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 spring semester. We will study 15 of these constellations which 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. You could make a random pattern by closing your eyes and spattering droplets of ink on a sheet of paper. If you look at the result for a while, your mind will start to group ink spots together, and some groups might even seem like pictures of things you know. Another person looking at the same sheet of paper might come up with some of the same groupings.

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. You can now buy a computer-controlled telescope for a few thousand dollars, although it takes time to learn how to use such an instrument. In this class we will mostly be using simple telescopes with manual controls, and a knowledge of the constellations will be very handy in finding targets 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 1-East shows the view looking East at about 20:30 (8:30 pm) in late January. The translucent overlay for each chart shows the outlines of the constellations and names some bright stars.

Once you've found a constellation, you can turn to the individual constellation charts in Stars & Planets for more detail. These charts show more stars than the ones in The Sky Tonight, and include Bayer letters (see Stars & Planets, p. 8) for the stars in each constellation. They also show the brightness of each star (using a system described below), and employ special symbols to indicate double and variable stars, as well as objects like clusters, nebulae, and galaxies.

APPARENT MAGNITUDES

The apparent magnitude of a star is a number which indicates how bright it appears in the sky. Briefly, bright stars have small aparent magnitudes, and faint stars have large apparent magnitudes (this may seem backward to you, but it made sense to astronomers thousands of years ago and we've been stuck with it ever since). More precisely, 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. 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
(Note: if you know about logarithms, you may recognize that the magnitude system is a logarithmic scale.)

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.

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 straight in front, your thumb pointing up, and your pinkie pointing down; now imagine two lines extending from your eye to the tips of your thumb and pinkie, as shown below. 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 your shaka up against the sky at arm's length.

To measure angular separations more accurately, we will use a device called a cross-staff, which is basically a stick with a ruler mounted on one end. The stick determines the distance from your eye to the ruler; if this distance is 57.3 cm, then 1 cm on the ruler corresponds to an angular separation of 1°. (Note: if you know about 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 angle 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.
  2. Make a rough sketch in your observing log.
  3. Pick two bright stars in the constellation, and measure their angular separation; record the results in your log.
  4. Plot those two stars on a sheet of sketch paper, using a scale of 0.5 cm per degree.
  5. Fill in the other stars, using the ones you've already plotted as reference points.
  6. Use larger points to indicate the brighter stars; note stellar colors or other interesting features.
  7. Later, use a colored pencil to outline the shape of the constellation, and write the Bayer letter next to each star.

OBSERVING SCHEDULE

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

  1. Early Winter (Jan 21 or Jan 28)

    Constellation Description in
    Stars & Planets
    Chart in
    The Sky Tonight
    Orionp. 1941-East
    Perseus
    p. 2021-North
    Cassiopeia
    p. 1061-North
    Gemini
    p. 1501-East
    Taurusp. 2362-West

    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.

  2. Late Winter (Feb 25 or Mar 04)

    Constellation Description in
    Stars & Planets
    Chart in
    The Sky Tonight
    Canis Major
    p. 982-South
    Cancer
    p. 942-East
    Auriga
    p. 862-North
    Leo
    p. 1662-East
    Ursa Major
    p. 2483-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.

  3. Early Spring (Apr 22 or Apr 29)

    Constellation Description in
    Stars & Planets
    Chart in
    The Sky Tonight
    Ursa Minor
    p. 2524-North
    Bootes
    p. 884-East
    Virgo
    p. 2564-East
    Centaurus
    p. 2125-South
    Crux
    p. 1325-South

    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.


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

Last modified: January 8, 2003
http://www.ifa.hawaii.edu/~barnes/ASTR110L_S03/constellations.html