Constellations & Magnitudes

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.

CONSTELLATION CHARTS

Each constellation is shown in a detailed chart in Stars & Planets. The charts show every star you can easily see with your naked eye. Individual stars are labeled with Greek letters (or sometimes 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. Special symbols indicate double stars, variable stars, star clusters, nebulae, and galaxies; these objects may be labeled with Roman letters, the letter "M" followed by a number, or just a 3 or 4-digit number. The charts in Stars & Planets also show the brightness of each star, using large dots for bright stars and small dots for faint ones; the system used to measure stellar brightness is described next.

APPARENT MAGNITUDES

A star's apparent magnitude is a number measuring 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. In modern terms, 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 10×10 = 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 to 5.0, and the faintest stars visible from a really dark location 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.

LEARNING CONSTELLATIONS

It's often fairly easy to see a constellation when it's 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. Since drawing a constellation ''from scratch'' is hard, we will help you get started by plotting a few of the brighter stars; your job will be to add other stars, distinguish bright stars from faint ones, and note any unusual objects you may observe in the constellation. In detail, here's what we want you to do:

  1. Write your name, the date, and the time on your worksheet.
  2. Identify the constellation in the sky; we will help you with this.
  3. Identify the bright stars already plotted on the worksheet for this constellation.
  4. Fill in the other stars, using the ones already plotted as reference points.
  5. Use larger dots to indicate the brighter stars. Try to show at least three levels of brightness (e.g., bright, medium, and faint).
  6. Note any stars with distinctive colors, and any other interesting objects (e.g., ``fuzzy'' stars).
  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. September

    Constellation Description in Stars & Planets
    Cygnus p. 134
    Sagittarius p. 220
    Scorpius p. 224

    These constellations are easy to see; you may already know some of them. Cygnus and Sagittarius lie along the Milky Way; Sagittarius and Scorpius lie along the ecliptic.

  2. October

    Constellation Description in Stars & Planets
    Lyra p. 178
    Cassiopeia p. 106
    Cepheus p. 112

    Lyra contains Vega, the brightest star in the summer and fall skies, as well as the variable star beta Lyrae. 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 type of variable stars.

  3. November

    Constellation Description in Stars & Planets
    Pegasus p. 200
    Andromeda p. 72
    Perseus p. 202

    The ``Great Square'' of Pegasus is good signpost for the Autumn sky. 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: August 31, 2005
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