An Evening Project

Most amateurs begin astronomy just as sightseers, and eventually they reach a plateau. After a few months or years of pleasantly touring the celestial scenery, they may feel a need for more meaningful involvement. After all, a tourist can't be expected to gape at the Grand Canyon forever. Lasting enthusiasm, in astronomy as in anything else, comes from purposeful activity. For thousands of people during the last century or more, variable stars have provided that outlet.

If you've never tried your eye on variable stars, do a simple project tonight (assuming it's before sunrise in September or October, 2003). If you can find the Sickle of Leo you can find R Leonis, one of the brightest red long-period variables. It brightens and fades between magnitudes 5 and 10 over a cycle of 312 days on average — about 10 months. A small telescope or even binoculars are all the equipment you'll need. (For a selection of bright targets available at various times of year, see "The Top 12 Naked-Eye Variable Stars.")

The first step in variable-star observing is finding the star. As shown on any chart that shows all stars down to 6th magnitude, R Leonis lies 5° west of bright Regulus very close to the pair of 6th-magnitude stars 18 and 19 Leonis. The low-power view in most finderscopes or binoculars shows them barely in the same field as Regulus. Work your way west from Regulus, carefully comparing the brightest star patterns on the chart to what you see. Be sure to use a correct-image finder, not one that gives a mirror image (take out any right-angle star diagonal).

If you have trouble finding your target star, make sure you know the size of your instrument's field of view, and make a wire ring of that size using the degree scale on the edge of your chart. Slide the ring across the paper to preview what star patterns will cross your eyepiece.

Be sure to turn the chart so north matches north in the eyepiece. Push your instrument slightly toward Polaris; north is the side from which new stars enter the field of view.

One help in identifying R Leonis is its deep orange-red color. Like other Mira-type variables, it is a pulsing red giant of spectral type M. It contrasts vividly with bluish white 19 Leonis (spectrum A3) and golden yellow 18 Leonis (gK4).

Many comparison stars around R have their magnitudes printed on the maps on this page. The magnitudes are traditionally given to the nearest tenth with the decimal point left out so that it won't be confused with a faint star.

Once you've identified the variable, find a comparison star that looks just a little brighter than it and one that's just a little fainter. Look back and forth among the three and judge where the variable's brightness falls with the respect to the two comparisons as best you can.

Suppose you decide R is two-thirds of the way in brightness from a star of magnitude 8.2 to another of magnitude 8.6. In other words it appears twice as far in brightness from magnitude 8.2 as it does from 8.6. The usual shorthand way of writing this in your notes at the scope is 82-2-R-1-86. Do the arithmetic later indoors: two-thirds of the way from 8.2 to 8.6 is 8.47, which you can round off to 8.5.

Don't be discouraged if at first you can hardly distinguish brightnesses 0.3 or 0.4 magnitude apart. Brightness estimating is a skill honed by practice and experience. But be warned: once you start, it's addictive. The second estimate of a particular star is much easier than the first, and the more familiar you become with a star the greater your curiosity will be about what it's doing tonight.