The American Association of Variable Star Observers

25 Birch Street · Cambridge, MA 02138 · U.S.A. · 617-354-0484 · aavso@aavso.org

Observing Variable Stars

Observing Variable Stars has been adapted from the curriculum material being developed by Hands-On Astrophysics, an educational project of the American Association of Variable Star Observers (AAVSO), supported by a grant from the National Science Foundation (U.S.A.).

Learning to Observe Variable Stars

Stars that vary in brightness are the neon lights of the Universe. Their brightness can change in minutes, in hours, in days, or years. Observers all over the world watch them carefully, compare their brightness with that of the "comparison" stars which do not vary, and send their estimates each month to the American Association of Variable Star Observers (AAVSO) or to other variable star organizations.

First, LEARN THE CONSTELLATIONS. For this a good Star Atlas will be your best friend. Although there are a number of variable stars visible to the naked eye, you may want to obtain a telescope (or binoculars for bright stars only) and some AAVSO Star Charts (or other standardized charts).

These charts show the "field" of a variable, and its position is given in its designation number. Just as we find a place on earth by Latitude and Longitude, we can find any point in the sky by Right Ascension and Declination. For example, Z Ursae Majoris 1151+58 (its designation number) is the star Z in constellation Ursa Major, in a position of 11 hours and 51 minutes Right Ascension (vertical lines on the chart), and +58° Declination (horizontal lines).

As you study your Atlas, the instructions become clearer. To help you remember the position of stars, find formations like squares, triangles, etc., to point the way to the variable, which is shown on the chart as either a small open circle (º) or a little circle with a dot--while other stars are shown as different-sized dots. An important thing to remember if you use a telescope with the usual eyepieces (not diagonals) is that the star fields are seen upside down. So your chart must be inverted; the end marked N must be pointed away from the North Pole. There are different kinds of charts for use with different size telescopes. Type "a" charts are used with binoculars and they need not be used upside down. They show variables 7th magnitude or brighter at maximum.

At first it may take you a long while to find a variable. But take your time--check back and forth between chart and sky to be sure you have the right position. When you are sure, compare the star's brightness with the known light of the comparison stars. Stars of magnitude 1 are brightest, 2nd magnitude not quite so bright, 3rd magnitude a little fainter, and so on. Sixth magnitude is the limit of naked-eye seeing and optical aid (binoculars, telescope) is needed to go fainter. The brightness in between magnitudes is given in tenths (with a decimal point, as 5.1, 5.2, 5.3, down to 5.9, which is followed by 6.0 magnitude.) For example, a star halfway between magnitude 5 and 6 will be given as 5.5, and a star a tiny bit brighter than 6.0 will be 5.9.

Image of imaginary variable and
  comp stars Let us take an imaginary variable within a triangle of 3 comparison stars: The numbers are magnitudes of comparison stars. One is 60 (6.0, decimal left out so you won't mistake it for another star), the next 63, the third 65. Quickly glance back and forth between the 60 star and the variable. Is the variable fainter? Try the 63. Fainter still? Perhaps 65 is the one. No, it's a little brighter than 65, but dimmer than 63. So 64 is the magnitude you will report. Don't stare at stars too long or they will seem brighter. Do as experts do: look back and forth quickly between the stars. In your notebook record the time you made observations, and comparison stars used.

You are cordially invited to use the report form at the AAVSO's FTP site to send us your estimates (you can also submit estimates by telephone, fax, or e-mail), giving the name of the variable, its designation number, date, time (please indicate wnat time zone you live in), and your final magnitude. Your estimated observations will be compiled with those of other observers and will become part of the AAVSO International Database. Professional astronomers and other members of the scientific community access this data in order to coordinate satellite and ground-based telescope observing programs, obtain information on different kinds of variable stars (including novae and supernovae), etc. You can also request access to your own observations by contacting the AAVSO.

Some additional suggestions

1. Beginners' observations are more accurate and their ability to successfully complete related projects is greatly enhanced if they can do their first observations in the presence of an experienced instructor who can explain the hows of star-finding, measuring and recording observations. Catching problems of misidentification, using wrong comparison stars, miscalculating magnitude, etc., are best caught and corrected early on.

Furthermore, observing can be more fun with other people along. One or more companions can help one another identify constellations and famous stars, share instruments, and so on. There are many members of the AAVSO in Europe, and most European countries have variable star associations. Local experienced observers may be good resources for additional equipment as well as for expertise about the technical aspects of observing (in addition to knowing how to throw a good "star party").

2. Find a suitably dark, but easily-accessible location ahead of time. If you will be on private property, obtain the owner's permission to be there. It may also be a good idea to let local authorities know about your activities.

3. Always be sure to allow enough time for yours eyes to adjust to the darkness (even using a flashlight to write down observations significantly affects light sensitivity).

4. In addition to being adequately dressed for weather conditions, make sure to bring along graph paper, something to write with, and rulers (as well as any optical aids, of course).

Next: Observing Delta Cephei