Stars & Planets (p. 135) incorporates several fainter stars with apparent magnitudes between 3.4 and 4.0; these will be harder to see unless you are looking from a fairly 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.

ANGULAR SEPARATIONS

The angular separation of two objects is an angle measuring how far apart the objects appear from your point of view. For example, make a ``shaka'' with your arm outstretched, thumb up, and pinkie down; now imagine two lines extending from your eye to the tips of your thumb and pinkie, as shown in Fig. 1. These lines meet at an angle of about 20°, so the angular separation between the tip of your thumb and the tip of your pinkie is about 20° (this depends on the length of your arm and the size of your hand, but 20° is average). If two stars have an angular separation of 20°, you should be just about able to cover one with your thumb and the other with your pinkie by holding a shaka up to the sky at arm's length.

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 single finger, at arm's length, is about 2° wide.

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.

LEARNING CONSTELLATIONS

It's often fairly easy to see a constellation when its pointed out in the sky, but harder to remember it so you can find it yourself. The best way to really learn constellations is to draw them; when you do this, your eyes often find geometrical patterns which will help you identify these constellations later. You should make your drawings to scale; this will give you a feeling for the sizes of constellations. (Many people confuse the Pleiades with the Little Dipper, not realizing that one is about ten times the size of the other!) Here's how to make an accurate drawing:

  1. Identify the constellation in the sky; we will help you with this.
  2. Pick two bright stars in the constellation, and measure their angular separation.
  3. Plot those two stars on a sheet of sketch paper, using a scale of 0.5 cm per degree (unless you can't fit the constellation on the paper at this scale.)
  4. In the margin of your sketch, name the two stars you measured, and give the angle between them in degrees.
  5. Fill in the other stars, using the ones you've already plotted as reference points.
  6. Use larger dots to indicate the brighter stars. Try to show at least three levels of brightness (e.g., bright, faint, and dim).
  7. Note any stars with distinctive colors, and any other interesting observations (e.g., ``fuzzy'' stars).
  8. Later, use a colored pencil to outline the shape of the constellation, and write the Bayer letter next to each star.

Make sure to include your name, the name of the constellation, the date, and the time on your sketch.

OBSERVING SCHEDULE

Because different constellations are visible at different times, we will return to the study of constellations throughout the semester.

  1. September

    Constellation Description in
    Stars & Planets
    Chart in
    The Sky Tonight
    Cygnus p. 134 9-North
    Lyra p. 178 9-North
    Sagittarius p. 220 9-South
    Scorpius p. 224 8-South

    Most of these constellations are easy to see; you may even know some of them. Cygnus and Sagittarius are bright constellations along the Milky Way. Sagittarius and Scorpius lie along the ecliptic. Lyra includes the star beta Lyrae, which is a famous variable star.

  2. October

    Constellation Description in
    Stars & Planets
    Chart in
    The Sky Tonight
    Cassiopeia p. 106 10-North
    Cepheus p. 112 10-North
    Capricornus p. 102 10-South
    Pegasus p. 200 9-East

    Cassiopeia continues the sequence of bright constellations along the Milky Way. When observing Cepheus, note the star delta Cephei, which is the prototype of an important class of variable stars. Capricornus, although faint, lies along the ecliptic. The ``Great Square'' of Pegasus is useful signpost for the Autumn sky.

  3. November

    Constellation Description in
    Stars & Planets
    Chart in
    The Sky Tonight
    Andromeda p. 72 10-East
    Perseus p. 202 11-North

    Andromeda, although not very striking, contains the famous Andromeda Galaxy. Perseus lies along the Milky Way; it contains the variable star Algol (beta Persei).


Joshua E. Barnes (barnes@ifa.hawaii.edu)

Last modified: September 9, 2003
http://www.ifa.hawaii.edu/~barnes/ASTR110L_F03/constellations.html